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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
skeleton The skeleton refers to the frames of support of animal bodies. There are several different skeletal types: the exoskeleton, which is the stable outer shell of an organism, the endoskeleton, which forms the support structure inside the body, th ...

skeleton
in most
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 Heterotroph, consume organic ma ...
animals. Bones protect the various organs of the body, produce
red Red is the color at the long wavelength end of the visible spectrum Laser beams with visible spectrum The visible spectrum is the portion of the electromagnetic spectrum that is visual perception, visible to the human eye. Electromagnetic ...

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. (20 ...

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. The hard tissues of humans are bone A bone is a Stiffness, rigid tissue (anatomy), tissue that constitutes part of the vertebrate s ...
, a type of specialized
connective tissue Connective tissue is one of the four basic types of animal tissue (biology), tissue, along with epithelial tissue, muscle tissue, and nervous tissue. It develops from the mesoderm. Connective tissue is found in between other tissues everywhere in ...
. It has a
honeycomb A honeycomb is a mass of hexagonal prismatic wax cells built by honey bee A honey bee (also spelled honeybee) is a eusocial flying insect within the genus ''Apis'' of the bee clade, all native to Eurasia. They are known for their constructi ...

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 vertebrate skeleton in animals. Bones protect the various organs of the body, produce red blood cell, red and white b ...
s.
Osteoblast Osteoblasts (from the Greek language, Greek combining forms for "bone", ὀστέο-, ''osteo-'' and βλαστάνω, ''blastanō'' "germinate") are cell (biology), cells with a single Cell nucleus, nucleus that synthesize bone. However, in the pro ...

Osteoblast
s and
osteocyte An osteocyte, an oblate shaped type of 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 b ...
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 vertebrate skeleton in animals. Bones protect the various organs of the body, ...

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 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 Proteins are large biomolecules and macromolecules th ...
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 component of bone tissue. It gives bone A bone is a Stiffness, rigid tissue (anatomy), tissue that constitutes part of the vertebrate skeleton in an ...
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 * ''Triphosa ...
,
endosteum The endosteum (plural endostea) is a thin vascular membrane of connective tissue Connective tissue is one of the four basic types of animal tissue (biology), tissue, along with epithelial tissue, muscle tissue, and nervous tissue. It develops from ...
,
periosteum The periosteum is a membrane that covers the outer surface of all 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 re ...
,
nerve A nerve is an enclosed, cable-like bundle of nerve fibers called axon An axon (from Greek ἄξων ''áxōn'', axis), or nerve fiber (or nerve fibre: see American and British English spelling differences#-re, -er, spelling differences), is a ...

nerve
s,
blood vessel The blood vessels are the components of the circulatory system that transport blood throughout the human body. These vessels transport blood cells, nutrients, and oxygen to the tissues of the body. They also take waste and carbon dioxide away fr ...
s and
cartilage Cartilage (cartilaginous tissue) is a resilient and smooth elastic tissue Elastic fibers (or yellow fibers) are an essential component of the extracellular matrix In biology Biology is the natural science that studies life and living org ...
. 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 body. It comprises a ...

human body
at birth, there are approximately 300 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 indi ...
s. The largest bone in the body is the
femur The femur (, pl. ''femurs'' or ''femora'' ), 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 ro ...

femur
or thigh-bone, and the smallest is the
stapes The stapes or stirrup is a 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 blood cells ...
in the
middle ear The middle ear is the portion of the ear internal to the eardrum, and external to the oval window of the inner ear. The mammalian middle ear contains three ossicles, which transfer the vibrations of the eardrum into waves in the fluid and membran ...

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 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 Proteins are large biomolecules and macromolecules th ...
fibers, also known as ossein, and the remainder is
ground substanceGround substance is an amorphous gel-like substance in the extracellular space that contains all components of the extracellular matrix (ECM) except for fibrous materials such as collagen and elastin. Ground substance is active in the development, ...
. The elasticity of
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 Proteins are large biomolecules and macromolecules th ...
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 In geology Geology (from the Ancient Greek γῆ, ''gē'' ("earth") and -λoγία, ''-logia'', ("study of", "discourse")) is an Earth science concerned ...
. It is the
bone mineral Bone mineral (also called inorganic bone phase, bone salt, or bone apatite) is the inorganic component of bone tissue. It gives bone A bone is a Stiffness, rigid tissue (anatomy), tissue that constitutes part of the vertebrate skeleton in an ...
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.


Cortex

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 The skeleton refers to the frames of support of animal bodies. There are several different skeletal types: the exoskeleton, which is the stable outer shell of an organism, the endoskeleton, which forms the support structure inside the body, th ...

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 consisting of a beam or rigid rod pivoted at a fixed hinge, or '':wikt:fulcrum, fulcrum''. A lever is a rigid body capable of rotating on a point on itself. On the basis of the locations of fulcrum, load and effo ...

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. Osteons are roughly cylindrical structures that are typically between 0.25 mm and 0.35 mm in diameter In geometry Ge ...

osteon
or Haversian system. Each column is multiple layers of
osteoblast Osteoblasts (from the Greek language, Greek combining forms for "bone", ὀστέο-, ''osteo-'' and βλαστάνω, ''blastanō'' "germinate") are cell (biology), cells with a single Cell nucleus, nucleus that synthesize bone. However, in the pro ...

osteoblast
s and
osteocyte An osteocyte, an oblate shaped type of 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 b ...
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 that covers the outer surface of all 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 re ...
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 four basic types of animal tissue (biology), tissue, along with epithelial tissue, muscle tissue, and nervous tissue. It develops from ...
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. Osteons are roughly cylindrical structures that are typically between 0.25 mm and 0.35 mm in diameter In geometry Ge ...

osteon
.


Trabecules

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 (composites), void (i.e. "empty") spaces in a material, and is a volume fraction, fraction of the volume of voids over the total volume, between 0 and 1, or as a percentage between 0% and 100%. Stri ...
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 * ''Triphosa ...
where
hematopoiesis Haematopoiesis (, from Greek , 'blood' and 'to make'; also hematopoiesis in American English; sometimes also h(a)emopoiesis) is the formation of blood Blood is a body fluid in humans and other animals that delivers necessary substances suc ...
, 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 element in the form of a small beam Beam may refer to: Streams of particles or energy *Light beam, or beam of light, a directional projection o ...
. The trabeculae are aligned towards the mechanical load distribution that a bone experiences within long bones such as the
femur The femur (, pl. ''femurs'' or ''femora'' ), 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 ro ...

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 language, Greek combining forms for "bone", ὀστέο-, ''osteo-'' and βλαστάνω, ''blastanō'' "germinate") are cell (biology), cells with a single Cell nucleus, nucleus that synthesize bone. However, in the pro ...

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 * ''Triphosa ...
and
hematopoietic stem cell Hematopoietic stem cells (HSCs) are the stem cells that give rise to other blood cells. This process is called haematopoiesis. This process occurs in the red bone marrow, in the core of most bones. In embryonic development, the red bone marro ...

hematopoietic stem cell
s that give rise to
platelet Platelets, also called thrombocytes (from 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 i ...

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 ''ky ...

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.


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 * ''Triphosa ...
, 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 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 ...
. 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.


Cells

Bone is a metabolically active tissue composed of several types of cells. These cells include
osteoblast Osteoblasts (from the Greek language, Greek combining forms for "bone", ὀστέο-, ''osteo-'' and βλαστάνω, ''blastanō'' "germinate") are cell (biology), cells with a single Cell nucleus, nucleus that synthesize bone. However, in the pro ...

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 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 b ...
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 vertebrate skeleton in animals. Bones protect the various organs of the body, ...

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 vertebrate skeleton in animals. Bones protect the various organs of the body, ...

osteoclast
s are derived from the same cells that differentiate to form
macrophage Macrophages (abbreviated as M φ, MΦ or MP) ( el, large eaters, from Greek ''μακρός'' (') = large, ''φαγεῖν'' (') = to eat) are a type of white blood cell of the immune system that engulfs and digests anything that does not have o ...

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 organis ...

monocyte
s. Within the marrow of the bone there are also
hematopoietic stem cell Hematopoietic stem cells (HSCs) are the stem cells that give rise to other blood cells. This process is called haematopoiesis. This process occurs in the red bone marrow, in the core of most bones. In embryonic development, the red bone marro ...

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 ''ky ...

red blood cell
s, and
platelet Platelets, also called thrombocytes (from 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 i ...

platelet
s.


Osteoblast

Osteoblast Osteoblasts (from the Greek language, Greek combining forms for "bone", ὀστέο-, ''osteo-'' and βλαστάνω, ''blastanō'' "germinate") are cell (biology), cells with a single Cell nucleus, nucleus that synthesize bone. However, in the pro ...

Osteoblast
s are mononucleate bone-forming cells. They are located on the surface of osteon seams and make a
protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residue (biochemistry), residues. Proteins perform a vast array of functions within organisms, including Enzyme catalysis, catalysing metabol ...

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 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 Proteins are large biomolecules and macromolecules th ...
. Osteoblasts also manufacture
hormone A hormone (from the Greek participle , "setting in motion") is any member of a class of signaling molecules in multicellular organisms, that are transported to distant organs to regulate physiology and / or behavior. Hormones are required for t ...

hormone
s, such as
prostaglandin The prostaglandins (PG) are a group of physiologically active lipid In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecul ...
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 polyatomic ion, diatomic anion with chemical formula OH−. It consists of an oxygen and hydrogen atom held together by a single covalent bond, and carries a negative electric charge. It is an important but usually Self-ionization o ...
and
bicarbonate In inorganic chemistry, bicarbonate (IUPAC The International Union of Pure and Applied Chemistry (IUPAC ) is an international federation of National Adhering Organizations that represents chemists in individual countries. It is a member of ...
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 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 b ...
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 vertebrate skeleton in animals. Bones protect the various organs of the body, ...

Osteoclast
s are very large
multinucleate Multinucleate cells (multinucleated or polynuclear cells) are eukaryotic cells that have more than one nucleus per cell, i.e., multiple nuclei share one common cytoplasm In cell biology, the cytoplasm is all of the material within a Cell (biol ...
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 organis ...

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 M φ, MΦ or MP) ( el, large eaters, from Greek ''μακρός'' (') = large, ''φαγεῖν'' (') = to eat) are a type of white blood cell of the immune system that engulfs and digests anything that does not have o ...

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 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 exposed to air. Its physical and chemical properties a ...

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 mechanisms ...
.


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 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, crystal ...
, the dominant
bone mineral Bone mineral (also called inorganic bone phase, bone salt, or bone apatite) is the inorganic component of bone tissue. It gives bone A bone is a Stiffness, rigid tissue (anatomy), tissue that constitutes part of the vertebrate skeleton in an ...
, 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 Protein ...

type I collagen
—"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 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, crystal ...
phase, include other compounds 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 exposed to air. Its physical and chemical properties a ...

calcium
and
phosphate In chemistry Chemistry is the scientific discipline involved with Chemical element, elements and chemical compound, compounds composed of atoms, molecules and ions: their composition, structure, properties, behavior and the changes they un ...

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 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 Proteins are large biomolecules and macromolecules th ...
fibers give bone its
tensile strength In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'nature'), , is the natural science that studies matter, its Motion (physics), motion and behavior through Spa ...
, and the interspersed crystals of
hydroxyapatite 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, crystal ...
give bone its
compressive strength Compressive strength or compression strength is the capacity of a material or structure to withstand loads tending to reduce size, as opposed to Tensile strength which withstands loads tending to elongate. In other words, compressive strength resis ...
. 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 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 exposed to air. Its physical and chemical properties a ...

calcium
to
phosphate In chemistry Chemistry is the scientific discipline involved with Chemical element, elements and chemical compound, compounds composed of atoms, molecules and ions: their composition, structure, properties, behavior and the changes they un ...

phosphate
varying between 1.3 and 2.0 (per weight), and trace minerals such as
magnesium Magnesium is a chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance consisting only of atoms that all have the same ...

magnesium
,
sodium Sodium is a chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance consisting only of atoms that all have the same num ...

sodium
,
potassium Potassium is a chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance consisting only of atoms that all have the same ...

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 min ...
also being found. Type I collagen composes 90–95% of the organic matrix, with remainder of the matrix being a homogenous liquid called
ground substanceGround substance is an amorphous gel-like substance in the extracellular space that contains all components of the extracellular matrix (ECM) except for fibrous materials such as collagen and elastin. Ground substance is active in the development, ...
consisting of
proteoglycan Proteoglycans are protein Proteins are large biomolecules or macromolecules that are comprised of one or more long chains of amino acid residue (biochemistry), residues. Proteins perform a vast array of functions within organisms, including E ...
s such as
hyaluronic acid Hyaluronic acid (; abbreviated HA; conjugate base hyaluronate), also called hyaluronan, is an anionic, nonsulfated glycosaminoglycan distributed widely throughout connective, epithelial, and neural tissues. It is unique among glycosaminogly ...

hyaluronic acid
and
chondroitin sulfateImage:Chondroitin Sulfate Structure NTP.png, 240px, Chemical structure of one unit in a chondroitin sulfate chain. Chondroitin-4-sulfate: R1 = H; R2 = SO3H; R3 = H. Chondroitin-6-sulfate: R1 = SO3H; R2, R3 = H. Chondroitin sulfate is a sulfated glyco ...
, 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 of an animal that develops from an embryo An embryo is the early stage of development of a multicellular organism. In general, in organism In b ...

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; ''wikt:γῆ, geo-'' "earth", ''wikt:μέτρον, -metron'' "measurement") is, with arithmetic, one of the oldest branches of mathematics. It is concerned with properties of space ...
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. In general, in organisms that Sexual reprodu ...

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 (typica ...

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 language, Greek combining forms for "bone", ὀστέο-, ''osteo-'' and βλαστάνω, ''blastanō'' "germinate") are cell (biology), cells with a single Cell nucleus, nucleus that synthesize bone. However, in the pro ...

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 ''wikt:poly-, poly-'', "many" + ''wikt:-mer, -mer'', "part") is a Chemical substance, substance or material consisting of very large molecules, or macromolecules, composed of many Repeat unit, repeating subunits. Due to their ...

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 Precipitation is the process of conversion of a chemical substance A chemical substance is a form of matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All ever ...
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 In biochemistry, a protein dimer is a macromolecular Complex (chemistry), complex formed by two protein monomers, or single proteins, which are usually n ...
. 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 bones: long, Short bone, short, Flat bone, flat, Irregular bone, irregular and Sesamoid bone, sesamoid. Long bones, especially the femur and tibia, are subje ...

Long bone
s are characterized by a shaft, the
diaphysis The diaphysis is the main or midsection (shaft) of a long bone. It is made up of cortical bone and usually contains bone marrow Bone marrow is a semi-solid tissue found within the spongy or cancellous portions of bone A bone is a Stiffnes ...
, 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 vertebrate skeleton in animals. Bones protect the various organs of the body, p ...
, 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 ...
of the
wrist In human anatomy, the wrist is variously defined as 1) the Carpal bones, carpus or carpal bones, the complex of eight bones forming the proximal skeletal segment of the hand;Behnke 2006, p. 76. "The wrist contains eight bones, roughly aligned i ...

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 and the human leg, leg meet. The ankle includes three joints: the ankle joint proper or talocrural joint, the subtalar joint, and the inferior tibiofibular joint. The movements prod ...
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; ''wikt:γῆ, geo-'' "earth", ''wikt:μέτρον, -metron'' "measurement") is, with arithmetic, one of the oldest branches of mathematics. It is concerned with properties of space ...
-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 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 blood cells, store minera ...
s are thin and generally curved, with two parallel layers of compact bone sandwiching a layer of spongy bone. Most of the bones of the
skull The skull is a 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 white blood cells, sto ...

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 indi ...
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 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, produc ...
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 o ...

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 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 re ...

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 organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is any individual conti ...
, anatomists use a number of Anatomical terminology, anatomical terms to describe the appearance, shape and function of bones. Other anatomical terms are also used to describe the Anatomical terms of location, location of bones. Like other anatomical terms, many of these derive from Latin and Greek language, Greek. 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 bones 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. During the prenatal development, fetal stage of development this occurs by two processes: intramembranous ossification and endochondral ossification. Intramembranous ossification involves the formation of bone from
connective tissue Connective tissue is one of the four basic types of animal tissue (biology), tissue, along with epithelial tissue, muscle tissue, and nervous tissue. It develops from the mesoderm. Connective tissue is found in between other tissues everywhere in ...
whereas endochondral ossification involves the formation of bone from
cartilage Cartilage (cartilaginous tissue) is a resilient and smooth elastic tissue Elastic fibers (or yellow fibers) are an essential component of the extracellular matrix In biology Biology is the natural science that studies life and living org ...
. Intramembranous ossification mainly occurs during formation of the flat bones of the
skull The skull is a 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 white blood cells, sto ...

skull
but also the mandible, maxilla, and clavicles; the bone is formed from connective tissue such as mesenchyme tissue rather than from cartilage. The process includes: the development of the ossification center, calcification, 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 centers, and the formation of articular cartilage and the epiphyseal plates. 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. 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 Epiphysis, epiphyses 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). 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.


Functions

Bones have a variety of functions:


Mechanical

Bones serve a variety of mechanical functions. Together the bones in the body form the
skeleton The skeleton refers to the frames of support of animal bodies. There are several different skeletal types: the exoskeleton, which is the stable outer shell of an organism, the endoskeleton, which forms the support structure inside the body, th ...

skeleton
. They provide a frame to keep the body supported, and an attachment point for skeletal muscles, tendons, ligaments and joints, 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). Bones protect internal organs, such as the
skull The skull is a 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 white blood cells, sto ...

skull
protecting the brain or the ribs protecting the heart and lungs. Because of the way that bone is formed, bone has a high
compressive strength Compressive strength or compression strength is the capacity of a material or structure to withstand loads tending to reduce size, as opposed to Tensile strength which withstands loads tending to elongate. In other words, compressive strength resis ...
of about , poor tensile strength of 104–121 MPa, and a very low 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 Brittleness, brittle, bone does have a significant degree of elasticity (physics), elasticity, contributed chiefly by
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 Proteins are large biomolecules and macromolecules th ...
. Mechanically, bones also have a special role in Hearing (sense), hearing. The ossicles are three small bones in the
middle ear The middle ear is the portion of the ear internal to the eardrum, and external to the oval window of the inner ear. The mammalian middle ear contains three ossicles, which transfer the vibrations of the eardrum into waves in the fluid and membran ...

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 * ''Triphosa ...
. Bone marrow produces blood cells in a process called
hematopoiesis Haematopoiesis (, from Greek , 'blood' and 'to make'; also hematopoiesis in American English; sometimes also h(a)emopoiesis) is the formation of blood Blood is a body fluid in humans and other animals that delivers necessary substances suc ...
. 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 ''ky ...

red blood cell
s,
platelet Platelets, also called thrombocytes (from 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 i ...

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 cells that give rise to other blood cells. This process is called haematopoiesis. This process occurs in the red bone marrow, in the core of most bones. In embryonic development, the red bone marro ...

hematopoietic stem cell
divide in a process called mitosis to produce precursor cells. These include precursors which eventually give rise to white blood cells, and erythroblasts which give rise to red blood cells. Unlike red and white blood cells, created by mitosis, platelets are shed from very large cells called megakaryocytes. This process of progressive differentiation occurs within the bone marrow. After the cells are matured, they enter the circulatory system, circulation. Every day, over 2.5 billion red blood cells and platelets, and 50–100 billion granulocytes 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 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 exposed to air. Its physical and chemical properties a ...

calcium
and phosphorus. Determined by the species, age, and the type of bone, bone cells make up to 15 percent of the bone. Growth factor storage—mineralized bone matrix stores important growth factors such as insulin-like growth factors, transforming growth factor, bone morphogenetic proteins and others. * 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 acids. * Acid-Base (chemistry), base balance – bone buffers the blood against excessive pH changes by absorbing or releasing Alkali salt, alkaline salts. * Detoxification – bone tissues can also store heavy metals and other foreign elements, removing them from the blood and reducing their effects on other tissues. These can later be gradually released for excretion. * Endocrine system, Endocrine organ – bone controls
phosphate In chemistry Chemistry is the scientific discipline involved with Chemical element, elements and chemical compound, compounds composed of atoms, molecules and ions: their composition, structure, properties, behavior and the changes they un ...

phosphate
metabolism by releasing fibroblast growth factor 23 (FGF-23), which acts on kidneys to reduce phosphate 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 (glucose) and Adipose tissue, fat deposition. Osteocalcin increases both the insulin secretion and sensitivity, in addition to boosting the number of beta cell, 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 Bone remodeling, remodeling. 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 paracrine, 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, repair Microdamage in bone, microdamaged bones from everyday stress, and to shape the skeleton during growth. Repeated stress, such as weight-bearing exercise or bone healing, results in the bone thickening at the points of maximum stress (Wolff's law). It has been hypothesized that this is a result of bone's piezoelectricity, 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 enzymes 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 signalling to control the activity of each other. For example, the rate at which osteoclasts resorb bone is inhibited by calcitonin and osteoprotegerin. Calcitonin is produced by parafollicular cells in the 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 Enzyme inhibitor, inhibiting the ability of osteoclasts to break down osseous tissue. Increased secretion of osteoid is stimulated by the secretion of growth hormone by the pituitary, thyroid hormone and the sex hormones (estrogens and androgens). These hormones also promote increased secretion of osteoprotegerin. Osteoblasts can also be induced to secrete a number of cytokines that promote reabsorption of bone by stimulating osteoclast activity and differentiation from progenitor cells. Vitamin D, parathyroid hormone and stimulation from osteocytes induce osteoblasts to increase secretion of RANK-ligand and interleukin 6, which cytokines then stimulate increased reabsorption of bone by osteoclasts. These same compounds also increase secretion of macrophage colony-stimulating factor by osteoblasts, which promotes the differentiation of progenitor cells into osteoclasts, and decrease secretion of osteoprotegerin.


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 tumors. Conditions relating to bone can be managed by a variety of doctors, including Rheumatology, rheumatologists for joints, and orthopedic surgeons, who may conduct surgery to fix broken bones. Other doctors, such as rehabilitation medicine, rehabilitation specialists may be involved in recovery, radiology, radiologists in interpreting the findings on imaging, and pathologists in investigating the cause of the disease, and family doctors 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. This might include ultrasound X-ray, CT scan, MRI scan and other imaging such as a 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 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 fractures and hip fractures, associated with osteoporosis, vertebral fractures 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, compartment syndromes or fat embolism. Compound fractures involve the bone's penetration through the skin. Some complex fractures can be treated by the use of bone grafting procedures that replace missing bone portions. Fractures and their underlying causes can be investigated by X-rays, CT scans and MRIs. 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. In addition, surgical measures such as internal fixation may be used. Because of the immobilisation, people with fractures are often advised to undergo Physical medicine and rehabilitation, rehabilitation.


Tumors

There are several types of tumor that can affect bone; examples of benign Bone tumor, bone tumors include osteoma, osteoid osteoma, osteochondroma, osteoblastoma, enchondroma, giant-cell tumor 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 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.


Other 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 Vitamin D. 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. Some 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. The extinct predatory fish ''Dunkleosteus'' had sharp edges of hard exposed bone along its jaws. 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
phosphate In chemistry Chemistry is the scientific discipline involved with Chemical element, elements and chemical compound, compounds composed of atoms, molecules and ions: their composition, structure, properties, behavior and the changes they un ...

phosphate
. 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, arrows, scrimshaw, ornaments, etc. 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