A bone is a
rigid organ
Organ may refer to:
Biology
* Organ (biology), a part of an organism
Musical instruments
* Organ (music), a family of keyboard musical instruments characterized by sustained tone
** Electronic organ, an electronic keyboard instrument
** Hammond ...
that constitutes part of the
skeleton in most
vertebrate animals. Bones protect the various other organs of the body, produce
red and
white blood cells, store
minerals, provide structure and support for the body, and enable
mobility. Bones come in a variety of shapes and sizes and have complex internal and external structures.
They are lightweight yet strong and hard and serve multiple
functions.
Bone tissue (osseous tissue), which is also called bone in the
uncountable sense of that word, is
hard tissue Hard tissue, refers to "normal" calcified tissue, is the tissue which is mineralized and has a firm intercellular matrix. The hard tissues of humans are bone, tooth enamel, dentin, and cementum. The term is in contrast to soft tissue.
Bone
B ...
, a type of specialized
connective tissue. It has a
honeycomb-like
matrix
Matrix most commonly refers to:
* ''The Matrix'' (franchise), an American media franchise
** '' The Matrix'', a 1999 science-fiction action film
** "The Matrix", a fictional setting, a virtual reality environment, within ''The Matrix'' (franchi ...
internally, which helps to give the bone rigidity. Bone tissue is made up of different types of
bone cells.
Osteoblasts and
osteocytes are involved in the formation and
mineralization
Mineralization may refer to:
* Mineralization (biology), when an inorganic substance precipitates in an organic matrix
** Biomineralization, a form of mineralization
** Mineralization of bone, an example of mineralization
** Mineralized tissues ar ...
of bone;
osteoclasts are involved in the
resorption 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 called ''
ossein'' and an inorganic component of
bone mineral made up of various salts. Bone tissue is
mineralized tissue of two types,
cortical bone and cancellous bone. Other types of tissue found in bones include
bone marrow,
endosteum,
periosteum,
nerves,
blood vessels and
cartilage.
In the
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 bones. The largest bone in the body is the
femur or thigh-bone, and the smallest is the
stapes in the
middle ear.
The Greek word for bone is ὀστέον ("osteon"), hence the many terms that use it as a prefix—such as
osteopathy. In
anatomical terminology, including the
Terminologia Anatomica international standard, the word for a bone is ''
os'' (for example,
os breve,
os longum,
os sesamoideum).
Structure
Bone is not uniformly solid, but consists of a flexible
matrix
Matrix most commonly refers to:
* ''The Matrix'' (franchise), an American media franchise
** '' The Matrix'', a 1999 science-fiction action film
** "The Matrix", a fictional setting, a virtual reality environment, within ''The Matrix'' (franchi ...
(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 fibers, also known as ossein, and the remainder is
ground substance.
The elasticity of
collagen improves fracture resistance.
The matrix is hardened by the binding of inorganic mineral salt,
calcium phosphate, in a chemical arrangement known as
bone mineral, a form of calcium
hydroxylapatite. It is the mineralization that gives 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, each with a 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. It facilitates bone's main functions—to support the whole body, to protect organs, to provide
levers for movement, and to store and release chemical elements, mainly calcium. It consists of multiple microscopic columns, each called an
osteon or Haversian system. Each column is multiple layers of
osteoblasts and
osteocytes 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. They allow blood vessels and nerves to travel through them to supply the osteocytes.
Structure
Each Haversia ...
.
Volkmann's canal
Volkmann's canals, also known as perforating holes or channels, are anatomic arrangements in cortical bones. Volkmann's canals are inside osteons. They interconnect the haversian canals with each other and the periosteum
The periosteum is a me ...
s 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 on its outer surface, and an
endosteum 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.
Trabecules
Cancellous bone, also called trabecular or spongy bone, is the internal tissue of the skeletal bone and is an open cell
porous network that follows the material properties of
biofoams. Cancellous bone has a higher
surface-area-to-volume ratio than cortical bone and it is less
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 and often contains red
bone marrow where
hematopoiesis, the production of blood cells, occurs. The primary anatomical and functional unit of cancellous bone is the
trabecula. The trabeculae are aligned towards the mechanical load distribution that a bone experiences within long bones such as the
femur. As far as short bones are concerned, trabecular alignment has been studied in the
vertebral pedicle. Thin formations of
osteoblasts covered in endosteum create an irregular network of spaces, known as trabeculae. Within these spaces are
bone marrow and
hematopoietic stem cells that give rise to
platelets,
red blood cells and
white blood cells. 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
Crisóstomo Martínez (1638–1694) was a Spanish painter, engraver, anatomist and microscopist from Valencia, known for his atlas of anatomy. His work has been ascribed to the Spanish intellectual movement called "''Novator''" which refers to th ...
.
Marrow
Bone marrow, also known as
myeloid tissue in red bone marrow, can be found in almost any bone that holds
cancellous tissue. In
newborns, all such bones are filled exclusively with red marrow or
hematopoietic marrow, but as the child ages the hematopoietic fraction decreases in quantity and the fatty/ yellow fraction called
marrow adipose tissue (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.
Vascular supply
Bone receives about 10% of cardiac output.
Blood enters the
endosteum, flows through the marrow, and exits through small vessels in the cortex.
In humans,
blood oxygen tension in bone marrow is about 6.6%, compared to about 12% in arterial blood, and 5% in venous and capillary blood.
Cells
Bone is metabolically active tissue composed of several types of cells. These cells include
osteoblasts, which are involved in the creation and
mineralization
Mineralization may refer to:
* Mineralization (biology), when an inorganic substance precipitates in an organic matrix
** Biomineralization, a form of mineralization
** Mineralization of bone, an example of mineralization
** Mineralized tissues ar ...
of bone tissue,
osteocytes, and
osteoclasts, which are involved in the reabsorption of bone tissue. Osteoblasts and osteocytes are derived from
osteoprogenitor cells, but
osteoclasts are derived from the same cells that differentiate to form
macrophages and
monocytes. Within the marrow of the bone there are also
hematopoietic stem cells. These cells give rise to other cells, including
white blood cells,
red blood cells, and
platelets.
Osteoblast
Osteoblasts 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 residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, res ...
mixture known as
osteoid, which mineralizes to become bone. The osteoid seam is a narrow region of a newly formed organic matrix, not yet mineralized, located on the surface of a bone. Osteoid is primarily composed of Type I
collagen. Osteoblasts also manufacture
hormones, such as
prostaglandins, 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 and
bicarbonate
In inorganic chemistry, bicarbonate (IUPAC-recommended nomenclature: hydrogencarbonate) is an intermediate form in the deprotonation of carbonic acid. It is a polyatomic anion with the chemical formula .
Bicarbonate serves a crucial biochemi ...
ions. The brand-new bone created by the osteoblast is called
osteoid. 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
Osteocytes are cells of mesenchymal origin and originate from osteoblasts that have migrated into and become trapped and surrounded by a 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
Osteoclasts are very large
multinucleate cells that are responsible for the breakdown of bones by the process of
bone resorption. 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 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 stem-cell
In multicellular organisms, stem cells are undifferentiated or partially differentiated cells that can differentiate into various types of cells and proliferate indefinitely to produce more of the same stem cell. They are the earliest type of ...
lineage, they are equipped with
phagocytic-like mechanisms similar to circulating
macrophages. Osteoclasts mature and/or migrate to discrete bone surfaces. Upon arrival, active enzymes, such as
tartrate-resistant acid phosphatase, 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 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 are most similar ...
homeostasis.
Composition
Bones consist of living cells (osteoblasts and osteocytes) embedded in a mineralized organic matrix. The primary inorganic component of human bone is
hydroxyapatite, the dominant
bone mineral, having the nominal composition of Ca
10(PO
4)
6(OH)
2. The organic components of this matrix consist mainly of
type I collagen—"organic" referring to materials produced as a result of the human body—and inorganic components, which alongside the dominant
hydroxyapatite phase, include other compounds of
calcium
Calcium is a chemical element with the 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 are most similar ...
and
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 fibers give bone its
tensile strength, and the interspersed crystals of
hydroxyapatite give bone its
compressive strength. 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 word συνεργία ' from ', , meaning "working together".
History
In Christia ...
. The exact composition of the matrix may be subject to change over time due to nutrition and
biomineralization, with the ratio of
calcium
Calcium is a chemical element with the 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 are most similar ...
to
phosphate varying between 1.3 and 2.0 (per weight), and trace minerals such as
magnesium,
sodium,
potassium and
carbonate also be found.
Type I collagen composes 90–95% of the organic matrix, with the remainder of the matrix being a homogenous liquid called
ground substance consisting of
proteoglycans such as
hyaluronic acid and
chondroitin sulfate, as well as non-collagenous proteins such as
osteocalcin,
osteopontin or
bone sialoprotein. 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 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 displ ...
or in
Paget's disease. 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 sheets with a much lower proportion of osteocytes to surrounding tissue. Lamellar bone, which makes its first appearance in humans in the
fetus during the third trimester,
[Salentijn, L. ''Biology of Mineralized Tissues: Cartilage and Bone'', Columbia University College of Dental Medicine 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
Cross section may refer to:
* Cross section (geometry)
** Cross-sectional views in architecture & engineering 3D
*Cross section (geology)
* Cross section (electronics)
* Radar cross section, measure of detectability
* Cross section (physics)
**Abs ...
, the fibers run in opposite directions in alternating layers, much like in
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
The micrometre ( international spelling as used by the International Bureau of Weights and Measures; SI symbol: μm) or micrometer ( American spelling), also commonly known as a micron, is a unit of length in the International System of Uni ...
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
osteoblasts, which secrete both collagen and ground substance. These synthesise collagen within the cell and then secrete collagen fibrils. The collagen fibers rapidly
polymerise to form collagen strands. At this stage, they are not yet mineralised, and are called "osteoid". Around the strands calcium and phosphate
precipitate 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 o ...
containing
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 bones are characterized by a shaft, the
diaphysis, that is much longer than its width; and by an
epiphysis, a rounded head at each end of the shaft. They are made up mostly of
compact bone, with lesser amounts of
marrow, located within the
medullary cavity, and areas of spongy, cancellous bone at the ends of the bones.
Most bones of the
limbs, including those of the
fingers and
toes
Toes are the digits (fingers) of the foot of a tetrapod. Animal species such as cats that walk on their toes are described as being ''digitigrade''. Humans, and other animals that walk on the soles of their feet, are described as being ''plan ...
, are long bones. The exceptions are the eight
carpal bones of the
wrist, the seven articulating
tarsal bones of the
ankle and the sesamoid bone of the
kneecap. Long bones such as the clavicle, that have a differently shaped shaft or ends are also called ''modified long bones''.
*
Short bones are roughly
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 bones 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 are flat bones, as is the
sternum.
*
Sesamoid bones 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 and the
pisiform
The pisiform bone ( or ), also spelled pisiforme (from the Latin ''pisifomis'', pea-shaped), is a small knobbly, sesamoid bone that is found in the wrist. It forms the ulnar border of the carpal tunnel.
Structure
The pisiform is a sesamoid bone, ...
.
*
Irregular bones 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
spine
Spine or spinal may refer to:
Science Biology
* Vertebral column, also known as the backbone
* Dendritic spine, a small membranous protrusion from a neuron's dendrite
* Thorns, spines, and prickles, needle-like structures in plants
* Spine (zoolo ...
,
pelvis, and some bones of the skull are irregular bones. Examples include the
ethmoid and
sphenoid bones.
Terminology
In the study of
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 a dialect spoken in the lower Tiber area (then known as Latium) around present-day Rome, but through ...
and
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, 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
fetal stage of development this occurs by two processes:
intramembranous ossification and
endochondral ossification. Intramembranous ossification involves the formation of bone from
connective tissue whereas endochondral ossification involves the formation of bone from
cartilage.
Intramembranous ossification mainly occurs during formation of the flat bones of the
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
Calcification is the accumulation of calcium salts in a body tissue. It normally occurs in the formation of bone, but calcium can be deposited abnormally in soft tissue,Miller, J. D. Cardiovascular calcification: Orbicular origins. ''Nature M ...
, 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
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. 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 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 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
brittle, bone does have a significant degree of
elasticity, contributed chiefly by
collagen.
Mechanically, bones also have a special role in
hearing. The
ossicles are three small bones in the
middle ear which are involved in sound transduction.
Synthetic
The cancellous part of bones contain
bone marrow. Bone marrow produces blood cells in a process called
hematopoiesis. Blood cells that are created in bone marrow include
red blood cells,
platelets and
white blood cells. Progenitor cells such as the
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
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 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 are most similar ...
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 (MAT) acts as a storage reserve of
fatty acids.
*
Acid-
base balance – bone buffers the blood against excessive
pH changes by absorbing or releasing
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 organ – bone controls
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, which contributes to the regulation of
blood sugar (
glucose) and
fat deposition. Osteocalcin increases both the
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. 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, repair
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
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
Calcitonin is a 32 amino acid peptide hormone secreted by parafollicular cells (also known as C cells) of the thyroid (or endostyle) in humans and other chordates. in the ultimopharyngeal body. It acts to reduce blood calcium (Ca2+), opposing the ...
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 and by 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
Vitamin D is a group of Lipophilicity, fat-soluble secosteroids responsible for increasing intestinal absorption of calcium, magnesium, and phosphate, and many other biological effects. In humans, the most important compounds in this group ar ...
, 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
The colony stimulating factor 1 (CSF1), also known as macrophage colony-stimulating factor (M-CSF), is a secreted cytokine which causes hematopoietic stem cells to differentiate into macrophages or other related cell types. Eukaryotic cells als ...
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 rheumatologists for joints, and orthopedic 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 to diagnose diseases and guide their treatment, within the bodies of humans and other animals. It began with radiography (which is why its name has a root referring to radiati ...
in interpreting the findings on imaging, and pathologist
Pathology is the study of the causes and effects of disease or injury. The word ''pathology'' also refers to the study of disease in general, incorporating a wide range of biology research fields and medical practices. However, when used in th ...
s 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
A bone scan or bone scintigraphy is a nuclear medicine imaging technique of the bone. It can help diagnose a number of bone conditions, including cancer of the bone or metastasis, location of bone inflammation and fractures (that may not be ...
, 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 displ ...
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 disease) 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
Flail chest is a life-threatening medical condition that occurs when a segment of the rib cage breaks due to trauma and becomes detached from the rest of the chest wall. Two of the symptoms of flail chest are chest pain and shortness of breath. ...
, 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
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 wit ...
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
Internal fixation is an operation in orthopedics that involves the surgical implementation of implants for the purpose of repairing a bone, a concept that dates to the mid-nineteenth century and was made applicable for routine treatment in the ...
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 retur ...
.
Tumors
There are several types of tumor that can affect bone; examples of benign bone tumor
A bone tumor is an abnormal growth of tissue in bone, traditionally classified as noncancerous (benign) or cancerous (malignant). Cancerous bone tumors usually originate from a cancer in another part of the body such as from lung, breast, thyro ...
s include 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 exostoses. It is characterized as a type of overgrowth that can occur in any bone w ...
, osteoblastoma, enchondroma, giant-cell tumor of bone, and aneurysmal bone cyst
Aneurysmal bone cyst (ABC) is a non-cancerous bone tumor composed of multiple varying sizes of spaces in a bone which are filled with blood. The term is a misnomer, as the lesion is neither an aneurysm nor a cyst. It generally presents with pai ...
.
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" cancer) or create bone (a " 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
A bruise, also known as a contusion, is a type of hematoma of tissue, the most common cause being capillaries damaged by trauma, causing localized bleeding that extravasates into the surrounding interstitial tissues. Most bruises occur close ...
, 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 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 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
Bone density, or bone mineral density, is the amount of bone mineral in bone tissue. The concept is of mass of mineral per volume of bone (relating to density in the physics sense), although clinically it is measured by proxy according to optical ...
, increasing the likelihood of 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 displ ...
. 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
A pathologic fracture is a bone fracture caused by weakness of the bone structure that leads to decrease mechanical resistance to normal mechanical loads. This process is most commonly due to osteoporosis, but may also be due to other pathologies ...
. 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 diseases or as a result of 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.
One of the most important risk factors for osteoporosis is advanced age
Old age refers to ages nearing or surpassing the life expectancy of human beings, and is thus the end of the human life cycle. Terms and euphemisms for people at this age include old people, the elderly (worldwide usage), OAPs (British usage ...
. Accumulation of oxidative DNA damage in osteoblastic and osteoclastic cells appears to be a key factor in age-related osteoporosis.
Osteoporosis treatment includes advice to stop smoking, decrease alcohol consumption, exercise regularly, and have a healthy diet. Calcium
Calcium is a chemical element with the 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 are most similar ...
and trace mineral
In the context of nutrition, a mineral is a chemical element required as an essential nutrient by organisms to perform functions necessary for life. However, the four major structural elements in the human body by weight (oxygen, hydrogen, carbon, ...
supplements may also be advised, as may Vitamin D
Vitamin D is a group of Lipophilicity, fat-soluble secosteroids responsible for increasing intestinal absorption of calcium, magnesium, and phosphate, and many other biological effects. In humans, the most important compounds in this group ar ...
. 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 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 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.
Typically anthropologists and archeologists study bone tool
In archaeology, a bone tool is a tool created from bone. A bone tool can conceivably be created from almost any bone, and in a variety of methods.
Bone tools have been documented from the advent of ''Homo sapiens'' and are also known from ''Hom ...
s 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 them being hollow.
A bird's beak is primarily made of bone as projections of the mandible
In anatomy, the mandible, lower jaw or jawbone is the largest, strongest and lowest bone in the human facial skeleton. It forms the lower jaw and holds the lower teeth in place. The mandible sits beneath the maxilla. It is the only movable bone ...
s which are covered in keratin
Keratin () is one of a family of structural fibrous proteins also known as ''scleroproteins''. Alpha-keratin (α-keratin) is a type of keratin found in vertebrates. It is the key structural material making up Scale (anatomy), scales, hair, Nail ...
.
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
Sea turtles (superfamily Chelonioidea), sometimes called marine turtles, are reptiles of the order Testudines and of the suborder Cryptodira. The seven existing species of sea turtles are the flatback, green, hawksbill, leatherback, loggerhead, ...
, or in various Mesozoic marine reptiles, such as ichthyosaurs, among others. This proportion can vary quickly in evolution; it often increases in early stages of returns to an aquatic lifestyle, as seen in early whales and pinnipeds, among others. It subsequently decreases in pelagic taxa, which typically acquire spongy bone, but aquatic taxa that live in shallow water can retain very thick, pachyostotic, osteosclerotic
Osteosclerosis is a disorder that is characterized by abnormal hardening of bone and an elevation in bone density. It may predominantly affect the medullary portion and/or cortex of bone. Plain radiographs are a valuable tool for detecting and ...
, or pachyosteosclerotic bones, especially if they move slowly, like sea cows. In some cases, even marine taxa that had acquired spongy bone can revert to thicker, compact bones if they become adapted to live in shallow water, or in hypersaline (denser) water.
Many animals, particularly herbivores, practice osteophagy—the eating of bones. This is presumably carried out in order to replenish lacking 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
Prehistory, also known as pre-literary history, is the period of human history between the use of the first stone tools by hominins 3.3 million years ago and the beginning of recorded history with the invention of writing systems. The use ...
, they have been used for making bone tool
In archaeology, a bone tool is a tool created from bone. A bone tool can conceivably be created from almost any bone, and in a variety of methods.
Bone tools have been documented from the advent of ''Homo sapiens'' and are also known from ''Hom ...
s. 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, 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 point the bone at someone is considered bad luck in some cultures, such as Australian aborigines, such as by the Kurdaitcha
A kurdaitcha, or kurdaitcha man, also spelt gadaidja, cadiche, kadaitcha, karadji, or kaditcha, is a type of shaman amongst the Arrernte people, an Aboriginal group in Central Australia. The name featherfoot is used to denote the same figure by ...
.
The wishbone
Wishbone commonly refers to:
* Furcula, a fork-shaped bone in birds and some dinosaurs
Wishbone may also refer to:
* Wish-Bone, an American salad dressing and condiment company
* Wishbone formation, a type of offense in American football
* Wishb ...
s 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
Artificial bone refers to bone-like material created in a laboratory that can be used in bone grafts, to replace human bone that was lost due to severe fractures, disease, etc.
Bone fracture, which is a complete or partial break in the bone, is a ...
* Bone health
* Distraction osteogenesis
* National Bone Health Campaign
The national bone health campaign teaches young girls habits for improving their bone health by encouraging them to eat more foods with calcium and vitamin D, and participating in physical activities that help the bones.
History
The National Bone ...
* Skeleton
References
Further reading
*
*
*
* – ''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
* ttp://www.scq.ubc.ca/?p=400 A good basic overview of bone biology from the Science Creative Quarterly*
Bone histology photomicrographs
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Bones
Skeletal system
Connective tissue