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A bone is a rigid
tissue Tissue may refer to: Biology * Tissue (biology), an ensemble of similar cells that together carry out a specific function * ''Triphosa haesitata'', a species of geometer moth found in North America * ''Triphosa dubitata'', a species of geometer mot ...
that constitutes part of the
vertebrate Vertebrates () comprise all species of animal Animals (also called Metazoa) are multicellular A multicellular organism is an organism In biology, an organism () is any organic, life, living system that functions as an indiv ...
skeleton A skeleton is a structural frame that supports an animal Animals (also called Metazoa) are multicellular eukaryotic organisms that form the Kingdom (biology), biological kingdom Animalia. With few exceptions, animals Heterotroph, consu ...

skeleton
in animals. Bones protect the various organs of the body, produce
red Red is the color at the long wavelength end of the visible spectrum of light, next to orange and opposite violet. It has a dominant wavelength Image:dominant wavelength.png, frame, Dominant/complementary wavelength example on the CIE color ...

red
and
white blood cell White blood cells (WBCs), also called leukocytes or leucocytes, are the cells of the immune system The immune system is a network of biological processes that protects an organism In biology, an organism (from Ancient Greek, Greek: ...
s, store
mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form.John P. Rafferty, ed. (2 ...

mineral
s, provide structure and support for the body, and enable
mobility Mobility may refer to: Social sciences and humanities * Economic mobility, ability of individuals or families to improve their economic status * Geographic mobility, the measure of how populations and goods move over time * Mobilities, a contempo ...
. Bones come in a variety of shapes and sizes and have a complex internal and external structure. They are lightweight yet strong and hard, and serve multiple
functions Function or functionality may refer to: Computing * Function key A function key is a key on a computer A computer is a machine that can be programmed to carry out sequences of arithmetic or logical operations automatically. Modern comp ...
. Bone tissue (osseous tissue) is a
hard tissueHard tissue (also termed calcified tissue) is tissue which is mineralized and has a firm intercellular matrix In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Bioc ...
, a type of specialized
connective tissue Connective tissue is one of the many basic types of animal Animals (also called Metazoa) are multicellular A multicellular organism is an organism In biology, an organism () is any organic, life, living system that functions ...
. It has a
honeycomb A honeycomb is a mass of hexagonal prismatic wax Waxes are a diverse class of organic compound In , organic compounds are generally any s that contain - . Due to carbon's ability to (form chains with other carbon s), millions of organi ...

honeycomb
-like
matrix Matrix or MATRIX may refer to: Science and mathematics * Matrix (mathematics), a rectangular array of numbers, symbols, or expressions * Matrix (logic), part of a formula in prenex normal form * Matrix (biology), the material in between a eukaryoti ...
internally, which helps to give the bone rigidity. Bone tissue is made up of different types of
bone cell An osteocyte, an oblate shaped type of bone A bone is a Stiffness, rigid tissue (anatomy), tissue that constitutes part of the skeleton in most vertebrate animals. Bones protect the various organs of the body, produce red blood cell, red and wh ...
s.
Osteoblast Osteoblasts (from the Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is app ...

Osteoblast
s and
osteocyte An osteocyte, an oblate shaped type of bone A bone is a rigid tissue Tissue may refer to: Biology * Tissue (biology), an ensemble of similar cells that together carry out a specific function * ''Triphosa haesitata'', a species of geometer m ...
s are involved in the formation and mineralization of bone;
osteoclast An osteoclast () is a type of bone cell An osteocyte, an oblate shaped type of bone A bone is a Stiffness, rigid tissue (anatomy), tissue that constitutes part of the skeleton in most vertebrate animals. Bones protect the various organs of the ...

osteoclast
s are involved in the
resorption Resorption is the absorption into the circulatory system of cells or tissue, usually by osteoclast, osteoclasts. Types of resorption include: * Bone resorption * Herniated Disc Resorption * Tooth resorption * Fetal resorption * Blood resorption S ...
of bone tissue. Modified (flattened) osteoblasts become the lining cells that form a protective layer on the bone surface. The mineralized matrix of bone tissue has an organic component of mainly
collagen Collagen () is the main structural protein Proteins are large biomolecule , showing alpha helices, represented by ribbons. This poten was the first to have its suckture solved by X-ray crystallography by Max Perutz and Sir John Cowder ...

collagen
called ''
osseinOssein is the organic extracellular matrix In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactio ...
'' and an inorganic component of
bone mineral Bone mineral (also called inorganic bone phase, bone salt, or bone apatite) is the inorganic In chemistry Chemistry is the scientific discipline involved with Chemical element, elements and chemical compound, compounds composed of atoms, mol ...
made up of various salts. Bone tissue is a
mineralized tissue, sea shells, conch, dentin, radiolarian, antler, bone Mineralized tissues are biological tissue (biology), tissues that incorporate minerals into soft matrices. Typically these tissues form a protective shield or structural support. Bone, mollusk ...
of two types, cortical bone and cancellous bone. Other types of tissue found in bones include
bone marrow Bone marrow is a semi-solid tissue Tissue may refer to: Biology * Tissue (biology), an ensemble of similar cells that together carry out a specific function * ''Triphosa haesitata'', a species of geometer moth found in North America * ''Triphos ...
,
endosteum The endosteum (plural endostea) is a thin vascular membrane of connective tissue Connective tissue is one of the many basic types of animal Animals (also called Metazoa) are multicellular eukaryotic organisms that form the Kingdom (biolo ...
,
periosteum The periosteum is a membrane Image:Schematic size.jpg, up150px, Schematic of size-based membrane exclusion A membrane is a selective barrier; it allows some things to pass through but stops others. Such things may be molecules, ions, or other sma ...
,
nerve A nerve is an enclosed, cable-like bundle of fibers (called axon An axon (from Greek ἄξων ''áxōn'', axis), or nerve fiber (or nerve fibre: see spelling differences Despite the various English dialects Dialect The term diale ...

nerve
s,
blood vessel The blood vessels are the components of the circulatory system The circulatory system, also called the cardiovascular system or the vascular system, is an organ system An organ system is a biological system A biological system is a comp ...
s and
cartilage Cartilage (cartilaginous tissue) is a resilient and smooth elastic tissue Elastic is a word often used to describe or identify certain types of elastomer An elastomer is a polymer A polymer (; Greek ''wikt:poly-, poly-'', "many" + ''wikt:-m ...

cartilage
. In the
human body The human body is the structure of a Human, human being. It is composed of many different types of Cell (biology), cells that together create Tissue (biology), tissues and subsequently organ systems. They ensure homeostasis and the life, viabi ...

human body
at birth, there are approximately 270 bones present; many of these fuse together during development, leaving a total of 206 separate bones in the adult, not counting numerous small
sesamoid bone In anatomy Anatomy (Greek ''anatomē'', 'dissection') is the branch of biology concerned with the study of the structure of organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is any ind ...
s. The largest bone in the body is the
femur The femur (; ), or thigh bone, is the proximal Standard anatomical terms of location deal unambiguously with the anatomy of animals, including humans. Terms used generally derive from Latin or Greek language, Greek roots and used to describe s ...

femur
or thigh-bone, and the smallest is the
stapes The ''stapes'' or stirrup is a bone in the middle ear of humans and other animals which is involved in the conduction of sound vibrations to the inner ear. This bone is connected to the oval window by its Annular ligament of stapes, annular ligam ...
in the
middle ear The middle ear is the portion of the ear The ear is the organ of hearing and, in mammals, balance. In mammals, the ear is usually described as having three parts—the outer ear The outer ear, external ear, or auris externa is t ...

middle ear
. The Greek word for bone is ὀστέον ("osteon"), hence the many terms that use it as a prefix—such as
osteopathy Osteopathy is a type of alternative medicine that emphasizes physical manipulation of the body's muscle tissue and bone A bone is a Stiffness, rigid tissue (anatomy), tissue that constitutes part of the vertebrate skeleton in animals. ...
.


Structure

Bone is not uniformly solid, but consists of a flexible
matrix Matrix or MATRIX may refer to: Science and mathematics * Matrix (mathematics), a rectangular array of numbers, symbols, or expressions * Matrix (logic), part of a formula in prenex normal form * Matrix (biology), the material in between a eukaryoti ...
(about 30%) and bound minerals (about 70%) which are intricately woven and endlessly remodeled by a group of specialized bone cells. Their unique composition and design allows bones to be relatively hard and strong, while remaining lightweight. Bone matrix is 90 to 95% composed of elastic
collagen Collagen () is the main structural protein Proteins are large biomolecule , showing alpha helices, represented by ribbons. This poten was the first to have its suckture solved by X-ray crystallography by Max Perutz and Sir John Cowder ...

collagen
fibers, also known as ossein, and the remainder is
ground substance Ground substance is an amorphous gel-like substance in the extracellular space that contains all components of the extracellular matrix In biology Biology is the natural science that studies life and living organisms, including their anatom ...
. The elasticity of
collagen Collagen () is the main structural protein Proteins are large biomolecule , showing alpha helices, represented by ribbons. This poten was the first to have its suckture solved by X-ray crystallography by Max Perutz and Sir John Cowder ...

collagen
improves fracture resistance. The matrix is hardened by the binding of inorganic mineral salt,
calcium phosphate Calcium phosphate is a family of materials and mineral In geology Geology (from the Ancient Greek γῆ, ''gē'' ("earth") and -λoγία, ''-logia'', ("study of", "discourse")) is an Earth science concerned with the solid Earth, the rock ...
, in a chemical arrangement known as calcium
hydroxylapatite Hydroxyapatite, also called hydroxylapatite (HA), is a naturally occurring mineral form of calcium apatite with the Chemical formula, formula Ca5(PO4)3(OH), but it is usually written Ca10(PO4)6(OH)2 to denote that the Crystal structure, crysta ...
. It is the
bone mineral Bone mineral (also called inorganic bone phase, bone salt, or bone apatite) is the inorganic In chemistry Chemistry is the scientific discipline involved with Chemical element, elements and chemical compound, compounds composed of atoms, mol ...
ization that give bones rigidity. Bone is actively constructed and remodeled throughout life by special bone cells known as osteoblasts and osteoclasts. Within any single bone, the tissue is woven into two main patterns, known as cortical and cancellous bone, and each with different appearance and characteristics.


Cortical bone

The hard outer layer of bones is composed of cortical bone, which is also called compact bone as it is much denser than cancellous bone. It forms the hard exterior (cortex) of bones. The cortical bone gives bone its smooth, white, and solid appearance, and accounts for 80% of the total bone mass of an adult human
skeleton A skeleton is a structural frame that supports an animal Animals (also called Metazoa) are multicellular eukaryotic organisms that form the Kingdom (biology), biological kingdom Animalia. With few exceptions, animals Heterotroph, consu ...

skeleton
. It facilitates bone's main functions—to support the whole body, to protect organs, to provide
lever A lever ( or ) is a simple machine A simple machine is a mechanical device that changes the direction or magnitude of a force In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge o ...

lever
s for movement, and to store and release chemical elements, mainly calcium. It consists of multiple microscopic columns, each called an
osteon The osteon or haversian system (named for Clopton Havers) is the fundamental functional unit of much compact bone A bone is a rigid tissue that constitutes part of the vertebrate Vertebrates () comprise all species of animal ...

osteon
or Haversian system. Each column is multiple layers of
osteoblast Osteoblasts (from the Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is app ...

osteoblast
s and
osteocyte An osteocyte, an oblate shaped type of bone A bone is a rigid tissue Tissue may refer to: Biology * Tissue (biology), an ensemble of similar cells that together carry out a specific function * ''Triphosa haesitata'', a species of geometer m ...
s around a central canal called the
haversian canal Haversian canals (sometimes canals of Havers) are a series of microscopic tubes in the outermost region of bone called cortical bone A bone is a rigid tissue that constitutes part of the vertebrate Vertebrates () comprise all species of ...
. Volkmann's canals at right angles connect the osteons together. The columns are metabolically active, and as bone is reabsorbed and created the nature and location of the cells within the osteon will change. Cortical bone is covered by a
periosteum The periosteum is a membrane Image:Schematic size.jpg, up150px, Schematic of size-based membrane exclusion A membrane is a selective barrier; it allows some things to pass through but stops others. Such things may be molecules, ions, or other sma ...
on its outer surface, and an
endosteum The endosteum (plural endostea) is a thin vascular membrane of connective tissue Connective tissue is one of the many basic types of animal Animals (also called Metazoa) are multicellular eukaryotic organisms that form the Kingdom (biolo ...
on its inner surface. The endosteum is the boundary between the cortical bone and the cancellous bone. The primary anatomical and functional unit of cortical bone is the
osteon The osteon or haversian system (named for Clopton Havers) is the fundamental functional unit of much compact bone A bone is a rigid tissue that constitutes part of the vertebrate Vertebrates () comprise all species of animal ...

osteon
.


Cancellous bone

Cancellous bone, also called trabecular or spongy bone, is the internal tissue of the skeletal bone and is an open cell
porous Porosity or void fraction is a measure of the void Void may refer to: Science, engineering, and technology * Void (astronomy) Cosmic voids are vast spaces between filaments (the largest-scale structures in the universe The universe ( ...
network. Cancellous bone has a higher
surface-area-to-volume ratio The surface-area-to-volume ratio, also called the surface-to-volume ratio and variously denoted sa/vol or SA:V, is the amount of surface area per unit volume of an object or collection of objects. In chemical reactions involving a solid material, t ...
than cortical bone and it is less
dense The density (more precisely, the volumetric mass density; also known as specific mass), of a substance is its mass Mass is both a property Property (''latin: Res Privata'') in the Abstract and concrete, abstract is what belongs to or ...

dense
. This makes it weaker and more flexible. The greater surface area also makes it suitable for metabolic activities such as the exchange of calcium ions. Cancellous bone is typically found at the ends of long bones, near joints and in the interior of vertebrae. Cancellous bone is highly
vascular The blood vessels are the components of the circulatory system The circulatory system, also called the cardiovascular system or the vascular system, is an Biological system, organ system that permits blood to circulate and transport nutrien ...
and often contains red
bone marrow Bone marrow is a semi-solid tissue Tissue may refer to: Biology * Tissue (biology), an ensemble of similar cells that together carry out a specific function * ''Triphosa haesitata'', a species of geometer moth found in North America * ''Triphos ...
where
hematopoiesis Haematopoiesis (, from Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is a ...
, the production of blood cells, occurs. The primary anatomical and functional unit of cancellous bone is the
trabecula A trabecula (plural trabeculae, from Latin for "small beam") is a small, often microscopic, tissue Tissue may refer to: Biology * Tissue (biology), an ensemble of similar cells that together carry out a specific function * ''Triphosa haesitata'', ...
. The trabeculae are aligned towards the mechanical load distribution that a bone experiences within long bones such as the
femur The femur (; ), or thigh bone, is the proximal Standard anatomical terms of location deal unambiguously with the anatomy of animals, including humans. Terms used generally derive from Latin or Greek language, Greek roots and used to describe s ...

femur
. As far as short bones are concerned, trabecular alignment has been studied in the
vertebral The vertebral column, also known as the backbone or spine, is part of the axial skeleton. The vertebral column is the defining characteristic of a vertebrate in which the notochord (a flexible rod of uniform composition) found in all chordata, c ...
pedicle. Thin formations of
osteoblast Osteoblasts (from the Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is app ...

osteoblast
s covered in endosteum create an irregular network of spaces, known as trabeculae. Within these spaces are
bone marrow Bone marrow is a semi-solid tissue Tissue may refer to: Biology * Tissue (biology), an ensemble of similar cells that together carry out a specific function * ''Triphosa haesitata'', a species of geometer moth found in North America * ''Triphos ...
and
hematopoietic stem cell Hematopoietic stem cells (HSCs) are the stem cell In multicellular organisms Multicellular organisms are organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is any individual contiguo ...

hematopoietic stem cell
s that give rise to
platelet Platelets, also called thrombocytes (from Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. ...

platelet
s,
red blood cell Red blood cells (RBCs), also referred to as red cells, red blood corpuscles (in humans or other animals not having nucleus in red blood cells), haematids, erythroid cells or erythrocytes (from Greek language, Greek ''erythros'' for "red" and ''k ...

red blood cell
s and
white blood cell White blood cells (WBCs), also called leukocytes or leucocytes, are the cells of the immune system The immune system is a network of biological processes that protects an organism In biology, an organism (from Ancient Greek, Greek: ...
s. Trabecular marrow is composed of a network of rod- and plate-like elements that make the overall organ lighter and allow room for blood vessels and marrow. Trabecular bone accounts for the remaining 20% of total bone mass but has nearly ten times the surface area of compact bone. The words ''cancellous'' and ''trabecular'' refer to the tiny lattice-shaped units (trabeculae) that form the tissue. It was first illustrated accurately in the engravings of Crisóstomo Martinez.


Bone marrow

Bone marrow Bone marrow is a semi-solid tissue Tissue may refer to: Biology * Tissue (biology), an ensemble of similar cells that together carry out a specific function * ''Triphosa haesitata'', a species of geometer moth found in North America * ''Triphos ...
, also known as
myeloid tissue Myeloid tissue, in the bone marrow Bone marrow is a semi-solid tissue found within the spongy or cancellous portions of bone A bone is a Stiffness, rigid tissue (anatomy), tissue that constitutes part of the vertebrate skeleton in anima ...
in red bone marrow, can be found in almost any bone that holds
cancellous tissue A bone is a rigid tissue Tissue may refer to: Biology * Tissue (biology), an ensemble of similar cells that together carry out a specific function * ''Triphosa haesitata'', a species of geometer moth found in North America * ''Triphosa dubit ...
. In
newborns 222x222px, Eight-month-old Fraternal twin, sororal twin sisters An infant (from the Latin word ''infans'', meaning 'unable to speak' or 'speechless') is the more formal or specialised synonym for the common term ''baby'', meaning the juve ...

newborns
, all such bones are filled exclusively with red marrow or
hematopoietic Haematopoiesis (, from Ancient Greek, Greek , 'blood' and 'to make'; also hematopoiesis in American English; sometimes also h(a)emopoiesis) is the formation of blood cellular components. All cellular blood components are derived from hematopoieti ...
marrow, but as the child ages the hematopoietic fraction decreases in quantity and the fatty/ yellow fraction called
marrow adipose tissue Marrow adipose tissue (MAT), also known as bone marrow adipose tissue (BMAT), is a type of fat deposit in bone marrow Bone marrow is a semi-solid tissue found within the spongy or cancellous portions of bone A bone is a Stiffness, rigid ti ...
(MAT) increases in quantity. In adults, red marrow is mostly found in the bone marrow of the femur, the ribs, the vertebrae and pelvic bones.


Bone cells

Bone is a metabolically active tissue composed of several types of cells. These cells include
osteoblast Osteoblasts (from the Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is app ...

osteoblast
s, which are involved in the creation and mineralization of bone tissue,
osteocyte An osteocyte, an oblate shaped type of bone A bone is a rigid tissue Tissue may refer to: Biology * Tissue (biology), an ensemble of similar cells that together carry out a specific function * ''Triphosa haesitata'', a species of geometer m ...
s, and
osteoclast An osteoclast () is a type of bone cell An osteocyte, an oblate shaped type of bone A bone is a Stiffness, rigid tissue (anatomy), tissue that constitutes part of the skeleton in most vertebrate animals. Bones protect the various organs of the ...

osteoclast
s, which are involved in the reabsorption of bone tissue. Osteoblasts and osteocytes are derived from osteoprogenitor cells, but
osteoclast An osteoclast () is a type of bone cell An osteocyte, an oblate shaped type of bone A bone is a Stiffness, rigid tissue (anatomy), tissue that constitutes part of the skeleton in most vertebrate animals. Bones protect the various organs of the ...

osteoclast
s are derived from the same cells that differentiate to form
macrophage Macrophages (abbreviated as Mφ, MΦ or MP) ( el, large eaters, from Greek ''μακρός'' (') = large, ''φαγεῖν'' (') = to eat) are a type of white blood cell White blood cells (WBCs), also called leukocytes or leucocytes, are the ...

macrophage
s and
monocyte Monocytes are a type of ''leukocyte'', or white blood cell White blood cells (WBCs), also called leukocytes or leucocytes, are the cells of the immune system The immune system is a network of biological processes that protects an organi ...

monocyte
s. Within the marrow of the bone there are also
hematopoietic stem cell Hematopoietic stem cells (HSCs) are the stem cell In multicellular organisms Multicellular organisms are organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is any individual contiguo ...

hematopoietic stem cell
s. These cells give rise to other cells, including
white blood cell White blood cells (WBCs), also called leukocytes or leucocytes, are the cells of the immune system The immune system is a network of biological processes that protects an organism In biology, an organism (from Ancient Greek, Greek: ...
s,
red blood cell Red blood cells (RBCs), also referred to as red cells, red blood corpuscles (in humans or other animals not having nucleus in red blood cells), haematids, erythroid cells or erythrocytes (from Greek language, Greek ''erythros'' for "red" and ''k ...

red blood cell
s, and
platelet Platelets, also called thrombocytes (from Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. ...

platelet
s.


Osteoblast

Osteoblast Osteoblasts (from the Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is app ...

Osteoblast
s are mononucleate bone-forming cells. They are located on the surface of osteon seams and make a
protein Proteins are large biomolecule , showing alpha helices, represented by ribbons. This poten was the first to have its suckture solved by X-ray crystallography by Max Perutz and Sir John Cowdery Kendrew in 1958, for which they received a No ...

protein
mixture known as
osteoid In histology Histology, also known as microscopic anatomy or microanatomy, is the branch of biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical ...
, which mineralizes to become bone. The osteoid seam is a narrow region of newly formed organic matrix, not yet mineralized, located on the surface of a bone. Osteoid is primarily composed of Type I
collagen Collagen () is the main structural protein Proteins are large biomolecule , showing alpha helices, represented by ribbons. This poten was the first to have its suckture solved by X-ray crystallography by Max Perutz and Sir John Cowder ...

collagen
. Osteoblasts also manufacture
hormone A hormone (from the Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is appr ...

hormone
s, such as
prostaglandin The prostaglandins (PG) are a group of physiologically active lipid In and , a lipid is a macro that is soluble in solvents. are typically s used to dissolve other naturally occurring hydrocarbon lipid s that do not (or do not easily) ...
s, to act on the bone itself. The osteoblast creates and repairs new bone by actually building around itself. First, the osteoblast puts up collagen fibers. These collagen fibers are used as a framework for the osteoblasts' work. The osteoblast then deposits calcium phosphate which is hardened by
hydroxide Hydroxide is a diatomic anion with chemical formula A chemical formula is a way of presenting information about the chemical proportions of s that constitute a particular or molecule, using symbols, numbers, and sometimes also other sym ...
and
bicarbonate In inorganic chemistry, bicarbonate (International Union of Pure and Applied Chemistry, IUPAC-recommended nomenclature: hydrogen carbonate) is an intermediate form in the deprotonation of carbonic acid. It is a Polyatomic ion, polyatomic anion w ...

bicarbonate
ions. The brand-new bone created by the osteoblast is called
osteoid In histology Histology, also known as microscopic anatomy or microanatomy, is the branch of biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical ...
. Once the osteoblast is finished working it is actually trapped inside the bone once it hardens. When the osteoblast becomes trapped, it becomes known as an osteocyte. Other osteoblasts remain on the top of the new bone and are used to protect the underlying bone, these become known as lining cells.


Osteocyte

Osteocyte An osteocyte, an oblate shaped type of bone A bone is a rigid tissue Tissue may refer to: Biology * Tissue (biology), an ensemble of similar cells that together carry out a specific function * ''Triphosa haesitata'', a species of geometer m ...
s are cells of mesenchymal origin and originate from osteoblasts that have migrated into and become trapped and surrounded by bone matrix that they themselves produced. The spaces the cell body of osteocytes occupy within the mineralized collagen type I matrix are known as lacunae, while the osteocyte cell processes occupy channels called canaliculi. The many processes of osteocytes reach out to meet osteoblasts, osteoclasts, bone lining cells, and other osteocytes probably for the purposes of communication. Osteocytes remain in contact with other osteocytes in the bone through gap junctions—coupled cell processes which pass through the canalicular channels.


Osteoclast

Osteoclast An osteoclast () is a type of bone cell An osteocyte, an oblate shaped type of bone A bone is a Stiffness, rigid tissue (anatomy), tissue that constitutes part of the skeleton in most vertebrate animals. Bones protect the various organs of the ...

Osteoclast
s are very large
multinucleate Multinucleate cell Cell most often refers to: * Cell (biology), the functional basic unit of life Cell may also refer to: Closed spaces * Monastic cell, a small room, hut, or cave in which a monk or religious recluse lives * Prison cell, a roo ...
cells that are responsible for the breakdown of bones by the process of
bone resorption Bone resorption is resorption Resorption is the absorption into the circulatory system of cells or tissue, usually by osteoclast, osteoclasts. Types of resorption include: * Bone resorption * Herniated Disc Resorption * Tooth resorption * Fetal re ...
. New bone is then formed by the osteoblasts. Bone is constantly remodeled by the resorption of osteoclasts and created by osteoblasts. Osteoclasts are large cells with multiple
nuclei ''Nucleus'' (plural nuclei) is a Latin word for the seed inside a fruit. It most often refers to: *Atomic nucleus, the very dense central region of an atom *Cell nucleus, a central organelle of a eukaryotic cell, containing most of the cell's DNA ...

nuclei
located on bone surfaces in what are called ''Howship's lacunae'' (or ''resorption pits''). These lacunae are the result of surrounding bone tissue that has been reabsorbed. Because the osteoclasts are derived from a
monocyte Monocytes are a type of ''leukocyte'', or white blood cell White blood cells (WBCs), also called leukocytes or leucocytes, are the cells of the immune system The immune system is a network of biological processes that protects an organi ...

monocyte
stem-cell lineage, they are equipped with
phagocytic Phagocytosis () is the process by which a cell (biology), cell uses its plasma membrane to engulf a large particle (≥ 0.5 μm), giving rise to an internal compartment called the phagosome. It is one type of endocytosis. A cell that performs pha ...

phagocytic
-like mechanisms similar to circulating
macrophage Macrophages (abbreviated as Mφ, MΦ or MP) ( el, large eaters, from Greek ''μακρός'' (') = large, ''φαγεῖν'' (') = to eat) are a type of white blood cell White blood cells (WBCs), also called leukocytes or leucocytes, are the ...

macrophage
s. Osteoclasts mature and/or migrate to discrete bone surfaces. Upon arrival, active enzymes, such as
tartrate-resistant acid phosphatase Tartrate-resistant acid phosphatase (TRAP or TRAPase), also called acid phosphatase 5, tartrate resistant (ACP5), is a glycosylated monomeric metalloprotein enzyme expressed in mammals. It has a molecular weight of approximately 35kDa, a basic isoel ...
, are secreted against the mineral substrate. The reabsorption of bone by osteoclasts also plays a role in
calcium Calcium is a chemical element In chemistry, an element is a pure Chemical substance, substance consisting only of atoms that all have the same numbers of protons in their atomic nucleus, nuclei. Unlike chemical compounds, chemical elem ...

calcium
homeostasis In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mechanis ...
.


Composition

Bones consist of living cells (osteoblasts and osteocytes) embedded in a mineralized organic matrix. The primary inorganic component of human bone is
hydroxyapatite Hydroxyapatite, also called hydroxylapatite (HA), is a naturally occurring mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition and ...
, the dominant
bone mineral Bone mineral (also called inorganic bone phase, bone salt, or bone apatite) is the inorganic In chemistry Chemistry is the scientific discipline involved with Chemical element, elements and chemical compound, compounds composed of atoms, mol ...
, having the nominal composition of Ca10(PO4)6(OH)2. The organic components of this matrix consist mainly of
type I collagen Type I collagen is the most abundant collagen upright=1.5, Tropocollagen molecule: three left-handed procollagens (red, green, blue) join to form a right-handed triple helical tropocollagen. Collagen () is the main structural protein Prote ...
—"organic" referring to materials produced as a result of the human body—and inorganic components, which alongside the dominant
hydroxyapatite Hydroxyapatite, also called hydroxylapatite (HA), is a naturally occurring mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition and ...
phase, include other compounds of
calcium Calcium is a chemical element In chemistry, an element is a pure Chemical substance, substance consisting only of atoms that all have the same numbers of protons in their atomic nucleus, nuclei. Unlike chemical compounds, chemical elem ...

calcium
and
phosphate In chemistry, a phosphate is an anion, salt (chemistry), salt, functional group or ester derived from a phosphoric acids and phosphates, phosphoric acid. It most commonly means orthophosphate, a derivative of phosphoric acid, orthophosphoric a ...

phosphate
including salts. Approximately 30% of the acellular component of bone consists of organic matter, while roughly 70% by mass is attributed to the inorganic phase. The
collagen Collagen () is the main structural protein Proteins are large biomolecule , showing alpha helices, represented by ribbons. This poten was the first to have its suckture solved by X-ray crystallography by Max Perutz and Sir John Cowder ...

collagen
fibers give bone its
tensile strength Ultimate tensile strength (UTS), often shortened to tensile strength (TS), ultimate strength, or F_\text within equations, is the maximum stress that a material can withstand while being stretched or pulled before breaking. In brittle tensi ...
, and the interspersed crystals of
hydroxyapatite Hydroxyapatite, also called hydroxylapatite (HA), is a naturally occurring mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition and ...
give bone its
compressive strength In mechanics Mechanics (Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is approximat ...
. These effects are
synergistic Synergy is an interaction or cooperation giving rise to a whole that is greater than the simple sum of its parts. The term ''synergy'' comes from the Attic Greek Attic Greek is the Greek language, Greek dialect of the regions of ancient Greec ...
. The exact composition of the matrix may be subject to change over time due to nutrition and
biomineralization Biomineralization, or biomineralisation is the process by which living organisms produce minerals, often to harden or stiffen existing tissues. Such tissues are called mineralized tissues. It is an extremely widespread phenomenon; all six taxono ...
, with the ratio of
calcium Calcium is a chemical element In chemistry, an element is a pure Chemical substance, substance consisting only of atoms that all have the same numbers of protons in their atomic nucleus, nuclei. Unlike chemical compounds, chemical elem ...

calcium
to
phosphate In chemistry, a phosphate is an anion, salt (chemistry), salt, functional group or ester derived from a phosphoric acids and phosphates, phosphoric acid. It most commonly means orthophosphate, a derivative of phosphoric acid, orthophosphoric a ...

phosphate
varying between 1.3 and 2.0 (per weight), and trace minerals such as
magnesium Magnesium is a chemical element upright=1.0, 500px, The chemical elements ordered by link=Periodic table In chemistry Chemistry is the science, scientific study of the properties and behavior of matter. It is a natural science ...

magnesium
,
sodium Sodium is a chemical element In chemistry, an element is a pure Chemical substance, substance consisting only of atoms that all have the same numbers of protons in their atomic nucleus, nuclei. Unlike chemical compounds, chemical eleme ...

sodium
,
potassium Potassium is a chemical element In chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, properties, b ...

potassium
and
carbonate In chemistry, a carbonate is a salt Salt is a mineral composed primarily of sodium chloride (NaCl), a chemical compound belonging to the larger class of Salt (chemistry), salts; salt in its natural form as a crystallinity, crystalline mi ...

carbonate
also being found. Type I collagen composes 90–95% of the organic matrix, with remainder of the matrix being a homogenous liquid called
ground substance Ground substance is an amorphous gel-like substance in the extracellular space that contains all components of the extracellular matrix In biology Biology is the natural science that studies life and living organisms, including their anatom ...
consisting of
proteoglycan Proteoglycans are protein Proteins are large biomolecule , showing alpha helices, represented by ribbons. This poten was the first to have its suckture solved by X-ray crystallography by Max Perutz and Sir John Cowdery Kendrew in 1958 ...
s such as
hyaluronic acid Hyaluronic acid (; abbreviated HA; conjugate base A conjugate acid, within the Brønsted–Lowry acid–base theory The Brønsted–Lowry theory (also called proton theory of acids and bases) is an acid–base reaction An acid–base ...

hyaluronic acid
and
chondroitin sulfate Chondroitin sulfate is a sulfated glycosaminoglycan . For polysaccharide nomenclature see oligosaccharide nomenclature, here. R1, R2, R3 may have different values. Glycosaminoglycans (GAGs) or mucopolysaccharides are long linear polysaccharides c ...

chondroitin sulfate
, as well as non-collagenous proteins such as
osteocalcin Osteocalcin, also known as bone gamma-carboxyglutamic acid-containing protein (BGLAP), is a small (49-amino-acid) noncollagenous protein Proteins are large biomolecules or macromolecules that are comprised of one or more long chains of amino a ...
,
osteopontin Osteopontin (OPN), also known as bone sialoprotein I (BSP-1 or BNSP), early T-lymphocyte activation (ETA-1), secreted phosphoprotein 1 (SPP1), 2ar and Rickettsia resistance (Ric), is a protein Proteins are large biomolecules or macromolecules t ...
or
bone sialoprotein Bone sialoprotein (BSP) is a component of mineralized tissues, sea shells, conch, dentin, radiolarian, antler, bone Mineralized tissues are biological tissue (biology), tissues that incorporate minerals into soft matrices. Typically these tissue ...
. Collagen consists of strands of repeating units, which give bone tensile strength, and are arranged in an overlapping fashion that prevents shear stress. The function of ground substance is not fully known. Two types of bone can be identified microscopically according to the arrangement of collagen: woven and lamellar. * Woven bone (also known as ''fibrous bone''), which is characterized by a haphazard organization of collagen fibers and is mechanically weak.Currey, John D. (2002)
"The Structure of Bone Tissue"
pp. 12–14 in ''Bones: Structure and Mechanics''. Princeton University Press. Princeton, NJ.
* Lamellar bone, which has a regular parallel alignment of collagen into sheets ("lamellae") and is mechanically strong. Woven bone is produced when osteoblasts produce osteoid rapidly, which occurs initially in all
fetal A fetus or foetus (; plural fetuses, feti, foetuses, or foeti) is the unborn offspring that develops from an animal embryo An embryo is the early stage of development of a multicellular organism A multicellular organism is an organism tha ...

fetal
bones, but is later replaced by more resilient lamellar bone. In adults woven bone is created after
fractures Fracture is the separation of an object or material into two or more pieces under the action of stress. The fracture of a solid usually occurs due to the development of certain displacement discontinuity surfaces within the solid. If a displa ...
or in
Paget's diseasePaget's disease may refer to several conditions described by Sir James Paget Sir James Paget, 1st Baronet FRS HFRSE (11 January 1814 – 30 December 1899) (, rhymes with "gadget") was an English surgeon and pathologist who is best remembered ...
. Woven bone is weaker, with a smaller number of randomly oriented collagen fibers, but forms quickly; it is for this appearance of the fibrous matrix that the bone is termed ''woven''. It is soon replaced by lamellar bone, which is highly organized in
concentric In geometry Geometry (from the grc, γεωμετρία; ' "earth", ' "measurement") is, with , one of the oldest branches of . It is concerned with properties of space that are related with distance, shape, size, and relative position ...

concentric
sheets with a much lower proportion of osteocytes to surrounding tissue. Lamellar bone, which makes its first appearance in humans in the
fetus A fetus or foetus (; plural fetuses, feti, foetuses, or foeti) is the unborn offspring that develops from an animal embryo An embryo is the early stage of development of a multicellular organism A multicellular organism is an organism tha ...

fetus
during the third trimester,Salentijn, L. ''Biology of Mineralized Tissues: Cartilage and Bone'',
Columbia University College of Dental Medicine The Columbia University College of Dental Medicine, often abbreviated CDM, is one of the twenty graduate and professional school Professional development is learning to earn or maintain professional credentials such as academic degrees to form ...
post-graduate dental lecture series, 2007
is stronger and filled with many collagen fibers parallel to other fibers in the same layer (these parallel columns are called osteons). In cross-section, the fibers run in opposite directions in alternating layers, much like in
plywood Plywood is a material manufactured from thin layers or "plies" of wood veneer In woodworking, veneer refers to thin slices of wood and sometimes bark, usually thinner than 3 mm (1/8 inch), that typically are glued onto core panels (typic ...

plywood
, assisting in the bone's ability to resist torsion forces. After a fracture, woven bone forms initially and is gradually replaced by lamellar bone during a process known as "bony substitution." Compared to woven bone, lamellar bone formation takes place more slowly. The orderly deposition of collagen fibers restricts the formation of osteoid to about 1 to 2  µm per day. Lamellar bone also requires a relatively flat surface to lay the collagen fibers in parallel or concentric layers.


Deposition

The extracellular matrix of bone is laid down by
osteoblast Osteoblasts (from the Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is app ...

osteoblast
s, which secrete both collagen and ground substance. These synthesise collagen within the cell, and then secrete collagen fibrils. The collagen fibers rapidly
polymer A polymer (; Greek ''poly- Poly, from the Greek :wikt:πολύς, πολύς meaning "many" or "much", may refer to: Businesses * China Poly Group Corporation, a Chinese business group, and its subsidiaries: ** Poly Property, a Hong Kong inc ...

polymer
ise to form collagen strands. At this stage they are not yet mineralised, and are called "osteoid". Around the strands calcium and phosphate
precipitate In aqueous solution, precipitation is the process of transforming a dissolved substance Substance may refer to: * Substance (Jainism), a term in Jain ontology to denote the base or owner of attributes * Chemical substance, a material with a de ...
on the surface of these strands, within days to weeks becoming crystals of hydroxyapatite. In order to mineralise the bone, the osteoblasts secrete
vesicles Vesicle may refer to: ; In cellular biology or chemistry * Vesicle (biology and chemistry), a supramolecular assembly of lipid molecules, like a cell membrane * Synaptic vesicle ; In human embryology * Vesicle (embryology), bulge-like features of ...
containing
alkaline phosphatase Alkaline phosphatase (ALP, ALKP, ALPase, Alk Phos) (), or basic phosphatase, is a homodimeric protein enzyme of 86 kilodaltons. Each monomer contains five cysteine residues, two zinc atoms and one magnesium atom crucial to its catalytic function, ...

alkaline phosphatase
. This cleaves the phosphate groups and acts as the foci for calcium and phosphate deposition. The vesicles then rupture and act as a centre for crystals to grow on. More particularly, bone mineral is formed from globular and plate structures.


Types

There are five types of bones in the human body: long, short, flat, irregular, and sesamoid. *
Long bone The long bones are those that are longer than they are wide. They are one of five types of bone A bone is a rigid tissue Tissue may refer to: Biology * Tissue (biology), an ensemble of similar cells that together carry out a specific funct ...

Long bone
s are characterized by a shaft, the
diaphysis The diaphysis is the main or midsection (shaft) of a long bone The long bones are those that are longer than they are wide. They are one of five types of bone A bone is a rigid tissue Tissue may refer to: Biology * Tissue (biology), an en ...
, that is much longer than its width; and by an
epiphysis The epiphysis is the rounded end of a long bone, at its joint A joint or articulation (or articular surface) is the connection made between bones in the body which link the skeletal system into a functional whole.Saladin, Ken. Anatomy & Physiol ...
, a rounded head at each end of the shaft. They are made up mostly of
compact bone A bone is a rigid tissue that constitutes part of the vertebrate Vertebrates () comprise all species of animal Animals (also called Metazoa) are multicellular eukaryotic organisms that form the Kingdom (biology), biological kingdo ...
, with lesser amounts of marrow, located within the
medullary cavity The medullary cavity (''medulla'', innermost part) is the central cavity of bone A bone is a Stiffness, rigid tissue (anatomy), tissue that constitutes part of the skeleton in most vertebrate animals. Bones protect the various organs of the bo ...
, and areas of spongy, cancellous bone at the ends of the bones. Most bones of the limbs, including those of the
fingers A finger is a limb of the human body and a type of digit (anatomy), digit, an organ of wikt:manipulate, manipulation and Wikt:sensation, sensation found in the hands of humans and other primates.#Cha1998, Chambers 1998 p. 603#OxfIll, Oxford Illus ...
and
toes Toes are the digits (fingers) of the foot of a tetrapod Tetrapods (; from Greek 'four' and 'foot') are four-limbed animals constituting the superclass Tetrapoda . It includes extant and extinct amphibians, reptiles (including dinosaurs ...

toes
, are long bones. The exceptions are the eight
carpal bones The carpal bones are the eight small bones that make up the wrist (or carpus) that connects the hand to the forearm. The term "carpus" is derived from the Latin wikt:carpus#Latin, carpus and the Greek language, Greek wikt:καρπός#Ancient Gree ...

carpal bones
of the
wrist In human anatomy The human body is the structure of a human being Humans (''Homo sapiens'') are the most populous and widespread species In biology, a species is the basic unit of biological classification, classification and ...

wrist
, the seven articulating
tarsal bone In the human body The human body is the structure of a human being. It is composed of many different types of cells that together create tissues and subsequently organ systems. They ensure homeostasis and the life, viability of the human b ...
s of the
ankle The ankle, or the talocrural region, is the region where the foot The foot (plural: feet) is an anatomical Anatomy (Greek ''anatomē'', 'dissection') is the branch of biology Biology is the natural science that studies life and liv ...
and the sesamoid bone of the
kneecap The patella, also known as the kneecap, is a flat, rounded triangular bone which articulates with the femur (thigh bone) and covers and protects the anterior articular surface of the knee In human Humans (''Homo sapiens'') are the most p ...
. Long bones such as the clavicle, that have a differently shaped shaft or ends are also called ''modified long bones''. *
Short bone Short bones are designated as those bone A bone is a Stiffness, rigid tissue (anatomy), tissue that constitutes part of the vertebrate skeleton in animals. Bones protect the various organs of the body, produce red blood cell, red and white blo ...
s are roughly
cube In geometry Geometry (from the grc, γεωμετρία; ' "earth", ' "measurement") is, with , one of the oldest branches of . It is concerned with properties of space that are related with distance, shape, size, and relative position ...

cube
-shaped, and have only a thin layer of compact bone surrounding a spongy interior. The bones of the wrist and ankle are short bones. *
Flat bone Flat bones are bone A bone is a rigid tissue Tissue may refer to: Biology * Tissue (biology), an ensemble of similar cells that together carry out a specific function * ''Triphosa haesitata'', a species of geometer moth found in North Amer ...
s are thin and generally curved, with two parallel layers of compact bones sandwiching a layer of spongy bone. Most of the bones of the
skull The skull is a bone A bone is a rigid tissue Tissue may refer to: Biology * Tissue (biology), an ensemble of similar cells that together carry out a specific function * ''Triphosa haesitata'', a species of geometer moth found in North A ...

skull
are flat bones, as is the
sternum The sternum or breastbone is a long flat bone located in the central part of the chest. It connects to the ribs via cartilage and forms the front of the rib cage, thus helping to protect the heart, human lung, lungs, and major blood vessels from in ...

sternum
. *
Sesamoid bone In anatomy Anatomy (Greek ''anatomē'', 'dissection') is the branch of biology concerned with the study of the structure of organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is any ind ...
s are bones embedded in tendons. Since they act to hold the tendon further away from the joint, the angle of the tendon is increased and thus the leverage of the muscle is increased. Examples of sesamoid bones are the
patella The patella, also known as the kneecap, is a flat, rounded triangular bone which articulates with the femur (thigh bone) and covers and protects the anterior articular surface of the knee joint. The patella is found in many tetrapods, such as mo ...
and the
pisiform The pisiform bone ( or ), also spelled pisiforme (from the Latin ''pisifomis'', pea-shaped), is a small knobbly, sesamoid bone In anatomy Anatomy (Greek ''anatomē'', 'dissection') is the branch of biology concerned with the study of the struc ...
. *
Irregular bone The irregular bones are bones which, from their peculiar form, cannot be grouped as long, short, flat or sesamoid bones. Irregular bones serve various purposes in the body, such as protection of nervous tissue (such as the vertebrae In the ve ...
s do not fit into the above categories. They consist of thin layers of compact bone surrounding a spongy interior. As implied by the name, their shapes are irregular and complicated. Often this irregular shape is due to their many centers of ossification or because they contain bony sinuses. The bones of the ,
pelvis The pelvis (plural pelves or pelvises) is the lower part of the trunk, between the abdomen The abdomen (colloquially called the belly, tummy, midriff or stomach) is the part of the body between the thorax (chest) and pelvis, in humans and in ...

pelvis
, and some bones of the skull are irregular bones. Examples include the
ethmoid The ethmoid bone (; from Greek ''ethmos'', "sieve") is an unpaired bone in the human skull, skull that separates the nasal cavity from the brain. It is located at the roof of the human nose, nose, between the two Orbit (anatomy), orbits. The cubic ...

ethmoid
and bones.


Terminology

In the study of
anatomy Anatomy (Greek ''anatomē'', 'dissection') is the branch of biology concerned with the study of the structure of organisms and their parts. Anatomy is a branch of natural science which deals with the structural organization of living things. It ...

anatomy
, anatomists use a number of anatomical terms to describe the appearance, shape and function of bones. Other anatomical terms are also used to describe the location of bones. Like other anatomical terms, many of these derive from
Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken in the area around Rome, known as Latium. Through the power of the Roman Republic, it became ...

Latin
and
Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is approximately 10.7 million as of ...
. Some anatomists still use Latin to refer to bones. The term "osseous", and the prefix "osteo-", referring to things related to bone, are still used commonly today. Some examples of terms used to describe bones include the term "foramen" to describe a hole through which something passes, and a "canal" or "meatus" to describe a tunnel-like structure. A protrusion from a bone can be called a number of terms, including a "condyle", "crest", "spine", "eminence", "tubercle" or "tuberosity", depending on the protrusion's shape and location. In general,
long bone The long bones are those that are longer than they are wide. They are one of five types of bone A bone is a rigid tissue Tissue may refer to: Biology * Tissue (biology), an ensemble of similar cells that together carry out a specific funct ...

long bone
s are said to have a "head", "neck", and "body". When two bones join together, they are said to "articulate". If the two bones have a fibrous connection and are relatively immobile, then the joint is called a "suture".


Development

The formation of bone is called
ossification Ossification (or osteogenesis) in bone remodeling is the process of laying down new bone material by Cell (biology), cells named osteoblasts. It is synonymous with bone tissue formation. There are two processes resulting in the formation of no ...
. During the this occurs by two processes:
intramembranous ossification Intramembranous ossification is one of the two essential processes during fetal A fetus American and British English spelling differences#ae and oe, or foetus (; plural fetuses, feti, foetuses, or foeti) is the unborn offspring that develops fr ...
and
endochondral ossification Endochondral ossification is one of the two essential processes during fetal A fetus or foetus (; plural fetuses, feti, foetuses, or foeti) is the unborn offspring of an animal that develops from an embryo An embryo is the early stage of ...
. Intramembranous ossification involves the formation of bone from
connective tissue Connective tissue is one of the many basic types of animal Animals (also called Metazoa) are multicellular A multicellular organism is an organism In biology, an organism () is any organic, life, living system that functions ...
whereas endochondral ossification involves the formation of bone from
cartilage Cartilage (cartilaginous tissue) is a resilient and smooth elastic tissue Elastic is a word often used to describe or identify certain types of elastomer An elastomer is a polymer A polymer (; Greek ''wikt:poly-, poly-'', "many" + ''wikt:-m ...

cartilage
. Intramembranous ossification mainly occurs during formation of the flat bones of the
skull The skull is a bone A bone is a rigid tissue Tissue may refer to: Biology * Tissue (biology), an ensemble of similar cells that together carry out a specific function * ''Triphosa haesitata'', a species of geometer moth found in North A ...

skull
but also the mandible, maxilla, and clavicles; the bone is formed from connective tissue such as
mesenchyme Mesenchyme () is a type of loosely organised animal embryonic ''Embryonic'' is the twelfth studio album by experimental rock band the Flaming Lips released on October 13, 2009, on Warner Bros. Records, Warner Bros. The band's first double album, ...
tissue rather than from cartilage. The process includes: the development of the
ossification center An ossification center is a point where ossification Image:Bonemetabolism.svg, 300 px, Bone is broken down by osteoclasts, and rebuilt by osteoblasts, both of which communicate through cytokine (TGF-β, Insulin-like growth factor, IGF) signall ...
,
calcification Calcification is the accumulation of calcium Calcium is a chemical element with the Symbol (chemistry), symbol Ca and atomic number 20. As an alkaline earth metal, calcium is a reactive metal that forms a dark oxide-nitride layer when expose ...
, trabeculae formation and the development of the periosteum. Endochondral ossification occurs in long bones and most other bones in the body; it involves the development of bone from cartilage. This process includes the development of a cartilage model, its growth and development, development of the primary and secondary
ossification center An ossification center is a point where ossification Image:Bonemetabolism.svg, 300 px, Bone is broken down by osteoclasts, and rebuilt by osteoblasts, both of which communicate through cytokine (TGF-β, Insulin-like growth factor, IGF) signall ...
s, and the formation of articular cartilage and the
epiphyseal plate The epiphyseal plate (or epiphysial plate, physis, or growth plate) is a hyaline cartilage Hyaline cartilage is the glass-like (hyaline) but translucent cartilage found on many joint surfaces. It is also most commonly found in the ribs, nose, la ...
s. Endochondral ossification begins with points in the cartilage called "primary ossification centers." They mostly appear during fetal development, though a few short bones begin their primary ossification after
birth Birth is the act or process of bearing or bringing forth offspring, also referred to in technical contexts as parturition. In mammals, the process is initiated by hormones which cause the muscular walls of the uterus to contract, expelling the fe ...

birth
. They are responsible for the formation of the diaphyses of long bones, short bones and certain parts of irregular bones. Secondary ossification occurs after birth, and forms the
epiphyses The epiphysis is the rounded end of a long bone, at its joint A joint or articulation (or articular surface) is the connection made between bones in the body which link the skeletal system into a functional whole.Saladin, Ken. Anatomy & Physiol ...
of long bones and the extremities of irregular and flat bones. The diaphysis and both epiphyses of a long bone are separated by a growing zone of cartilage (the
epiphyseal plate The epiphyseal plate (or epiphysial plate, physis, or growth plate) is a hyaline cartilage Hyaline cartilage is the glass-like (hyaline) but translucent cartilage found on many joint surfaces. It is also most commonly found in the ribs, nose, la ...
). At skeletal maturity (18 to 25 years of age), all of the cartilage is replaced by bone, fusing the diaphysis and both epiphyses together (epiphyseal closure). In the upper limbs, only the diaphyses of the long bones and scapula are ossified. The epiphyses, carpal bones, coracoid process, medial border of the scapula, and acromion are still cartilaginous. The following steps are followed in the conversion of cartilage to bone: # Zone of reserve cartilage. This region, farthest from the marrow cavity, consists of typical hyaline cartilage that as yet shows no sign of transforming into bone. # Zone of cell proliferation. A little closer to the marrow cavity, chondrocytes multiply and arrange themselves into longitudinal columns of flattened lacunae. # Zone of cell hypertrophy. Next, the chondrocytes cease to divide and begin to hypertrophy (enlarge), much like they do in the primary ossification center of the fetus. The walls of the matrix between lacunae become very thin. # Zone of calcification. Minerals are deposited in the matrix between the columns of lacunae and calcify the cartilage. These are not the permanent mineral deposits of bone, but only a temporary support for the cartilage that would otherwise soon be weakened by the breakdown of the enlarged lacunae. # Zone of bone deposition. Within each column, the walls between the lacunae break down and the chondrocytes die. This converts each column into a longitudinal channel, which is immediately invaded by blood vessels and marrow from the marrow cavity. Osteoblasts line up along the walls of these channels and begin depositing concentric lamellae of matrix, while osteoclasts dissolve the temporarily calcified cartilage.


Function

Bones have a variety of functions:


Mechanical

Bones serve a variety of mechanical functions. Together the bones in the body form the
skeleton A skeleton is a structural frame that supports an animal Animals (also called Metazoa) are multicellular eukaryotic organisms that form the Kingdom (biology), biological kingdom Animalia. With few exceptions, animals Heterotroph, consu ...

skeleton
. They provide a frame to keep the body supported, and an attachment point for
skeletal muscle Skeletal muscles (commonly referred to as muscles) are Organ (biology), organs of the vertebrate muscular system that are mostly attached by tendons to bones of the skeleton. The muscle cells of skeletal muscles are much longer than in the other ...

skeletal muscle
s,
tendon A tendon or sinew is a tough, high-tensile-strength band of dense fibrous connective tissue that connects muscle Skeletal muscles (commonly referred to as muscles) are organs An organ is a group of tissues with similar functions. Plant ...

tendon
s,
ligament A ligament is the Connective tissue#Types, fibrous connective tissue that connects bones to other bones. It is also known as ''articular ligament'', ''articular larua'', ''fibrous ligament'', or ''true ligament''. Other ligaments in the body incl ...

ligament
s and
joint A joint or articulation (or articular surface) is the connection made between bone A bone is a Stiffness, rigid tissue (anatomy), tissue that constitutes part of the skeleton in most vertebrate animals. Bones protect the various organs of th ...

joint
s, which function together to generate and transfer forces so that individual body parts or the whole body can be manipulated in three-dimensional space (the interaction between bone and muscle is studied in
biomechanics Biomechanics is the study of the structure, function and motion of the mechanical aspects of biological systems, at any level from whole organism In biology, an organism () is any organic, life, living system that functions as an indivi ...
). Bones protect internal organs, such as the
skull The skull is a bone A bone is a rigid tissue Tissue may refer to: Biology * Tissue (biology), an ensemble of similar cells that together carry out a specific function * ''Triphosa haesitata'', a species of geometer moth found in North A ...

skull
protecting the
brain A brain is an organ Organ may refer to: Biology * Organ (anatomy) An organ is a group of Tissue (biology), tissues with similar functions. Plant life and animal life rely on many organs that co-exist in organ systems. A given organ's tis ...

brain
or the
ribs The rib cage is the arrangement of ribs attached to the vertebral column and sternum in the thorax of most vertebrates that encloses and protects the vital organs such as the heart, lungs and great vessels. In humans, the rib cage and the sternum ...

ribs
protecting the
heart The heart is a muscular MUSCULAR (DS-200B), located in the United Kingdom The United Kingdom of Great Britain and Northern Ireland, commonly known as the United Kingdom (UK) or Britain,Usage is mixed. The Guardian' and Telegraph' use ...

heart
and
lungs The lungs are the primary organs of the respiratory system The respiratory system (also respiratory apparatus, ventilatory system) is a biological system consisting of specific organs and structures used for gas exchange in animal ...

lungs
. Because of the way that bone is formed, bone has a high
compressive strength In mechanics Mechanics (Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is approximat ...
of about , poor
tensile strength Ultimate tensile strength (UTS), often shortened to tensile strength (TS), ultimate strength, or F_\text within equations, is the maximum stress that a material can withstand while being stretched or pulled before breaking. In brittle tensi ...
of 104–121 MPa, and a very low
shear stress Shear stress, often denoted by (Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is approx ...

shear stress
strength (51.6 MPa). This means that bone resists pushing (compressional) stress well, resist pulling (tensional) stress less well, but only poorly resists shear stress (such as due to torsional loads). While bone is essentially
brittle tensile testpieces A material is brittle if, when subjected to stress (physics), stress, it fracture Fracture is the separation of an object or material into two or more pieces under the action of stress. The fracture of a solid usually ...
, bone does have a significant degree of
elasticity Elasticity often refers to: *Elasticity (physics), continuum mechanics of bodies that deform reversibly under stress Elasticity may also refer to: Information technology * Elasticity (data store), the flexibility of the data model and the clu ...
, contributed chiefly by
collagen Collagen () is the main structural protein Proteins are large biomolecule , showing alpha helices, represented by ribbons. This poten was the first to have its suckture solved by X-ray crystallography by Max Perutz and Sir John Cowder ...

collagen
. Mechanically, bones also have a special role in
hearing Schematic diagram of the human ear Hearing, or auditory perception, is the ability to perceive Sound, sounds through an organ, such as an ear, by detecting Vibration, vibrations as periodic changes in the pressure of a surrounding medium. Th ...
. The
ossicles The ossicles (also called auditory ossicles) are three bone A bone is a rigid tissue Tissue may refer to: Biology * Tissue (biology), an ensemble of similar cells that together carry out a specific function * ''Triphosa haesitata'', a spe ...
are three small bones in the
middle ear The middle ear is the portion of the ear The ear is the organ of hearing and, in mammals, balance. In mammals, the ear is usually described as having three parts—the outer ear The outer ear, external ear, or auris externa is t ...

middle ear
which are involved in sound transduction.


Synthetic

The cancellous part of bones contain
bone marrow Bone marrow is a semi-solid tissue Tissue may refer to: Biology * Tissue (biology), an ensemble of similar cells that together carry out a specific function * ''Triphosa haesitata'', a species of geometer moth found in North America * ''Triphos ...
. Bone marrow produces blood cells in a process called
hematopoiesis Haematopoiesis (, from Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is a ...
. Blood cells that are created in bone marrow include
red blood cell Red blood cells (RBCs), also referred to as red cells, red blood corpuscles (in humans or other animals not having nucleus in red blood cells), haematids, erythroid cells or erythrocytes (from Greek language, Greek ''erythros'' for "red" and ''k ...

red blood cell
s,
platelet Platelets, also called thrombocytes (from Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. ...

platelet
s and
white blood cell White blood cells (WBCs), also called leukocytes or leucocytes, are the cells of the immune system The immune system is a network of biological processes that protects an organism In biology, an organism (from Ancient Greek, Greek: ...
s. Progenitor cells such as the
hematopoietic stem cell Hematopoietic stem cells (HSCs) are the stem cell In multicellular organisms Multicellular organisms are organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is any individual contiguo ...

hematopoietic stem cell
divide in a process called
mitosis In cell biology Cell biology (also cellular biology or cytology) is a branch of biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical proce ...

mitosis
to produce precursor cells. These include precursors which eventually give rise to
white blood cells White blood cells, also called leukocytes or leucocytes, are the cell (biology), cells of the immune system that are involved in protecting the body against both infectious disease and foreign invaders. All white blood cells are produced and de ...
, and
erythroblast A nucleated red blood cell (NRBC), also known by #Nomenclature, several other names, is a red blood cell that contains a cell nucleus. Almost all vertebrate organisms have hemoglobin-containing cells in their blood, and with the exception of mammal ...
s which give rise to red blood cells. Unlike red and white blood cells, created by mitosis, platelets are shed from very large cells called
megakaryocyte A megakaryocyte ('' mega-'' + '' karyo-'' + '' -cyte'', "large-nucleus cell") is a large bone marrow Bone marrow is a semi-solid tissue found within the spongy or cancellous portions of bone A bone is a Stiffness, rigid tissue (anatomy), ti ...

megakaryocyte
s. This process of progressive differentiation occurs within the bone marrow. After the cells are matured, they enter the
circulation Circulation may refer to: Science and technology * Atmospheric circulation, the large-scale movement of air * Circulation (physics), the path integral of the fluid velocity around a closed curve in a fluid flow field * Circulatory system, a biolo ...
. Every day, over 2.5 billion red blood cells and platelets, and 50–100 billion
granulocyte Granulocytes are cells in the innate immune system characterized by the presence of specific granules in their cytoplasm. They are also called polymorphonuclear leukocytes (PMN, PML, or PMNL) because of the varying shape of the cell nucleus, nucl ...
s are produced in this way. As well as creating cells, bone marrow is also one of the major sites where defective or aged red blood cells are destroyed.


Metabolic

* Mineral storage – bones act as reserves of minerals important for the body, most notably
calcium Calcium is a chemical element In chemistry, an element is a pure Chemical substance, substance consisting only of atoms that all have the same numbers of protons in their atomic nucleus, nuclei. Unlike chemical compounds, chemical elem ...

calcium
and
phosphorus Phosphorus is a chemical element In chemistry, an element is a pure Chemical substance, substance consisting only of atoms that all have the same numbers of protons in their atomic nucleus, nuclei. Unlike chemical compounds, chemical el ...

phosphorus
. Determined by the species, age, and the type of bone, bone cells make up to 15 percent of the bone.
Growth factor A growth factor is a naturally occurring substance capable of stimulating cell proliferation Cell proliferation is the process by which ''a cell grows and divides to produce two daughter cells''. Cell proliferation leads to an exponential gro ...
storage—mineralized bone matrix stores important growth factors such as
insulin Insulin (, from Latin ''insula'', 'island') is a peptide hormone produced by beta cells of the pancreatic islets; it is considered to be the main Anabolism, anabolic hormone of the body. It regulates the metabolism of carbohydrates, fats and p ...

insulin
-like growth factors, transforming growth factor,
bone morphogenetic protein Bone morphogenetic proteins (BMPs) are a group of growth factors also known as cytokines and as metabologens. Originally discovered by their ability to induce the formation of bone and cartilage, BMPs are now considered to constitute a group of piv ...
s and others. *
Fat In nutrition Nutrition is the biochemical Biochemistry or biological chemistry, is the study of chemical processes within and relating to living organisms. A sub-discipline of both chemistry and biology, biochemistry may be divided ...

Fat
storage –
marrow adipose tissue Marrow adipose tissue (MAT), also known as bone marrow adipose tissue (BMAT), is a type of fat deposit in bone marrow Bone marrow is a semi-solid tissue found within the spongy or cancellous portions of bone A bone is a Stiffness, rigid ti ...
(MAT) acts as a storage reserve of
fatty acid In chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, properties, behavior and the changes they undergo during ...
s. *
Acid An acid is a or capable of donating a (hydrogen ion H+) (a ), or, alternatively, capable of forming a with an (a ). The first category of acids are the proton donors, or s. In the special case of , proton donors form the H3O+ and are ...
-
base Base or BASE may refer to: Brands and enterprises * Base (mobile telephony provider), a Belgian mobile telecommunications operator *Base CRM Base CRM (originally Future Simple or PipeJump) is an enterprise software company based in Mountain Vie ...
balance – bone buffers the blood against excessive changes by absorbing or releasing alkaline salts. * Detoxification – bone tissues can also store
heavy metals upright=1.2, Crystals of osmium, a heavy metal nearly twice as dense as lead">lead.html" ;"title="osmium, a heavy metal nearly twice as dense as lead">osmium, a heavy metal nearly twice as dense as lead Heavy metals are generally defined as m ...
and other foreign elements, removing them from the blood and reducing their effects on other tissues. These can later be gradually released for
excretion Excretion is a process in which metabolic waste Metabolic wastes or excrements are Chemical substance, substances left over from metabolism, metabolic processes (such as cellular respiration) which cannot be used by the organism (they are surplus ...
. *
Endocrine The endocrine system is a messenger system comprising feedback loops of the hormone A hormone (from the Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Re ...

Endocrine
organ – bone controls
phosphate In chemistry, a phosphate is an anion, salt (chemistry), salt, functional group or ester derived from a phosphoric acids and phosphates, phosphoric acid. It most commonly means orthophosphate, a derivative of phosphoric acid, orthophosphoric a ...

phosphate
metabolism by releasing
fibroblast growth factor 23 Fibroblast growth factor 23 or FGF23 is a protein Proteins are large biomolecule , showing alpha helices, represented by ribbons. This poten was the first to have its suckture solved by X-ray crystallography by Max Perutz and Sir John Co ...
(FGF-23), which acts on
kidney The kidneys are two reddish-brown bean-shaped organs An organ is a group of tissues with similar functions. Plant life and animal life rely on many organs that co-exist in organ systems. A given organ's tissues can be broadly categorized ...

kidney
s to reduce phosphate
reabsorption {{Multiple issues, {{Unreferenced, date=December 2009 {{Confusing, date=February 2007 In renal physiology 300px, This illustration demonstrates the normal kidney physiology, including the Proximal Convoluted Tubule (PCT), Loop of Henle, and Dis ...

reabsorption
. Bone cells also release a hormone called
osteocalcin Osteocalcin, also known as bone gamma-carboxyglutamic acid-containing protein (BGLAP), is a small (49-amino-acid) noncollagenous protein Proteins are large biomolecules or macromolecules that are comprised of one or more long chains of amino a ...
, which contributes to the regulation of
blood sugar The blood sugar level, blood sugar concentration, or blood glucose level is the concentration of glucose Glucose is a simple with the . Glucose is the most abundant , a subcategory of s. Glucose is mainly made by and most during from water ...
(
glucose Glucose is a simple with the . Glucose is the most abundant , a subcategory of s. Glucose is mainly made by and most during from water and carbon dioxide, using energy from sunlight, where it is used to make in s, the most abundant carbohydr ...

glucose
) and . Osteocalcin increases both the
insulin Insulin (, from Latin ''insula'', 'island') is a peptide hormone produced by beta cells of the pancreatic islets; it is considered to be the main Anabolism, anabolic hormone of the body. It regulates the metabolism of carbohydrates, fats and p ...

insulin
secretion and sensitivity, in addition to boosting the number of insulin-producing cells and reducing stores of fat. * Calcium balance – the process of bone resorption by the osteoclasts releases stored calcium into the systemic circulation and is an important process in regulating calcium balance. As bone formation actively ''fixes'' circulating calcium in its mineral form, removing it from the bloodstream, resorption actively ''unfixes'' it thereby increasing circulating calcium levels. These processes occur in tandem at site-specific locations.


Remodeling

Bone is constantly being created and replaced in a process known as
remodeling Renovations (also called remodeling) is the process of improving a broken, damaged, or outdated structure. Renovations are typically either commercial or residential. Additionally, renovation can refer to making something new, or bringing someth ...
. This ongoing turnover of bone is a process of resorption followed by replacement of bone with little change in shape. This is accomplished through osteoblasts and osteoclasts. Cells are stimulated by a variety of signals, and together referred to as a remodeling unit. Approximately 10% of the skeletal mass of an adult is remodelled each year. The purpose of remodeling is to regulate
calcium homeostasis Calcium metabolism is the movement and regulation of calcium ions (Ca2+) ''in'' (via the gastrointestinal tract, gut) and ''out'' (via the gut and kidneys) of the body, and ''between'' body compartments: the blood plasma, the extracellular fluid, ex ...
, repair microdamaged bones from everyday stress, and to shape the skeleton during growth. Repeated stress, such as weight-bearing
exercise Exercise is any bodily activity that enhances or maintains physical fitness Physical fitness is a state of health Health is a state of physical, mental and social well-being Well-being, also known as ''wellness'', ''prudential value ...

exercise
or bone healing, results in the bone thickening at the points of maximum stress (
Wolff's law Wolff's law, developed by the German anatomist and surgeon Julius Wolff (1836–1902) in the 19th century, states that bone in a healthy person or animal will adapt to the loads under which it is placed. If loading on a particular bone increases, ...
). It has been hypothesized that this is a result of bone's
piezoelectric Piezoelectricity (, ) is the electric charge Electric charge is the physical property of matter that causes it to experience a force when placed in an electromagnetic field. Electric charge can be ''positive'' or ''negative'' (commonly carried ...

piezoelectric
properties, which cause bone to generate small electrical potentials under stress. The action of osteoblasts and osteoclasts are controlled by a number of chemical
enzyme Enzymes () are proteins that act as biological catalysts (biocatalysts). Catalysts accelerate chemical reactions. The molecules upon which enzymes may act are called substrate (chemistry), substrates, and the enzyme converts the substrates in ...

enzyme
s that either promote or inhibit the activity of the bone remodeling cells, controlling the rate at which bone is made, destroyed, or changed in shape. The cells also use
paracrine signallingParacrine signaling is form of cell signaling or cell-to-cell communication in which a cell produces a signal to induce changes in nearby cells, altering the behaviour of those cells. Signaling molecules known as paracrine factors diffuse over a rel ...
to control the activity of each other. For example, the rate at which osteoclasts resorb bone is inhibited by
calcitonin Calcitonin is a 32 amino acid peptide hormonePeptide hormones or protein hormones are hormones whose molecules are peptides or proteins, respectively. The latter have longer amino acid chain lengths than the former. These hormones have an effect on ...

calcitonin
and
osteoprotegerin Osteoprotegerin (OPG), also known as osteoclastogenesis inhibitory factor (OCIF) or tumour necrosis factor receptor superfamily member 11B (TNFRSF11B), is a cytokine receptor of the tumour necrosis factor (TNF) receptor superfamily encoded by the ' ...
. Calcitonin is produced by
parafollicular cell Parafollicular cells, also called C cells, are neuroendocrine cell Neuroendocrine cells are cells that receive neuronal input (neurotransmitter Neurotransmitters are chemical messengers that transmit a signal from a neuron across the synapse to ...
s in the
thyroid gland The thyroid, or thyroid gland, is an endocrine gland Endocrine glands are ductless glands of the endocrine system The endocrine system is a messenger system comprising feedback loops of the hormones released by internal glands of an organis ...

thyroid gland
, and can bind to receptors on osteoclasts to directly inhibit osteoclast activity. Osteoprotegerin is secreted by osteoblasts and is able to bind RANK-L, inhibiting osteoclast stimulation. Osteoblasts can also be stimulated to increase bone mass through increased secretion of
osteoid In histology Histology, also known as microscopic anatomy or microanatomy, is the branch of biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical ...
and by inhibiting the ability of osteoclasts to break down
osseous tissue A bone is a rigid tissue Tissue may refer to: Biology * Tissue (biology), an ensemble of similar cells that together carry out a specific function * ''Triphosa haesitata'', a species of geometer moth found in North America * ''Triphosa dubit ...
. Increased secretion of osteoid is stimulated by the secretion of
growth hormone Growth hormone (GH) or somatotropin, also known as human growth hormone (hGH or HGH) in its human form, is a peptide hormone that stimulates growth, cell (biology), cell reproduction, and cell regeneration in humans and other animals. It is th ...
by the
pituitary In vertebrate Vertebrates () comprise all species of animal Animals (also called Metazoa) are multicellular eukaryotic organisms that form the Kingdom (biology), biological kingdom Animalia. With few exceptions, animals Hetero ...
,
thyroid hormone File:Thyroid_system.svg, upright=1.5, The thyroid The thyroid, or thyroid gland, is an endocrine gland in vertebrates. In humans it is in the neck and consists of two connected lobe (anatomy), lobes. The lower two thirds of the lobes are connec ...
and the sex hormones (
estrogen Estrogens or oestrogens, are a class of natural or synthetic s responsible for the development and regulation of the female and s. There are three major estrogens that have estrogenic hormonal activity: (E1), (E2), and (E3). Estradiol, an ...

estrogen
s and
androgen An androgen (from Greek ''andr-'', the stem of the word meaning "man") is any natural or synthetic steroid hormone A steroid hormone is a steroid , hypothetical a steroid with 32 carbon atoms. Its core ring system (ABCD), composed of 17 c ...
s). These hormones also promote increased secretion of osteoprotegerin. Osteoblasts can also be induced to secrete a number of
cytokine Cytokines are a broad and loose category of small protein Proteins are large biomolecule , showing alpha helices, represented by ribbons. This poten was the first to have its suckture solved by X-ray crystallography by Max Perutz and ...

cytokine
s that promote reabsorption of bone by stimulating osteoclast activity and differentiation from progenitor cells.
Vitamin D Vitamin D is a group of fat-soluble secosteroid 250px, The parent steroid skeleton. The B-ring of the parent steroid is broken between C9 and C10 to yield D vitamins. A secosteroid () is a type of steroid , hypothetical a steroid with ...
,
parathyroid hormone Parathyroid hormone (PTH), also called parathormone or parathyrin, is a peptide hormone secreted by the parathyroid glands that regulates the serum calcium concentration through its effects on bone, kidney, and intestine. PTH influences bone rem ...
and stimulation from osteocytes induce osteoblasts to increase secretion of RANK-
ligand In coordination chemistry A coordination complex consists of a central atom An atom is the smallest unit of ordinary matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by havi ...
and
interleukin 6 Interleukin 6 (IL-6) is an interleukin that acts as both a pro-inflammatory cytokine and an anti-inflammatory myokine. In humans, it is encoded by the ''IL6'' gene. In addition, osteoblasts secrete IL-6 to stimulate osteoclast formation. Smooth ...
, which cytokines then stimulate increased reabsorption of bone by osteoclasts. These same compounds also increase secretion of
macrophage colony-stimulating factor The colony stimulating factor 1 (CSF1), also known as macrophage colony-stimulating factor (M-CSF), is a secreted cytokine Cytokines are a broad and loose category of small proteins (~5–25 kDa) important in cell signaling. Cytokines are peptid ...
by osteoblasts, which promotes the differentiation of progenitor cells into osteoclasts, and decrease secretion of osteoprotegerin.


Bone volume

Bone volume is determined by the rates of bone formation and bone resorption. Recent research has suggested that certain growth factors may work to locally alter bone formation by increasing osteoblast activity. Numerous bone-derived growth factors have been isolated and classified via bone cultures. These factors include insulin-like growth factors I and II, transforming growth factor-beta, fibroblast growth factor, platelet-derived growth factor, and bone morphogenetic proteins. Evidence suggests that bone cells produce growth factors for extracellular storage in the bone matrix. The release of these growth factors from the bone matrix could cause the proliferation of osteoblast precursors. Essentially, bone growth factors may act as potential determinants of local bone formation. Research has suggested that cancellous bone volume in postmenopausal osteoporosis may be determined by the relationship between the total bone forming surface and the percent of surface resorption.


Clinical significance

A number of diseases can affect bone, including arthritis, fractures, infections, osteoporosis and tumours. Conditions relating to bone can be managed by a variety of doctors, including rheumatologists for joints, and
orthopedic Orthopaedic surgery or orthopaedics, is the branch of surgery Surgery ''cheirourgikē'' (composed of χείρ, "hand", and ἔργον, "work"), via la, chirurgiae, meaning "hand work". is a medical or dental specialty that uses operative ma ...
surgeons, who may conduct surgery to fix broken bones. Other doctors, such as rehabilitation specialists may be involved in recovery,
radiologists Radiology is the medical discipline that uses medical imaging Medical imaging is the technique and process of imaging the interior of a body for clinical analysis and medical intervention, as well as visual representation of the function o ...

radiologists
in interpreting the findings on imaging, and
pathologist Pathology is the study of the causesCauses, or causality, is the relationship between one event and another. It may also refer to: * Causes (band), an indie band based in the Netherlands * Causes (company), an online company See also * Ca ...

pathologist
s in investigating the cause of the disease, and
family doctor Family medicine is a medical specialty Specialty or speciality may refer to: * Deed, a contract in law * Index of speciality, a geometrical invariant * Speciality (album), ''Speciality'' (album), an album by J-Pop singer Nami Tamaki * Specialty ...
s may play a role in preventing complications of bone disease such as osteoporosis. When a doctor sees a patient, a history and exam will be taken. Bones are then often imaged, called
radiography Radiography is an imaging technique using X-ray An X-ray, or, much less commonly, X-radiation, is a penetrating form of high-energy electromagnetic radiation. Most X-rays have a wavelength ranging from 10 Picometre, picometers to 10 ...
. This might include
ultrasound Ultrasound is sound wave In physics Physics is the that studies , its , its and behavior through , and the related entities of and . "Physical science is that department of knowledge which relates to the order of nature, or, ...

ultrasound
X-ray An X-ray, or, much less commonly, X-radiation, is a penetrating form of high-energy electromagnetic radiation In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Moti ...

X-ray
,
CT scan A CT scan or computed tomography scan (formerly known as computed axial tomography or CAT scan) is a medical image, imaging Scientific technique, technique used in radiology to obtain detailed internal images of the body noninvasively for Diagno ...

CT scan
,
MRI scan Magnetic resonance imaging (MRI) is a medical imaging Medical imaging is the technique and process of imaging the interior of a body for clinical analysis and medical intervention, as well as visual representation of the function of some or ...

MRI scan
and other imaging such as a
Bone scan A bone scan or bone scintigraphy is a nuclear medicine Nuclear medicine is a medical specialty involving the application of radioactive substances in the diagnosis and treatment of disease. Nuclear medicine imaging, in a sense, is "radiology ...

Bone scan
, which may be used to investigate cancer. Other tests such as a blood test for autoimmune markers may be taken, or a
synovial fluid Synovial fluid, also called synovia, elp 1/sup> is a viscous, non-Newtonian fluid A non-Newtonian fluid is a fluid In physics, a fluid is a substance that continually Deformation (mechanics), deforms (flows) under an applied shear stress, ...
aspirate may be taken.


Fractures

In normal bone,
fractures Fracture is the separation of an object or material into two or more pieces under the action of stress. The fracture of a solid usually occurs due to the development of certain displacement discontinuity surfaces within the solid. If a displa ...
occur when there is significant force applied, or repetitive trauma over a long time. Fractures can also occur when a bone is weakened, such as with osteoporosis, or when there is a structural problem, such as when the bone remodels excessively (such as
Paget's diseasePaget's disease may refer to several conditions described by Sir James Paget Sir James Paget, 1st Baronet FRS HFRSE (11 January 1814 – 30 December 1899) (, rhymes with "gadget") was an English surgeon and pathologist who is best remembered ...
) or is the site of the growth of cancer. Common fractures include
wrist fracture A distal radius fracture, also known as wrist fracture, is a fracture (bone), break of the part of the radius (bone), radius bone which is close to the wrist. Symptoms include pain, bruising, and rapid-onset swelling. The wrist may be deformed. Th ...
s and
hip fracture A hip fracture is a break that occurs in the upper part of the femur (thigh bone). Symptoms may include pain around the hip, particularly with movement, and shortening of the leg. Usually the person cannot walk. They most often occur as a re ...
s, associated with
osteoporosis Osteoporosis is a systemic skeletal disorder characterized by low bone massImage:Bone density scanner.jpg, A scanner used to measure bone density using dual energy X-ray absorptiometry Bone density, or bone mineral density (BMD), is the amount ...
,
vertebral fracture The vertebral column, also known as the backbone or spine, is part of the axial skeleton Axial may refer to: * one of the anatomical directions describing relationships in an animal body * Axial Seamount and submarine volcano off Oregon, USA ...
s associated with high-energy trauma and cancer, and fractures of long-bones. Not all fractures are painful. When serious, depending on the fractures type and location, complications may include
flail chest Flail chest is a life-threatening medical condition that occurs when a segment of the Human rib cage, rib cage breaks due to major trauma, trauma and becomes detached from the rest of the Thoracic wall, chest wall. Two of the symptoms of flail che ...
,
compartment syndrome Compartment syndrome is a condition in which increased pressure within one of the body's anatomical compartments results in insufficient blood supply to tissue Tissue may refer to: Biology * Tissue (biology), an ensemble of similar cells that t ...
s or
fat embolism Fat embolism syndrome occurs when fat In nutrition Nutrition is the biochemical Biochemistry or biological chemistry, is the study of chemical processes within and relating to living organisms. A sub-discipline of both chemistry ...
.
Compound fracture A bone fracture (abbreviated FRX or Fx, Fx, or #) is a medical condition in which there is a partial or complete break in the continuity of the bone A bone is a Stiffness, rigid tissue (anatomy), tissue that constitutes part of the vertebra ...
s involve the bone's penetration through the skin. Some complex fractures can be treated by the use of
bone grafting Bone grafting is a surgical procedure that replaces missing bone in order to repair bone fractures that are extremely complex, pose a significant health risk to the patient, or fail to heal properly. Some small or acute fractures can be cured with ...
procedures that replace missing bone portions. Fractures and their underlying causes can be investigated by
X-ray An X-ray, or, much less commonly, X-radiation, is a penetrating form of high-energy electromagnetic radiation In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Moti ...

X-ray
s,
CT scans A CT scan or computed tomography scan (formerly known as computed axial tomography or CAT scan) is a medical imaging technique used in radiology Radiology is the medical discipline that uses medical imaging to diagnose and treat diseases wi ...
and
MRI Magnetic resonance imaging (MRI) is a medical imaging Medical imaging is the technique and process of imaging Imaging is the representation or reproduction of an object's form; especially a visual representation (i.e., the formation of a ...

MRI
s. Fractures are described by their location and shape, and several classification systems exist, depending on the location of the fracture. A common long bone fracture in children is a Salter–Harris fracture. When fractures are managed, pain relief is often given, and the fractured area is often immobilised. This is to promote
bone healing thumbnail, 200px, Bone healing of a fracture by forming a callus as shown by X-ray. Bone healing, or fracture healing, is a proliferative physiological Physiology (; ) is the scientific study of functions and mechanisms in a living system. ...
. In addition, surgical measures such as
internal fixation Internal fixation is an operation in orthopedics Orthopaedic surgery or orthopaedics, is the branch of surgery concerned with conditions involving the musculoskeletal system. Orthopaedic surgeons use both surgical and nonsurgical means to tre ...
may be used. Because of the immobilisation, people with fractures are often advised to undergo
rehabilitation Rehabilitation or Rehab may refer to: Health * Rehabilitation (neuropsychology), therapy to regain or improve neurocognitive function that has been lost or diminished * Rehabilitation (wildlife), treatment of injured wildlife so they can be returne ...
.


Tumours

There are several types of tumour that can affect bone; examples of benign
bone tumour A bone tumor is a neoplastic growth of tissue in bone. Abnormal growths found in the bone can be either benign (noncancerous) or malignant (cancerous). Average five-year survival in the United States after being diagnosed with bone and joint canc ...
s include
osteoma An osteoma (plural: "osteomata") is a new piece of bone A bone is a rigid tissue Tissue may refer to: Biology * Tissue (biology), an ensemble of similar cells that together carry out a specific function * ''Triphosa haesitata'', a species o ...

osteoma
, osteoid osteoma,
osteochondroma Osteochondromas are the most common benign tumors of the bones. The tumors take the form of cartilage-capped bony projections or outgrowth on the surface of bones exostosis, exostoses. It is characterized as a type of overgrowth that can occur in a ...

osteochondroma
,
osteoblastoma Osteoblastoma is an uncommon osteoid tissue-forming primary neoplasm A neoplasm () is a type of abnormal and excessive growth, called neoplasia, of tissue. The growth of a neoplasm is uncoordinated with that of the normal surrounding tissue, a ...
,
enchondroma An enchondroma is a benign cartilage Cartilage (cartilaginous tissue) is a resilient and smooth elastic tissue, rubber-like padding that covers and protects the ends of long bone A bone is a Stiffness, rigid tissue (anatomy), tissue that con ...
, giant cell tumour of bone, and aneurysmal bone cyst.


Cancer

Cancer can arise in bone tissue, and bones are also a common site for other cancers to spread (metastasise) to. Cancers that arise in bone are called "primary" cancers, although such cancers are rare. Metastases within bone are "secondary" cancers, with the most common being breast cancer, lung cancer, prostate cancer, thyroid cancer, and kidney cancer. Secondary cancers that affect bone can either destroy bone (called a "lytic cycle, lytic" cancer) or create bone (a "sclerosis (medicine), sclerotic" cancer). Cancers of the bone marrow inside the bone can also affect bone tissue, examples including leukemia and multiple myeloma. Bone may also be affected by cancers in other parts of the body. Cancers in other parts of the body may release
parathyroid hormone Parathyroid hormone (PTH), also called parathormone or parathyrin, is a peptide hormone secreted by the parathyroid glands that regulates the serum calcium concentration through its effects on bone, kidney, and intestine. PTH influences bone rem ...
or parathyroid hormone-related peptide. This increases bone reabsorption, and can lead to bone fractures. Bone tissue that is destroyed or altered as a result of cancers is distorted, weakened, and more prone to fracture. This may lead to compression of the spinal cord, destruction of the marrow resulting in bruising, bleeding and immunosuppression, and is one cause of bone pain. If the cancer is metastatic, then there might be other symptoms depending on the site of the original cancer. Some bone cancers can also be felt. Cancers of the bone are managed according to their type, their cancer staging, stage, prognosis, and what symptoms they cause. Many primary cancers of bone are treated with radiotherapy. Cancers of bone marrow may be treated with chemotherapy, and other forms of targeted therapy such as immunotherapy may be used. Palliative care, which focuses on maximising a person's quality of life, may play a role in management, particularly if the likelihood of 5-year survival, survival within five years is poor.


Painful conditions

* Osteomyelitis is inflammation of the bone or bone marrow due to bacterial infection. *Osteomalacia is a painful softening of adult bone caused by severe vitamin D deficiency. * Osteogenesis imperfecta * Osteochondritis dissecans * Ankylosing spondylitis * Skeletal fluorosis is a bone disease caused by an excessive accumulation of fluoride in the bones. In advanced cases, skeletal fluorosis damages bones and joints and is painful.


Osteoporosis

Osteoporosis is a disease of bone where there is reduced bone mineral density, increasing the likelihood of bone fracture, fractures. Osteoporosis is defined in women by the World Health Organization as a bone mineral density of 2.5 standard deviations below peak bone mass, relative to the age and sex-matched average. This density is measured using dual energy X-ray absorptiometry (DEXA), with the term "established osteoporosis" including the presence of a fragility fracture. Osteoporosis is most common in women after menopause, when it is called "postmenopausal osteoporosis", but may develop in men and premenopausal women in the presence of particular hormonal disorders and other Chronic (medicine), chronic diseases or as a result of tobacco smoking, smoking and medications, specifically glucocorticoids. Osteoporosis usually has no symptoms until a fracture occurs. For this reason, DEXA scans are often done in people with one or more risk factors, who have developed osteoporosis and are at risk of fracture. Osteoporosis treatment includes advice to stop smoking, decrease alcohol consumption, exercise regularly, and have a healthy diet. Calcium and trace mineral supplements may also be advised, as may
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. When medication is used, it may include bisphosphonates, Strontium ranelate, and hormone replacement therapy.


Osteopathic medicine

Osteopathic medicine is a school of medical thought originally developed based on the idea of the link between the musculoskeletal system and overall health, but now very similar to mainstream medicine. , over 77,000 physicians Osteopathic medicine in the United States, in the United States are trained in osteopathic medical schools.


Osteology

The study of bones and teeth is referred to as osteology. It is frequently used in anthropology, archeology and forensics, forensic science for a variety of tasks. This can include determining the nutritional, health, age or injury status of the individual the bones were taken from. Preparing fleshed bones for these types of studies can involve the process of maceration (bone), maceration. Typically anthropologists and archeologists study bone tools made by ''Homo sapiens'' and ''Homo neanderthalensis''. Bones can serve a number of uses such as projectile points or artistic pigments, and can also be made from external bones such as antlers.


Other animals

Bird skeletons are very lightweight. Their bones are smaller and thinner, to aid flight. Among mammals, bats come closest to birds in terms of bone density, suggesting that small dense bones are a flight adaptation. Many bird bones have little marrow due to their being hollow. A bird's beak is primarily made of bone as projections of the mandibles which are covered in keratin.


Extra-skeletal bones in mammals

These bones, primarily formed separately in subcutaneous tissues, include headgears (such as bony core of horns, antlers, ossicones), osteoderm, and os penis/ os clitoris. A deer's antlers are composed of bone which is an unusual example of bone being outside the skin of the animal once the velvet is shed.


Other mineralized structures

The extinct predatory fish ''Dunkleosteus'' had sharp edges of hard exposed bone along its jaws. Many animals possess an exoskeleton that is not made of bone. These include insects and crustaceans. The proportion of cortical bone that is 80% in the human skeleton may be much lower in other animals, especially in marine mammals and marine turtles, or in various Mesozoic marine reptiles, such as ichthyosaurs, among others. Many animals, particularly herbivores, practice osteophagy—the eating of bones. This is presumably carried out in order to replenish lacking
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. Many bone diseases that affect humans also affect other vertebrates—an example of one disorder is skeletal fluorosis.


Society and culture

Bones from slaughtered animals have a number of uses. In prehistoric times, they have been used for making bone tools. They have further been used in bone carving, already important in prehistoric art, and also in modern time as crafting materials for buttons, beads, handles, bobbins, Napier's bones, calculation aids, head nuts, dice, poker chips, pick-up sticks, ornaments, etc. A special genre is scrimshaw. Bone glue can be made by prolonged boiling of ground or cracked bones, followed by filtering and evaporation to thicken the resulting fluid. Historically once important, bone glue and other animal glues today have only a few specialized uses, such as in antiques restoration. Essentially the same process, with further refinement, thickening and drying, is used to make gelatin. Broth is made by simmering several ingredients for a long time, traditionally including bones. Bone char, a porous, black, granular material primarily used for filtration and also as a black pigment, is produced by charring mammal bones. Oracle bone script was a writing system used in Ancient China based on inscriptions in bones. Its name originates from oracle bones, which were mainly ox clavicle. The Ancient Chinese (mainly in the Shang dynasty), would write their questions on the oracle bone, and burn the bone, and where the bone cracked would be the answer for the questions. To :wikt:point the bone, point the bone at someone is considered bad luck in some cultures, such as Australian aborigines, such as by the Kurdaitcha#Bone pointing, Kurdaitcha. The Furcula, wishbones of fowl have been used for divination, and are still customarily used in a tradition to determine which one of two people pulling on either prong of the bone may make a wish. Various cultures throughout history have adopted the custom of shaping an infant's head by the practice of artificial cranial deformation. A widely practised custom in China was that of foot binding to limit the normal growth of the foot.


Additional images

File:Gray72-en.svg, Cells in bone marrow File:Bertazzo S - SEM deproteined trabecular - wistar rat - x100.tif, Scanning electron microscope of bone at 100× magnification File:Bone structure marco photo.jpg, Structure detail of an animal bone


See also

*Artificial bone *Bone health *Distraction osteogenesis *National Bone Health Campaign *Skeleton


References


Footnotes

* * * * – ''drawings by Philip J.'' * * – ''Anthony edits the current version; Harrison edited previous versions.''


External links


Educational resource materials (including animations) by the American Society for Bone and Mineral Research


* [http://www.scq.ubc.ca/?p=400 A good basic overview of bone biology from the Science Creative Quarterly] *
Bone histology photomicrographs
{{Authority control Bones Skeletal system Connective tissue