Bone
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{{two other uses|the skeletal organs|the tissue|Osseous tissue|other uses|Bone (disambiguation)}}
[[Image:Gray252.png|thumb|[[Gray's Anatomy]] illustration of a human [[femur]].]]
'''Bones''' are rigid [[organ (anatomy)|organs]] that form part of the [[endoskeleton]] of [[vertebrate]]s. They function to move, support, and protect the various organs of the body, produce [[red blood cell|red]] and [[white blood cell]]s and store minerals. Because bones come in a variety of shapes and have a complex internal and external structure, they are lightweight, yet strong and hard, in addition to fulfilling their many other functions. One of the types of tissues that makes up bone is the mineralized [[osseous tissue]], also called bone tissue, that gives it rigidity and honeycomb-like three-dimensional internal structure. Other types of tissue found in bones include [[bone marrow|marrow]], [[endosteum]] and [[periosteum]], [[nerve]]s, [[blood vessel]]s and [[cartilage]]. There are 206 bones in the adult human body.
== Functions==
Bones have eight main functions:
* Protection — Bones can serve to protect internal organs, such as the [[skull]] protecting the [[brain]] or the [[rib]]s protecting the [[heart]] and [[lungs]].
* Shape — Bones provide a frame to keep the body supported.
* Blood production — The [[Bone marrow|marrow]], located within the [[medullary cavity]] of long bones and interstices of cancellous bone, produces blood cells in a process called [[haematopoiesis]].
* Mineral storage — Bones act as reserves of minerals important for the body, most notably [[calcium]] and [[phosphorus]].
* Movement — Bones, [[skeletal muscle]]s, [[tendons]], [[ligament]]s and [[joint]]s 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]].
* 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.{{fact|date=May 2007}}
* Sound transduction — Bones are important in the mechanical aspect of [[Hearing (sense)|hearing]].
==Characteristics==
The primary tissue of bone, [[osseous tissue]], is a relatively [[Rockwell scale|hard]] and lightweight [[composite material]], formed mostly of [[calcium phosphate]] in the chemical arrangement termed calcium [[hydroxylapatite]] (this is the [[osseous tissue]] that gives bones their rigidity). It has relatively high [[compressive strength]] but poor [[tensile strength]], meaning it resists pushing forces well, but not pulling forces. While bone is essentially brittle, it does have a significant degree of [[elasticity (physics)|elasticity]], contributed chiefly by [[collagen]]. All bones consist of living [[Cell (biology)|cells]] embedded in the mineralized organic ''matrix'' that makes up the osseous tissue.
==Masto-structure==
===Gross anatomy===
{{Seealso|Human skeleton|List of bones of the human skeleton}}
{{Bonesofskeleton}}
{{Facial bones}}
{{Cranium}}
{{Sutures}}
{{Bones of upper extremity}}
{{Spine}}
{{Bones of lower extremity}}
{{Pelvis}}
===Individual bones===
Bone is not a uniformly solid material, but rather has some spaces between its hard elements.
[[Image:Illu compact spongy bone.jpg|350px|right]]
[[Image:Caput femoris cortex medulla.jpg|right|thumb|Section through the head of the femur, showing the outer layer of compact bone and the soft center of trabecular bone, filled with red bone marrow and a spot of yellow bone marrow (white bar = 1 centimeter)]]
====Compact bone====
The hard outer layer of bones is composed of [[Cortical bone|compact bone]] tissue, so-called due to its minimal gaps and spaces. This tissue gives bones their smooth, white, and solid appearance, and accounts for 80% of the total bone mass of an adult skeleton. Compact bone may also be referred to as dense bone or cortical bone.
====Trabecular bone====
Filling the interior of the organ is the [[Cancellous bone|trabecular bone]] tissue (an open cell [[Porosity|porous]] network also called cancellous or spongy bone) which is composed of a network of rod- and plate-like elements that make the overall organ lighter and allowing 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.
==Cellular structure==
There are several types of cells constituting the bone;
* [[Osteoblast]]s are mononucleate bone-forming cells which descend from [[Osteoblast#Osteoblastogenesis|osteoprogenitor]] cells. They are located on the surface of osteoid seams and make a protein mixture known as [[osteoid]], which mineralizes to become bone. Osteoid is primarily composed of Type I [[collagen]]. Osteoblasts also manufacture [[hormone]]s, such as [[prostaglandin]]s, to act on the bone itself. They robustly produce [[alkaline phosphatase]], an enzyme that has a role in the mineralisation of bone, as well as many [[extracellular matrix|matrix proteins]]. Osteoblasts are the immature bone cells.
* Bone lining cells are essentially inactive osteoblasts. They cover all of the available bone surface and function as a barrier for certain ions.
* [[Osteocyte]]s originate from osteoblasts which have migrated into and become trapped and surrounded by bone matrix which they themselves produce. The spaces which they occupy are known as lacunae. Osteocytes have many processes which reach out to meet osteoblasts and other osteocytes probably for the purposes of communication. Their functions include to varying degrees: formation of bone, matrix maintenance and calcium homeostasis. They have also been shown to act as mechano-sensory receptors—regulating the bone's response to stress and mechanical load. They are mature bone cells.
* [[Osteoclast]]s are the cells responsible for [[bone resorption]] (remodeling of bone to reduce its volume). Osteoclasts are large, multinucleated cells located on bone surfaces in what are called [[Howship's lacunae]] or resorption pits. These lacunae, or resorption pits, are left behind after the breakdown of bone and often present as ''scalloped'' surfaces. Because the osteoclasts are derived from a [[monocyte]] [[stem cell|stem-cell]] lineage, they are equipped with engulfment strategies 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.
==Molecular structure==
=== Matrix ===
The matrix is the major constituent of bone, surrounding the cells. It has inorganic and organic parts.
====Inorganic====
The inorganic is mainly crystalline mineral salts and calcium, which is present in the form of [[hydroxylapatite|hydroxyapatite]]. The matrix is initially laid down as unmineralized osteoid (manufactured by osteoblasts). Mineralisation involves osteoblasts secreting [[vesicle (biology)|vesicles]] 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.
====Organic====
The organic part of matrix is mainly composed of Type I [[collagen]]. This is synthesised intracellularly as tropocollagen and then exported. It then associates into [[fibril]]s. Also making up the organic part of matrix include various growth factors, the functions of which are not fully known. Other factors present include [[glycosaminoglycan]]s, [[osteocalcin]], [[osteonectin]], [[bone sialo protein]] and Cell Attachment Factor. One of the main things that distinguishes the matrix of a bone from that of another cell is that the matrix in bone is hard.
===Woven or lamellar===
[[Image:Woven bone matrix.jpg|thumb|200px|right|Collagen fibres of woven bone]]
Bone is first deposited as woven bone, in a disorganized structure with a high proportion of [[osteocytes]] in young and in healing injuries. Woven bone is weaker, with a small number of randomly oriented collagen fibers, but forms quickly. It is replaced by lamellar bone, which is highly organized in concentric sheets with a low proportion of osteocytes. Lamellar bone is stronger and filled with many collagen fibers parallel to other fibers in the same layer (these parallel columns are called osteons). The fibers run in opposite directions in alternating layers, much like plywood, assisting in the bone's ability to resist [[torsion]] forces. After a break, woven bone quickly forms and is gradually replaced by slow-growing [[lamellar bone]] on pre-existing calcified [[Cartilage#Hyaline cartilage|hyaline cartilage]] through a process known as [["bony substitution."]]
== Five types of bones ==
[[Image:Illu long bone.jpg|250px|right]]
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 greater in length than width. They are comprised mostly of [[Cortical bone|compact bone]] and lesser amounts of [[Bone marrow|marrow]], which is located within the [[medullary cavity]], and [[Cancellous bone|spongy bone]]. Most bones of the limbs, including those of the [[metacarpus|fingers]] and [[metatarsus|toes]], are long bones. The exceptions are those of the [[carpal|wrist]], [[tarsus (skeleton)|ankle]] and [[patella|kneecap]].
* [[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, as are the [[sesamoid bones]].
* [[Flat bone]]s are thin and generally curved, with two parallel layers of compact bones sandwiching a layer of spongy bone. Most of the bones of the [[skull]] are flat bones, as is the [[sternum]].
* [[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. The bones of the spine and hips are irregular bones.
* [[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 force of the muscle is increased. Examples of sesamoid bones are the patella and the [[pisiform]]
== Formation ==
The formation of bone during the fetal stage of development occurs by two processes: [[Intramembranous ossification|intramembranous]] and [[endochondral ossification]].
Intramembranous ossification mainly occurs during formation of the flat bones of the [[skull]]; the bone is formed from [[mesenchyme]] tissue. The steps in intramembranous ossification are:
# Development of ossification center
# Calcification
# Formation of trabeculae
# Development of periosteum
===Endochondral ossification===
[[Image:Bone growth.png|450px|right|thumb|Endochondrial ossification]]
Endochondral ossification, on the other hand, occurs in long bones, such as limbs; the bone is formed from cartilage. The steps in endochondral ossification are:
# Development of cartilage model
# Growth of cartilage model
# Development of the primary ossification center
#
# Development of the secondary ossification center
# Formation of articular cartilage and [[epiphyseal plate]]
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]]). When the child reaches 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).
===Bone marrow===
There are two types of bone marrow, yellow and red, most commonly seen is 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 , but as the child ages it is mostly replaced by yellow, or ''fatty'' marrow. In adults, red marrow is mostly found in the flat bones of the skull, the ribs, the vertebrae and pelvic bones.
==Remodeling==
''Remodeling'' or [[bone turnover]] is the process of resorption followed by replacement of bone with little change in shape and occurs throughout a person's life. Osteoblasts and osteoclasts, coupled together via [[paracrine|paracrine cell signalling]], are referred to as bone remodeling units.
===Purpose===
The purpose of remodeling is to regulate [[calcium homeostasis]], repair micro-damaged bones (from everyday stress) but also to shape and sculpture the skeleton during growth.
====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.
====Repair====
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.<ref> Netter, p. 187-189, A scholarly review.</ref>
==Paracrine cell signalling==
The action of [[osteoblast]]s and [[osteoclast]]s are contolled by a number of chemical [[Enzyme|factors]] which either promote or inhibit the activity of the bone remodelling 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.
===Osteoblast stimulation===
Osteoblasts can be stimulated to increase bone mass through increased secretion of [[osteoid]] and by [[Enzyme inhibitor|inhibiting]] the ability of osteoclasts to break down [[osseous tissue]].
Bone building through increased secretion of osteoid is stimulated by the secretion of [[growth hormone]] by the [[pituitary]], [[thyroid hormone]] and the sex hormones ([[estrogen]]s and [[androgen]]s). These hormones also promote increased secretion of [[osteoprotegerin]].<ref name = Boron>{{cite book |author=Boulpaep, Emile L.; Boron, Walter F. |title=Medical physiology: a cellular and molecular approach |publisher=Saunders |location=Philadelphia |year=2005 |pages=p.1089-1091 |isbn=1416023283 |oclc= |doi=}}</ref> Osteoblasts can also be induced to secrete a number of [[cytokine]]s that promote reabsorbtion 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 reabsorbtion 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]].
===Osteoclast inhibition===
The rate at which osteoclasts resorb bone is inhibited by [[calcitonin]] and osteoprotegerin. Calcitonin is produced by [[parafollicular cell]]s 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.<ref name = Boron/>
== Disorders ==
{{Seealso|List of skeletal disorders}}
There are many disorders of the skeleton. One of the more prominent is [[osteoporosis]].
===Osteoporosis===
{{Main|Osteoporosis}}
Osteoporosis is a [[disease]] of bone - leading to an increased risk of [[bone fracture|fracture]]. In osteoporosis, the [[bone mineral density]] (BMD) is reduced, bone microarchitecture is disrupted, and the amount and variety of non-[[collagen]]ous proteins in bone is altered. Osteoporosis is defined by the [[World Health Organization]] (WHO) in women as a bone mineral density 2.5 [[standard deviation]]s below peak bone mass (20-year-old sex-matched healthy person average) as measured by [[Dual energy X-ray absorptiometry|DXA]]; the term "established osteoporosis" includes the presence of a [[fragility fracture]].<ref name=WHO1994>{{cite journal |author=WHO |title=Assessment of fracture risk and its application to screening for postmenopausal osteoporosis. Report of a WHO Study Group |journal=World Health Organization technical report series |volume=843 |issue= |pages=1–129 |year=1994 |pmid=7941614 |doi=}}</ref> Osteoporosis is most common in women after the [[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 [[glucocorticoid]]s, when the disease is called '''steroid-''' or '''glucocorticoid-induced osteoporosis''' (SIOP or GIOP).
Osteoporosis can be prevented with lifestyle advice and medication, and preventing falls in people with known or suspected osteoporosis is an established way to prevent fractures. Osteoporosis can be treated with [[bisphosphonate]]s and various other medical treatments.
===Other===
Other disorders of bone include:
*[[Bone fracture]]
*[[Osteomyelitis]]
*[[Osteosarcoma]]
*[[Osteogenesis imperfecta]]
==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 [[maceration (bone)|maceration]] - boiling fleshed bones to remove large particles, then hand-cleaning.
Typically anthropologists and archeologists study [[bone tool]]s made by [[Human|''Homo sapiens'']] and [[Neanderthal|''Homo neanderthalensis'']]. Bones can serve a number of uses such as projectile points or artistic pigments, and can be made from endoskeletal or external bones such as antler or tusk.
==Alternatives to bony endoskeletons==
There are several [[evolution]]ary alternatives to mammilary bone; though they have some similar functions, they are not completely functionally analogous to bone.
* [[Exoskeleton]]s offer support, protection and levers for movement similar to endoskeletal bone. Different types of exoskeletons include [[animal shell|shell]]s, [[carapace]]s (consisting of [[Calcium in biology|calcium compounds]] or [[silica]]) and [[chitin|chitinous exoskeletons]].
* A true [[endoskeleton]] (that is, protective tissue derived from mesoderm) is also present in [[Echinoderm]]s. [[Porifera]] (sponges) possess simple endoskeletons that consist of calcareous or siliceous [[spicule]]s and a spongin fiber network.
==Exposed bone==
Bone penetrating the skin and being exposed to the outside can be both a natural process in some animals, and due to injury:
*A deer's [[antler]]s are composed of bone
*Instead of teeth, The extinct predatory fish [[Dunkleosteus]] had sharp edges of hard exposed bone along its jaws
*A [[Bone fracture#Closed vs. open|compound fracture]] occurs when the edges of a broken bone puncture the skin
*Though not strictly speaking exposed, a bird's [[beak]] is primarily bone covered in a layer of [[keratin]]
== Terminology ==<!-- This section is linked from [[Rib]] -->
Several terms are used to refer to features and components of bones throughout the body:
{| border="1"
!Bone feature||Definition
|-
| ''articular process''|| A projection that contacts an adjacent bone.
|-
| ''articulation''|| The region where adjacent bones contact each other—a [[joint]].
|-
| ''canal''|| A long, tunnel-like foramen, usually a passage for notable nerves or blood vessels.
|-
| ''condyle''|| A large, rounded articular process.
|-
| ''crest''|| A prominent ridge.
|-
| ''eminence''|| A relatively small projection or bump.
|-
| ''epicondyle''|| A projection near to a condyle but not part of the joint.
|-
| ''facet''|| A small, flattened articular surface.
|-
| ''foramen''|| An opening through a bone.
|-
| ''fossa''|| A broad, shallow depressed area.
|-
| ''fovea''|| A small pit on the head of a bone.
|-
| ''labyrinth''|| A cavity within a bone.
|-
| ''line''|| A long, thin projection, often with a rough surface. Also known as a ''ridge''.
|-
| ''malleolus''|| One of two specific protuberances of bones in the [[ankle]].
|-
| ''meatus''|| A short canal.
|-
| ''process''|| A relatively large projection or prominent bump.(gen.)
|-
| ''ramus''|| An arm-like branch off the body of a bone.
|-
| ''sinus''|| A cavity within a cranial bone.
|-
| ''spine''|| A relatively long, thin projection or bump.
|-
| ''suture''|| Articulation between cranial bones.
|-
| ''trochanter''|| One of two specific tuberosities located on the [[femur]].
|-
| ''tubercle''|| A projection or bump with a roughened surface, generally smaller than a tuberosity.
|-
| ''tuberosity''|| A projection or bump with a roughened surface.
|}
Several terms are used to refer to specific features of long bones:
{|border="1"
! Bone feature||Definition
|-
| ''diaphysis''
| The long, relatively straight main body of a long bone; region of primary ossification. Also known as the ''shaft''.
|-
| ''epiphysis''
| The end regions of a long bone; regions of secondary ossification.
|-
| ''epiphyseal plate''
| Also known as the ''growth plate'' or ''physis''. In a long bone it is a thin disc of hyaline cartilage that is positioned transversely between the epiphysis and metaphysis. In the long bones of humans, the epiphyseal plate disappears by twenty years of age.
|-
| ''head''
| The proximal articular end of the bone.
|-
| ''metaphysis''
| The region of a long bone lying between the epiphysis and diaphysis.
|-
| ''neck''
| The region of bone between the head and the shaft.
|}
== See also ==
* [[List of bones of the human skeleton]]
* [[Terms for anatomical location]]
* [[Orthopedic surgery|Orthopaedics]]
==Footnotes==
{{reflist}}
== References ==
* {{cite book |author=Katja Hoehn; Marieb, Elaine Nicpon |title=Human Anatomy & Physiology (7th Edition) |publisher=Benjamin Cummings |location=San Francisco |year= |pages= |isbn=0805359095 |oclc= |doi=}}
* [[Frank H. Netter|Netter, Frank H.]] (1987), Musculoskeletal system: anatomy, physiology, and metabolic disorders. Summit, New Jersey: Ciba-Geigy Corporation ISBN 0914168886
* {{cite book |author=Bryan H. Derrickson; Tortora, Gerard J. |title=Principles of anatomy and physiology |publisher=Wiley |location=New York |year=2005 |pages= |isbn=0471689343 |oclc= |doi=}}
== External links ==
* [http://silver.neep.wisc.edu/~lakes/BoneElectr.html Review (including references) of piezoelectricity''' and bone remodelling]
* [http://www.scq.ubc.ca/?p=400 A good basic overview of bone biology from the Science Creative '''Quarterly]
* [http://gotbones.healthdiaries.com/ Bone Health] at Got Bones?
* [http://www.bonesmd.com/doctor.html Osteopathic physicians]
'''
{{Bone and cartilage}}
{{Bonesofskeleton}}
{{Facial bones}}'''
{{Cranium}}
[[Category:Bone products]]
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