View source for Bone

{{Short description|Rigid organs of the skeleton of vertebrates}}
{{hatnote group|
{{other uses}}
{{redirect|Bones}}
}}
{{pp-move}}
{{cs1 config|name-list-style=vanc|display-authors=6}}
{{Use dmy dates|date=March 2020}}
{{Infobox anatomy
|Name = Bone
|Image = Left femur of extinct elephant, Alaska, Ice Age Wellcome L0057714.jpg
|Caption = A bone dating from the [[Last Glacial Period|Pleistocene Ice Age]] of an extinct species of an [[elephant]], possibly a [[mammoth]]
|Image2 = Bertazzo S - SEM deproteined bone - wistar rat - x10k.tif
|Caption2 = A [[scanning electron microscope|scanning electronic micrograph]] of a [[Laboratory rat#Wistar rat|Wistar rat]]'s bone at 10,000× magnification
}}

A '''bone''' is a [[Stiffness|rigid]] [[Organ (biology)|organ]] that constitutes part of the [[skeleton]] in most [[vertebrate]] animals.<ref name="BoneOrgan">{{Cite book |url=https://www.sciencedirect.com/science/article/pii/B9780124708624500027 |title=The Bone Organ System: Form and Function |vauthors=Lee C |date=January 2001 |publisher=Academic Press |isbn=978-0-12-470862-4 |pages=3–20 |doi=10.1016/B978-012470862-4/50002-7 |access-date=30 January 2022 |via=Science Direct}}</ref> Bones protect the organs of the body, produce [[red blood cell|red]] and [[white blood cell]]s, store [[Mineral (nutrient)|minerals]], help regulate acid-base homeostasis, provide structure and support for the body, and enable [[animal locomotion|mobility]] and hearing. Bones come in a variety of shapes and sizes and have complex internal and external structures.<ref name="de_Buffrénil_2021">{{cite book | vauthors = de Buffrénil V, de Ricqlès AJ, Zylberberg L, Padian K, Laurin M, Quilhac A |title=Vertebrate skeletal histology and paleohistology |date=2021 |publisher=CRC Press |location=Boca Raton, FL |isbn=978-1-351-18957-6 |pages=xii + 825 |edition=First |url=https://books.google.com/books?id=tJcwEAAAQBAJ&dq=Vertebrate+Skeletal+Histology+and+Paleohistology&pg=PT8}}</ref>

'''Bone tissue''' (also known as osseous tissue or '''bone''' in the [[mass noun|uncountable]]) is a form of [[hard tissue]], specialised [[connective tissue]] that is [[mineralized tissues|mineralized]] and has an intercellular [[honeycomb]]-like [[matrix (biology)|matrix]],<ref>{{Cite book |title=Forensic Anthropology: A Comprehensive Approach |publisher=CRC Press |year=2017 |isbn=978-1-315-30003-0 |editor=Langley, Natalie |edition=2nd |page=82 |editor2=Tersigni-Terrant, Maria-Teresa}}</ref> which helps to give the bone rigidity. Bone tissue is made up of different types of bone cells: [[osteoblast]]s and [[osteocyte]]s (bone formation and [[mineralization (biology)|mineralisation]]); [[osteoclast]]s (bone [[bone resorption|resorption]]); modified or flattened osteoblasts (lining cells that form a protective layer on the bone surface). The mineralised matrix of bone tissue has an [[Organic chemistry|organic]] component of mainly [[ossein]], a form of [[collagen]], and an inorganic component of [[bone mineral]], made up of various salts. Bone tissue comprises '''cortical bone''' and '''cancellous bone''', although bones may also contain other kinds of [[Tissue (biology)|tissue]] including [[bone marrow]], [[endosteum]], [[periosteum]], [[nerve]]s, [[blood vessel]]s, and [[cartilage]].

In the [[human body]] at birth, approximately 300 bones are present. Many of these fuse together during development, leaving a total of 206 separate bones in the adult, not counting numerous small [[sesamoid bone]]s.<ref>{{cite book |title = The Anatomy and Biology of the Human Skeleton |url = https://archive.org/details/anatomybiologyo00stee |url-access = registration | vauthors = Steele DG, Bramblett CA |year = 1988 |publisher = Texas A&M University Press |page = [https://archive.org/details/anatomybiologyo00stee/page/4 4]|isbn = 978-0-89096-300-5}}</ref><ref>{{cite book |title=Mammal anatomy: an illustrated guide |date=2010 |publisher=Marshall Cavendish |location=New York |isbn=978-0-7614-7882-9 |page=129}}</ref> The largest bone in the body is the [[femur]] or thigh-bone, and the smallest is the ''[[stapes]]'' in the [[middle ear]].

The [[Ancient Greek]] word for bone is ὀστέον ("''osteon''"). In [[anatomical terminology]], including in the ''[[Terminologia Anatomica]]'', the word for a bone is ''[[wikt:os#Noun|os]]'' (for example, ''[[short bone|os breve]]'', ''[[long bone|os longum]]'', ''[[sesamoid bone|os sesamoideum]]'').

== Gross anatomy ==
Five types of bones are found in the human body: long, short, flat, irregular, and sesamoid.<ref>{{cite web |title=Types of bone |url=http://www.mananatomy.com/basic-anatomy/types-bone |access-date=6 February 2016 |publisher=mananatomy.com}}</ref>
[[File:Blausen 0229 ClassificationofBones.png|300px|right|One way to classify bones is by their shape or appearance.]]

* [[Long bone]]s 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 [[Cortical bone|compact bone]], with lesser amounts of [[Bone marrow|marrow]], located within the [[medullary cavity]], and areas of spongy, cancellous bone at the ends of the bones.<ref name="TLP">{{cite web |title=DoITPoMS – TLP Library Structure of bone and implant materials – Structure and composition of bone |url=https://www.doitpoms.ac.uk/tlplib/bones/structure.php |work=Dissemination of IT for the Promotion of Materials Science (DoITPoMS) |publisher=University of Cambridge |location=Cambridge, UK}}</ref> 
** Most bones of the [[Limb (anatomy)|limbs]], including those of the [[metacarpus|fingers]] and [[metatarsus|toes]], are long bones. The exceptions are the eight [[carpal bones]] of the [[wrist]], the seven articulating [[tarsal bone]]s of the [[tarsus (skeleton)|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 bone]]s are roughly [[cube]]-shaped, and have only a thin layer of compact bone surrounding a spongy interior. Short bones provide stability and support as well as some limited motion.<ref name="Openstax Anatomy & Physiology attribution">{{CC-notice|cc=by4|url=https://openstax.org/books/anatomy-and-physiology/pages/6-2-bone-classification}} {{cite book |title=Anatomy & Physiology |vauthors=Betts JG, Desaix P, Johnson E, Johnson JE, Korol O, Kruse D, Poe B, Wise J, Womble MD, Young KA |date=June 8, 2023 |publisher=OpenStax CNX |isbn=978-1-947172-04-3 |location=Houston |at=6.2 Bone classification}}</ref> 
** The bones of the wrist and ankle are short bones.
* [[Flat bone]]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]] are flat bones, as is the [[sternum]].<ref>{{Citation |title=Normal Bone Anatomy and Physiology |vauthors=Clarke B |journal=Clinical Journal of the American Society of Nephrology |volume=3 |issue=Suppl 3 |pages=S131–S139 |year=2008 |doi=10.2215/CJN.04151206 |pmc=3152283 |pmid=18988698}}</ref>
* [[Sesamoid bone]]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]] and the [[pisiform]].<ref>{{Citation |title=Occurrence and distribution of sesamoid bones in squamates: a comparative approach |vauthors=Jerez A, Mangione S, Abdala V |journal=Acta Zoologica |volume=91 |issue=3 |pages=295–305 |year=2010 |doi=10.1111/j.1463-6395.2009.00408.x |hdl=11336/74304 |hdl-access=free}}</ref>
* [[Irregular bone]]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 [[Vertebral column|spine]], [[pelvis]], and some bones of the skull are irregular bones. Examples include the [[ethmoid]] and [[sphenoid bone|sphenoid]] bones.<ref>{{cite web |title=Bone as an Organ |url=https://www.zoylo.com/diagnostics/organs/bones |archive-url=https://web.archive.org/web/20191030122832/https://www.zoylo.com/diagnostics/organs/bones |archive-date=2019-10-30 |access-date=2012-09-28 |work=AnatomyOne |publisher=Amirsys, Inc. |vauthors=Pratt R}}</ref>

=== Terminology ===
{{Main|Anatomical terms of bone}}
[[File:603 Anatomy of Long Bone.jpg|thumb|left|220px|Structure of a long bone]]
Anatomists use a number of [[Anatomical terminology|anatomical terms]] to describe the appearance, shape and function 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 bone]]s 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".

=== Functions ===

==== Mechanical ====
{{See also|Skeleton|Human skeleton|List of bones of the human skeleton}}
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 muscle]]s, [[tendon]]s, [[ligament]]s and [[joint]]s, which function together to generate and transfer forces so that individual body parts or the whole body can be manipulated in three-dimensional space (the interaction between bone and muscle is studied in [[biomechanics]]).

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 {{Cvt|170|MPa|kgf/cm2|lk=on}},<ref name="Schmidt-Nielsen" /> poor [[tensile strength]] of 104–121 MPa, and a very low [[shear stress]] strength (51.6 MPa).<ref>{{cite book |title=BENG 112A Biomechanics, Winter Quarter, 2013 |vauthors=Vincent K |publisher=Department of Bioengineering, University of California |chapter=Topic 3: Structure and Mechanical Properties of Bone |access-date=24 March 2015 |chapter-url=http://cmrg.ucsd.edu/Courses/be112a/Topics |archive-url=https://web.archive.org/web/20180528103922/http://cmrg.ucsd.edu/Courses/be112a/Topics |archive-date=28 May 2018}}</ref><ref>{{cite journal |vauthors=Turner CH, Wang T, Burr DB |date=December 2001 |title=Shear strength and fatigue properties of human cortical bone determined from pure shear tests |journal=Calcified Tissue International |volume=69 |issue=6 |pages=373–378 |doi=10.1007/s00223-001-1006-1 |pmid=11800235 |s2cid=30348345}}</ref> 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]].

Mechanically, bones also have a special role in [[Hearing (sense)|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]].<ref>{{cite journal |vauthors=Fernández KS, de Alarcón PA |date=December 2013 |title=Development of the hematopoietic system and disorders of hematopoiesis that present during infancy and early childhood |journal=Pediatric Clinics of North America |volume=60 |issue=6 |pages=1273–1289 |doi=10.1016/j.pcl.2013.08.002 |pmid=24237971}}</ref> Blood cells that are created in bone marrow include [[red blood cell]]s, [[platelet]]s and [[white blood cell]]s.{{sfn|Young|2006|pp=60–61}} 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 [[erythroblast]]s which give rise to red blood cells.{{sfn|Young|2006|p=60}} Unlike red and white blood cells, created by mitosis, platelets are shed from very large cells called [[megakaryocyte]]s.{{sfn|Young|2006|p=57}} This process of progressive differentiation occurs within the bone marrow. After the cells are matured, they enter the [[circulatory system|circulation]].{{sfn|Young|2006|p=46}} Every day, over 2.5 billion red blood cells and platelets, and 50–100 billion [[granulocyte]]s are produced in this way.{{sfn|Young|2006|p=58}}

As well as creating cells, bone marrow is also one of the major sites where defective or aged red blood cells are destroyed.{{sfn|Young|2006|p=58}}

==== Metabolic ====
* Mineral storage – bones act as reserves of minerals important for the body, most notably [[calcium]] and [[phosphorus]].<ref>{{cite journal |vauthors=Doyle ME, Jan de Beur SM |date=December 2008 |title=The skeleton: endocrine regulator of phosphate homeostasis |journal=Current Osteoporosis Reports |volume=6 |issue=4 |pages=134–141 |doi=10.1007/s11914-008-0024-6 |pmid=19032923 |s2cid=23298442}}</ref><ref>{{Cite web |date=2016-11-07 |title=Bone Health In Depth |url=https://lpi.oregonstate.edu/mic/health-disease/bone-health |access-date=2022-09-13 |website=Linus Pauling Institute |language=en}}</ref><ref>{{cite web |title=Bone |url=https://www.britannica.com/science/bone-anatomy/Chemical-composition-and-physical-properties |access-date=5 October 2017 |website=Encyclopedia Britannica |vauthors=Walker K}}</ref>

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 protein]]s and others.<ref>{{cite book |title=Ciba Foundation Symposium 136 - Cell and Molecular Biology of Vertebrate Hard Tissues |vauthors=Hauschka PV, Chen TL, Mavrakos AE |date=1988 |isbn=978-0-470-51363-7 |series=Novartis Foundation Symposia |volume=136 |pages=207–225 |chapter=Polypeptide Growth Factors in Bone Matrix |doi=10.1002/9780470513637.ch13 |pmid=3068010}}</ref>
* [[Fat]] storage – [[marrow adipose tissue]] (MAT) acts as a storage reserve of [[fatty acid]]s.<ref>{{cite journal |vauthors=Styner M, Pagnotti GM, McGrath C, Wu X, Sen B, Uzer G, Xie Z, Zong X, Styner MA, Rubin CT, Rubin J |date=August 2017 |title=Exercise Decreases Marrow Adipose Tissue Through β-Oxidation in Obese Running Mice |journal=Journal of Bone and Mineral Research |volume=32 |issue=8 |pages=1692–1702 |doi=10.1002/jbmr.3159 |pmc=5550355 |pmid=28436105}}</ref>
* [[Acid]]-[[Base (chemistry)|base]] balance – bone buffers the blood against excessive [[pH]] changes by absorbing or releasing [[Alkali salt|alkaline salts]].<ref name="fogelman">{{Cite book |url=https://books.google.com/books?id=C0K5BAAAQBAJ&q=bone+buffers+the+blood+against+excessive+pH+changes+by+absorbing+or+releasing+alkaline+salts.&pg=PA38 |title=Radionuclide and Hybrid Bone Imaging |vauthors=Fogelman I, Gnanasegaran G, van der Wall H |publisher=Springer |year=2013 |isbn=978-3-642-02400-9 |language=en}}</ref>
* 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]].<ref>{{Cite web |last=Lerro |first=Elena |date=2007 |title=Bone |url=http://flipper.diff.org/app/items/info/350 |website=flipper.diff.org}}</ref>
* [[Endocrine system|Endocrine]] organ – bone controls [[phosphate]] metabolism by releasing [[fibroblast growth factor 23]] (FGF-23), which acts on [[kidney]]s to reduce phosphate [[reabsorption]]. Bone cells also release a hormone called [[osteocalcin]], 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.<ref>{{cite journal |vauthors=Lee NK, Sowa H, Hinoi E, Ferron M, Ahn JD, Confavreux C, Dacquin R, Mee PJ, McKee MD, Jung DY, Zhang Z, Kim JK, Mauvais-Jarvis F, Ducy P, Karsenty G |date=August 2007 |title=Endocrine regulation of energy metabolism by the skeleton |journal=Cell |volume=130 |issue=3 |pages=456–469 |doi=10.1016/j.cell.2007.05.047 |pmc=2013746 |pmid=17693256 }}</ref>
* 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.<ref>{{Cite web |last1=Wilkin |first1=Douglas |last2=Gray-Wilson |first2=Niamh |date=8 February 2024 |title=Bones |url=https://www.ck12.org/biology/bones/lesson/Bones-Advanced-BIO-ADV/ |work=CK-12 Foundation |language=en}}</ref> 
[[File:Skeletal system.svg|thumb|Skeletal System of Human Body]]
== Tissue ==

Bone is not uniformly solid, but consists of a flexible [[matrix (biology)|matrix]] (about 30%) and bound minerals (about 70%), which are intricately woven and continuously remodeled by a group of specialized bone cells. Their unique composition and design allows bones to be relatively [[Rockwell scale|hard]] and strong, while remaining lightweight. Bone matrix is 90 to 95% composed of elastic [[collagen]] fibers, also known as ossein,<ref>{{cite web|url=http://medical-dictionary.thefreedictionary.com/ossein|title=Ossein| work = The Free Dictionary}}</ref> and the remainder is [[ground substance]].<ref name="Hall">{{cite book | vauthors = Hall J |url=https://archive.org/details/textbookofmedica00guyt_1/page/957/mode/2up |title=Textbook of Medical Physiology |date=2011 |publisher=Elsevier |isbn=978-08089-2400-5 |edition=12th |location=Philadelphia |pages=957–960 |url-access=registration}}</ref> The elasticity of [[collagen]] improves fracture resistance.<ref name="Schmidt-Nielsen">{{Cite book| vauthors = Schmidt-Nielsen K |author-link=Knut Schmidt-Nielsen|year=1984|title=Scaling: Why Is Animal Size So Important?|publisher=Cambridge University Press|page=[https://archive.org/details/scalingwhyisanim0000schm/page/6 6]|isbn=978-0-521-31987-4|place=Cambridge|url=https://archive.org/details/scalingwhyisanim0000schm/page/6}}</ref> 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 [[apatite]].<ref>{{cite journal | doi=10.1016/j.msec.2005.01.008 | title=A mineralogical perspective on the apatite in bone | year=2005 | vauthors = Wopenka B, Pasteris JD | journal=Materials Science and Engineering: C | volume=25 | issue=2 | pages=131–143 | doi-access=free }}</ref><ref>{{cite journal | vauthors = Wang B, Zhang Z, Pan H | title = Bone Apatite Nanocrystal: Crystalline Structure, Chemical Composition, and Architecture | journal = Biomimetics | volume = 8 | issue = 1 | page = 90 | date = February 2023 | pmid = 36975320 | pmc = 10046636 | doi = 10.3390/biomimetics8010090 | doi-access = free }}</ref> It is the mineralization that gives bones rigidity.

Within any single bone, the tissue is woven into two main patterns: cortical and cancellous bone, each with distinct appearances and characteristics. Bone is actively constructed and remodeled throughout life by specialized bone cells known as osteoblasts and osteoclasts. 

===Cortex===
[[File:Illu compact spongy bone.jpg|thumb|Cross-section details of a long bone]]

The hard outer layer of bones is composed of '''cortical bone''', which is also called '''compact bone''' as it is much denser than cancellous bone. It forms the hard exterior (cortex) of bones. The cortical bone gives bone its smooth, white, and solid appearance, and accounts for 80% of the total bone mass of an adult [[human skeleton]].<ref>{{Cite web|title=Structure of Bone|publisher=CK12-Foundation|url=https://flexbooks.ck12.org/cbook/ck-12-college-human-biology-flexbook-2.0/section/13.4/primary/lesson/structure-of-bone-chumbio|website=flexbooks.ck12.org|access-date=2020-05-28}}</ref> It facilitates bone's main functions—to support the whole body, to protect organs, to provide [[lever]]s 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 [[osteoblast]]s and [[osteocyte]]s around a central canal called the [[Haversian canal|osteonic canal]]. [[Volkmann's canal]]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.{{sfn|Young|2006|p=192}} The primary anatomical and functional unit of cortical bone is the [[osteon]].

=== Trabeculae <span class="anchor" id="Cancellous bone"></span>===
{{Further|Trabecula#Bone trabecula}}
[[File:Spongy bone - trabecules.jpg|thumb|Micrograph of cancellous bone]]

'''Cancellous bone''' or '''spongy bone''',<ref name="SEER">{{cite web |title=Structure of Bone Tissue | work = SEER Training |url=https://training.seer.cancer.gov/anatomy/skeletal/tissue.html |publisher = Surveillance, Epidemiology, and End Results Program (SEER) U.S. National Cancer Institute |access-date=25 January 2023}}</ref>{{sfn|Young|2006|p=192}} also known as '''trabecular bone''', is the internal tissue of the skeletal bone and is an open cell [[Porosity|porous]] network that follows the material properties of [[biofoams]].<ref>{{Cite journal | vauthors = Meyers MA, Chen PY, Lin AY, Seki Y |date= January 2008 |title=Biological materials: Structure and mechanical properties |url=https://www.sciencedirect.com/science/article/pii/S0079642507000254 |journal=Progress in Materials Science |language=en |volume=53 |issue=1 |pages=1–206 |doi=10.1016/j.pmatsci.2007.05.002 |issn=0079-6425|url-access=subscription }}</ref><ref name="Buss_2022">{{cite journal | vauthors = Buss DJ, Kröger R, McKee MD, Reznikov N | title = Hierarchical organization of bone in three dimensions: A twist of twists | journal = Journal of Structural Biology | volume = 6 | article-number = 100057 | date = 2022 | pmid = 35072054 | pmc = 8762463 | doi = 10.1016/j.yjsbx.2021.100057 }}</ref> 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 [[Blood vessel|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 of vertebral arch|pedicle]].<ref>{{cite journal | vauthors = Gdyczynski CM, Manbachi A, Hashemi S, Lashkari B, Cobbold RS | title = On estimating the directionality distribution in pedicle trabecular bone from micro-CT images | journal = Physiological Measurement | volume = 35 | issue = 12 | pages = 2415–2428 | date = December 2014 | pmid = 25391037 | doi = 10.1088/0967-3334/35/12/2415 | s2cid = 206078730 | bibcode = 2014PhyM...35.2415G }}</ref> Thin formations of [[osteoblast]]s covered in endosteum create an irregular network of spaces,{{sfn|Young|2006|p=195}} known as trabeculae. Within these spaces are [[bone marrow]] and [[hematopoietic stem cell]]s that give rise to [[platelet]]s, [[red blood cell]]s and [[white blood cell]]s.{{sfn|Young|2006|p=195}} 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.<ref>{{cite book| vauthors = Hall SJ |title=Basic Biomechanics with OLC.|date=2007|publisher=McGraw-Hill Higher Education|location=Burr Ridge|isbn=978-0-07-126041-1|page=88|edition=5th }}</ref>

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]].<ref>{{cite journal | vauthors = Gomez S | title = Crisóstomo Martinez, 1638-1694: the discoverer of trabecular bone | journal = Endocrine | volume = 17 | issue = 1 | pages = 3–4 | date = February 2002 | pmid = 12014701 | doi = 10.1385/ENDO:17:1:03 | s2cid = 46340228 }}</ref>

===Marrow===
[[Bone marrow]], also known as [[myeloid tissue]] in red bone marrow, can be found in almost any bone that holds [[cancellous tissue]]. In [[Infant|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]].<ref>{{cite book| vauthors = Barnes-Svarney PL, Svarney TE |title=The Handy Anatomy Answer Book: Includes Physiology|date=2016|publisher=Visible Ink Press|location=Detroit|isbn=978-1-57859-542-6|pages=90–91}}</ref>

===Vascular supply===
Bone receives about 10% of cardiac output.<ref name="pmid26273504">{{cite journal | vauthors = Marenzana M, Arnett TR | title = The Key Role of the Blood Supply to Bone | journal = Bone Research | volume = 1 | issue = 3 | pages = 203–215 | date = September 2013 | pmid = 26273504 | pmc = 4472103 | doi = 10.4248/BR201303001 }}</ref> Blood enters the [[endosteum]], flows through the marrow, and exits through small vessels in the cortex.<ref name="pmid26273504" /> In humans, [[Blood gas tension#Oxygen tension|blood oxygen tension]] in bone marrow is about 6.6%, compared to about 12% in arterial blood, and 5% in venous and capillary blood.<ref name="pmid26273504" />

== Histology and physiology ==
[[File:604 Bone cells.jpg|thumb|Bone cells]]

Bone is metabolically active tissue composed of several types of cells. These cells include [[osteoblast]]s, which are involved in the creation and [[mineralized tissue|mineralization]] of bone tissue, [[osteocyte]]s, and [[osteoclast]]s, which are involved in the reabsorption of bone tissue. Osteoblasts and osteocytes are derived from [[osteoprogenitor]] cells, but [[osteoclast]]s are derived from the same cells that differentiate to form [[macrophage]]s and [[monocyte]]s.{{sfn|Young|2006|p=189}} Within the marrow of the bone there are also [[hematopoietic stem cell]]s. These cells give rise to other cells, including [[white blood cell]]s, [[red blood cell]]s, and [[platelet]]s.{{sfn|Young|2006|p=58}}

=== Osteoblast ===
[[File:Active osteoblasts.jpg|thumb|[[Micrograph|Light micrograph]] of [[Bone decalcification|decalcified]] cancellous bone tissue displaying osteoblasts actively synthesizing osteoid, containing two osteocytes.]]

[[Osteoblast]]s are mononucleate bone-forming cells. They are located on the surface of osteon seams and make a [[protein]] mixture known as [[osteoid]], which mineralizes to become bone.{{sfn|Young|2006|pp=189–190}} 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 [[hormone]]s, such as [[prostaglandin]]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]] and [[bicarbonate]] ions. The brand-new bone created by the osteoblast is called [[osteoid]].<ref>{{cite web | url = http://depts.washington.edu/bonebio/bonAbout/bonecells.html | archive-url = https://web.archive.org/web/20110807200120/http://depts.washington.edu/bonebio/bonAbout/bonecells.html | archive-date = 7 August 2011 | title = The O' Cells | work = Bone Cells | publisher = The University of Washington | date = 3 April 2013 }}</ref> 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 bone lining cells.<ref>{{cite journal | vauthors = Wein MN |date=28 April 2017 |title= Bone Lining Cells: Normal Physiology and Role in Response to Anabolic Osteoporosis Treatments |journal=Current Molecular Biology Reports |volume=3 |issue= 2|pages= 79–84 |doi= 10.1007/s40610-017-0062-x|s2cid= 36473110 }}</ref>

=== Osteocyte ===
[[Osteocyte]]s 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.{{sfn|Young|2006|p=192}} The spaces the cell body of osteocytes occupy within the mineralized collagen type I matrix are known as [[lacuna (histology)|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.<ref>{{cite journal | vauthors = Sims NA, Vrahnas C | title = Regulation of cortical and trabecular bone mass by communication between osteoblasts, osteocytes and osteoclasts | journal = Archives of Biochemistry and Biophysics | volume = 561 | pages = 22–28 | date = November 2014 | pmid = 24875146 | doi = 10.1016/j.abb.2014.05.015 }}</ref> Osteocytes remain in contact with other osteocytes in the bone through gap junctions—coupled cell processes which pass through the canalicular channels.

=== Osteoclast ===
[[Osteoclast]]s 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 [[bone remodeling|remodeled]] by the resorption of osteoclasts and created by osteoblasts.{{sfn|Young|2006|p=189}} Osteoclasts are large cells with multiple [[Cell nucleus|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.{{sfn|Young|2006|p=190}} Because the osteoclasts are derived from a [[monocyte]] [[stem cell|stem-cell]] lineage, they are equipped with [[Phagocytosis|phagocytic]]-like mechanisms similar to circulating [[macrophage]]s.{{sfn|Young|2006|p=189}} Osteoclasts mature and/or migrate to discrete bone surfaces. Upon arrival, active enzymes, such as [[tartrate-resistant acid phosphatase]], are [[Secretion|secreted]] against the mineral substrate.{{citation needed|date=September 2013}} The reabsorption of bone by osteoclasts also plays a role in [[calcium]] [[homeostasis]].{{sfn|Young|2006|p=190}}

===Composition===
{{Main|Extracellular matrix}}

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<sub>10</sub>(PO<sub>4</sub>)<sub>6</sub>(OH)<sub>2</sub>.<ref>[https://arxiv.org/ftp/arxiv/papers/2001/2001.11808.pdf#page=2 Enhancement of Hydroxyapatite Dissolution] Journal of Materials Science & Technology,38, 148-158</ref> The organic components of this matrix consist mainly of [[Collagen#Types|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]] 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.{{sfn|Hall|2005|p=981}} The [[collagen]] fibers give bone its [[ultimate tensile strength|tensile strength]], and the interspersed crystals of [[hydroxyapatite]] give bone its [[compressive strength]]. These effects are [[synergy|synergistic]].{{sfn|Hall|2005|p=981}} The exact composition of the matrix may be subject to change over time due to nutrition and [[biomineralization]], with the ratio of [[calcium]] to [[phosphate]] varying between 1.3 and 2.0 (per weight), and trace minerals such as [[magnesium]], [[sodium]], [[potassium]] and [[carbonate]] also be found.{{sfn|Hall|2005|p=981}}

{{anchor|Woven vs. lamellar bone}}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 [[proteoglycan]]s such as [[hyaluronic acid]] and [[chondroitin sulfate]],{{sfn|Hall|2005|p=981}} 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.{{sfn|Hall|2005|p=981}} 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.<ref name="Curry2006">Currey, John D. (2002). [http://press.princeton.edu/chapters/s7313.html "The Structure of Bone Tissue"] {{Webarchive|url=https://web.archive.org/web/20170425052316/http://press.princeton.edu/chapters/s7313.html |date=25 April 2017 }}, pp.  12–14 in ''Bones: Structure and Mechanics''. Princeton University Press. Princeton, NJ. {{ISBN|978-1-4008-4950-5}}</ref>
* Lamellar bone, which has a regular parallel alignment of collagen into sheets ("lamellae") and is mechanically strong.<ref name="Buss_2022" /><ref name="Curry2006" />

[[File:Woven bone matrix.jpg|thumb|right|[[Transmission electron microscopy|Transmission]] [[electron micrograph]] of decalcified woven bone matrix displaying characteristic irregular orientation of collagen fibers]]

Woven bone is produced when osteoblasts produce osteoid rapidly, which occurs initially in all [[fetus|fetal]] bones, but is later replaced by more resilient lamellar bone. In adults, woven bone is created after [[Bone fracture|fractures]] or in [[Paget's disease of bone|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,<ref name="Salentijn">Salentijn, L. ''Biology of Mineralized Tissues: Cartilage and Bone'', [[Columbia University College of Dental Medicine]] post-graduate dental lecture series, 2007</ref> 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 (geometry)|cross-section]], the fibers run in opposite directions in alternating layers, much like in [[plywood]], assisting in the bone's ability to resist [[torsion (mechanics)|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&nbsp;[[Micrometre|μm]] per day. Lamellar bone also requires a relatively flat surface to lay the collagen fibers in parallel or concentric layers.<ref>{{Cite book| vauthors = Royce PM, Steinmann B |url=https://books.google.com/books?id=x-Z-cXUGlL8C&q=Lamella+bone+requires+a+relatively+flat+surface&pg=PA70|title=Connective Tissue and Its Heritable Disorders: Molecular, Genetic, and Medical Aspects|date=2003-04-14|publisher=John Wiley & Sons|isbn=978-0-471-46117-3|language=en}}</ref>

====Deposition====
The extracellular matrix of bone is laid down by [[osteoblast]]s, which secrete both collagen and ground substance. These cells synthesise collagen alpha polypeptide chains and then secrete collagen molecules. The collagen molecules associate with their neighbors and crosslink via lysyl oxidase to form collagen fibrils. At this stage, they are not yet mineralized, and this zone of unmineralized collagen fibrils is called "osteoid". Around and inside collagen fibrils calcium and phosphate eventually [[Precipitation (chemistry)|precipitate]] within days to weeks becoming then fully mineralized bone with an overall carbonate substituted hydroxyapatite inorganic phase.<ref>{{cite journal | vauthors = Buss DJ, Reznikov N, McKee MD | title = Crossfibrillar mineral tessellation in normal and Hyp mouse bone as revealed by 3D FIB-SEM microscopy | journal = Journal of Structural Biology | volume = 212 | issue = 2 | article-number = 107603 | date = November 2020 | pmid = 32805412 | doi = 10.1016/j.jsb.2020.107603 | s2cid = 221164596 | url = https://escholarship.mcgill.ca/concern/articles/vq27zt432 }}</ref>{{sfn|Hall|2005|p=981}}

In order to mineralise the bone, the osteoblasts secrete alkaline phosphatase, some of which is carried by [[Vesicle (biology and chemistry)|vesicles]]. This cleaves the inhibitory pyrophosphate and simultaneously generates free phosphate ions for mineralization, acting as the foci for calcium and phosphate deposition. Vesicles may initiate some of the early mineralization events by rupturing and acting as a centre for crystals to grow on. Bone mineral may be formed from globular and plate structures, and via initially amorphous phases.<ref name="r1">{{cite journal| vauthors = Bertazzo S, Bertran CA |year=2006|title=Morphological and dimensional characteristics of bone mineral crystals|journal= Key Engineering Materials|volume=309-311 |pages=3–6 |doi=10.4028/www.scientific.net/kem.309-311.3|s2cid=136883011 }}</ref><ref>{{cite journal|doi=10.4028/www.scientific.net/kem.309-311.11|title=Morphological Characterization of Femur and Parietal Bone Mineral of Rats at Different Ages|year=2006| vauthors = Bertazzo S, Bertran C, Camilli J |journal=Key Engineering Materials|volume=309–311|pages=11–14|s2cid=135813389}}</ref>

==Development==
[[File:Bone growth.png|300px|thumb|left|Endochondral ossification]]
[[File:Bone (1).jpg|thumb|Light micrograph of a section through a juvenile knee joint (rat) showing the cartilagineous growth plates]]
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]].<ref>{{cite book | vauthors = Betts JG, Young KA, Wise JA, Johnson E, Poe B, Kruse DH, Korol O, Johnson JE, Womble M, DeSaix P | chapter = 6.4 Bone Formation and Development | title = Anatomy and Physiology | date = 25 April 2013 | publisher = OpenStax | access-date = 26 February 2016 | chapter-url = https://openstax.org/books/anatomy-and-physiology/pages/6-4-bone-formation-and-development }}</ref> 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]], trabeculae formation and the development of the periosteum.<ref>{{Cite web|title=Bone Growth and Development | work = Biology for Majors II|url=https://courses.lumenlearning.com/wm-biology2/chapter/bone-growth-and-development/| publisher = lumenlearning.com |access-date=2020-05-28}}</ref>

'''Endochondral ossification''' occurs in long bones and most other bones in the body; it involves the development of bone from cartilage. This process includes the development of a cartilage model, its growth and development, development of the primary and secondary [[ossification center]]s, and the formation of articular cartilage and the [[epiphyseal plate]]s.<ref>{{Cite book| vauthors = Tortora GJ, Derrickson BH |url=https://books.google.com/books?id=aSaVDwAAQBAJ&q=Endochondral+ossification&pg=PA181|title=Principles of Anatomy and Physiology |year=2018|publisher=John Wiley & Sons|isbn=978-1-119-44445-9|language=en}}</ref>

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).<ref>{{Cite web|title=6.4B: Postnatal Bone Growth|url=https://med.libretexts.org/Bookshelves/Anatomy_and_Physiology/Book%3A_Anatomy_and_Physiology_(Boundless)/6%3A_Skeletal_System/6.4%3A_Bone_Formation/6.4B%3A_Postnatal_Bone_Growth|date=2018-07-19|website=Medicine LibreTexts|language=en|access-date=2020-05-28}}</ref> 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.<ref>{{cite book| vauthors = Agur A |title=Grant's Atlas of Anatomy|year=2009|publisher=Lippincott Williams & Wilkins|location=Philadelphia|isbn=978-0-7817-7055-2|page=598}}</ref>

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.<ref name="Saladin 2012 217">{{cite book| vauthors = Saladin K |title=Anatomy and Physiology: The Unity of Form and Function|year=2012|publisher=McGraw-Hill|location=New York|isbn=978-0-07-337825-1|page=217}}</ref>
# Zone of cell proliferation. A little closer to the marrow cavity, chondrocytes multiply and arrange themselves into longitudinal columns of flattened lacunae.<ref name="Saladin 2012 217"/>
# 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.<ref name="Saladin 2012 217"/>
# 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.<ref name="Saladin 2012 217"/>
# 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.<ref name="Saladin 2012 217"/>
Bone development in youth is extremely important in preventing future complications of the skeletal system. Regular exercise during childhood and adolescence can help improve bone architecture, making bones more resilient and less prone to fractures in adulthood. Physical activity, specifically resistance training, stimulates growth of bones by increasing both bone density and strength. Studies have shown a positive correlation between the adaptations of resistance training and bone density.<ref name="Layne_1999">{{cite journal | vauthors = Layne JE, Nelson ME | title = The effects of progressive resistance training on bone density: a review | language = en-US | journal = Medicine and Science in Sports and Exercise | volume = 31 | issue = 1 | pages = 25–30 | date = January 1999 | pmid = 9927006 | doi = 10.1097/00005768-199901000-00006 }}</ref> While nutritional and pharmacological approaches may also improve bone health, the strength and balance adaptations from resistance training are a substantial added benefit.<ref name="Layne_1999" /> Weight-bearing exercise may assist in osteoblast (bone-forming cells) formation and help to increase bone mineral content. High-impact sports, which involve quick changes in direction, jumping, and running, are particularly effective with stimulating bone growth in the youth.<ref name="López-García_2019">{{Cite journal | vauthors = López-García R, Cruz-Castruita R, Morales-Corral P, Banda-Sauceda N, Lagunés-Carrasco J |date=2019-12-16 |title=Evaluación del mineral óseo con la dexa en futbolistas juveniles |url=http://cdeporte.rediris.es/revista/revista76/artevaluacion1098.htm  |journal=Revista Internacional de Medicina y Ciencias de la Actividad Física y del Deporte |volume=19 |issue=76 |pages=617–626 |doi=10.15366/rimcafd2019.76.004 |doi-broken-date=4 September 2025 |issn=1577-0354|hdl=10486/689625 |hdl-access=free }}</ref> Sports such as soccer, basketball, and tennis have shown to have positive effects on bone mineral density as well as bone mineral content in teenagers.<ref name="López-García_2019" /> Engaging in physical activity during childhood years, particularly in these high-impact osteogenic sports, can help to positively influence bone mineral density in adulthood.<ref name="Van Langendonck_2003">{{cite journal | vauthors = Van Langendonck L, Lefevre J, Claessens AL, Thomis M, Philippaerts R, Delvaux K, Lysens R, Renson R, Vanreusel B, Vanden Eynde B, Dequeker J, Beunen G | title = Influence of participation in high-impact sports during adolescence and adulthood on bone mineral density in middle-aged men: a 27-year follow-up study | journal = American Journal of Epidemiology | volume = 158 | issue = 6 | pages = 525–533 | date = September 2003 | pmid = 12965878 | doi = 10.1093/aje/kwg170 }}</ref> Children and adolescents who participate in regular physical activity will place the groundwork for bone health later in life, reducing the risk of bone-related conditions such as osteoporosis.<ref name="Van Langendonck_2003" />

=== Remodeling ===
{{Main|Bone 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.<ref>{{cite journal | vauthors = Manolagas SC | title = Birth and death of bone cells: basic regulatory mechanisms and implications for the pathogenesis and treatment of osteoporosis | journal = Endocrine Reviews | volume = 21 | issue = 2 | pages = 115–137 | date = April 2000 | pmid = 10782361 | doi = 10.1210/edrv.21.2.0395 | doi-access = free }}</ref> 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.<ref>{{cite journal | vauthors = Hadjidakis DJ, Androulakis II | title = Bone remodeling | journal = Annals of the New York Academy of Sciences | volume = 1092 | pages = 385–396 | date = December 2006 | issue = 1 | pmid = 17308163 | doi = 10.1196/annals.1365.035 | bibcode = 2006NYASA1092..385H | s2cid = 39878618 }}</ref> 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>{{cite book| veditors = Woodburne RT |title=Anatomy, physiology, and metabolic disorders|date=1999|publisher=Novartis Pharmaceutical Corp.|location=Summit, N.J.|isbn=978-0-914168-88-1|pages=187–189|edition=5th }}</ref>

The action of osteoblasts and osteoclasts are controlled by a number of chemical [[enzyme]]s that either promote or inhibit the activity of the bone remodeling cells, controlling the rate at which bone is made, destroyed, or changed in shape. The cells also use [[paracrine signalling]] to control the activity of each other.<ref name="fogelman" /><ref>{{Cite web|title=Introduction to cell signaling (article)|url=https://www.khanacademy.org/science/ap-biology/cell-communication-and-cell-cycle/cell-communication/a/introduction-to-cell-signaling|access-date=2020-12-24|website=Khan Academy|language=en}}</ref> For example, 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/>

Osteoblasts can also 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]].{{citation needed|date=September 2013}} 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 | vauthors = Boulpaep EL, Boron WF |title=Medical physiology: a cellular and molecular approach |publisher=Saunders |location=Philadelphia |year=2005 |pages=1089–1091 |isbn=978-1-4160-2328-9 }}</ref> Osteoblasts can also be induced to secrete a number of [[cytokine]]s 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.{{citation needed|date=September 2013}}

=== Volume ===
Bone volume is determined by the rates of bone formation and bone resorption. 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.<ref name="ukpmc.ac.uk">{{cite journal | vauthors = Mohan S, Baylink DJ | title = Bone growth factors | journal = Clinical Orthopaedics and Related Research | volume = 263 | issue = 263 | pages = 30–48 | date = February 1991 | pmid = 1993386 | doi = 10.1097/00003086-199102000-00004 }}</ref> 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.<ref name="ukpmc.ac.uk"/> Cancellous bone volume in postmenopausal osteoporosis may be determined by the relationship between the total bone forming surface and the percent of surface resorption.<ref name="pmid6114324">{{cite journal | vauthors = Nordin BE, Aaron J, Speed R, Crilly RG | title = Bone formation and resorption as the determinants of trabecular bone volume in postmenopausal osteoporosis | journal = Lancet | volume = 2 | issue = 8241 | pages = 277–279 | date = August 1981 | pmid = 6114324 | doi = 10.1016/S0140-6736(81)90526-2 | s2cid = 29646037 }}</ref>

==Clinical significance==
{{See also|Bone disease}}
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 [[pathologist]]s in investigating the cause of the disease, and [[family doctor]]s may play a role in preventing complications of bone disease such as osteoporosis.

When a doctor sees a patient, a history and exam will be taken. Bones are then often imaged, called [[radiography]]. 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.{{sfn|Davidson|2010|pp=1059–1062}} Other tests such as a blood test for autoimmune markers may be taken, or a [[synovial fluid]] aspirate may be taken.{{sfn|Davidson|2010|pp=1059–1062}}

===Fractures===
[[File:Xraymachine.JPG|thumb|[[Radiography]] used to identify possible [[bone fracture]]s after a knee injury]]
{{main|Bone fracture}}
In normal bone, [[Bone fracture|fractures]] 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 of bone|Paget's disease]]) or is the site of the growth of cancer.{{sfn|Davidson|2010|p=1068}} Common fractures include [[wrist fracture]]s and [[hip fracture]]s, associated with [[osteoporosis]], [[vertebral fracture]]s associated with high-energy trauma and cancer, and fractures of long-bones. Not all fractures are painful.{{sfn|Davidson|2010|p=1068}} When serious, depending on the fractures type and location, complications may include [[flail chest]], [[compartment syndrome]]s or [[fat embolism]].
[[Compound fracture]]s 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-ray]]s, [[CT scans]] and [[MRI]]s.{{sfn|Davidson|2010|p=1068}} 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]].<ref>{{cite journal |vauthors=Salter RB, Harris WR |year=1963 |title=Injuries Involving the Epiphyseal Plate |journal=J Bone Joint Surg Am |volume=45 |issue=3 |pages=587–622 |url=http://jbjs.org/content/45/3/587 |doi=10.2106/00004623-196345030-00019 |s2cid=73292249 |access-date=2 December 2016 |archive-url=https://web.archive.org/web/20161202172951/http://jbjs.org/content/45/3/587 |archive-date=2 December 2016 |url-access=subscription }}</ref> 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]].{{sfn|Davidson|2010|p=1068}}

===Tumors===
{{main|Bone tumor}}

Tumor that can affect bone in several ways. Examples of benign [[bone tumor]]s include [[osteoma]], [[osteoid osteoma]], [[osteochondroma]], [[osteoblastoma]], [[enchondroma]], [[giant-cell tumor of bone]], and [[aneurysmal bone cyst]].<ref>{{Cite web|url=http://my.clevelandclinic.org/health/articles/benign-bone-tumors|title=Benign Bone Tumours|date=2017|website=Cleveland Clinic|access-date=29 March 2017}}</ref>

===Cancer===
{{main|Bone metastases}}
[[Cancer]] can arise in bone tissue, and bones are also a common site for other cancers to spread ([[metastasise]]) to.{{sfn|Davidson|2010|p=1125}} Cancers that arise in bone are called "primary" cancers, although such cancers are rare.{{sfn|Davidson|2010|p=1125}} Metastases within bone are "secondary" cancers, with the most common being [[breast cancer]], [[lung cancer]], [[prostate cancer]], [[thyroid cancer]], and [[kidney cancer]].{{sfn|Davidson|2010|p=1125}} 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.{{sfn|Davidson|2010|p=1032}} [[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.

===Diabetes===

[[Type 1 diabetes]] is an autoimmune disease in which the body attacks the insulin-producing pancreas cells causing the body to not make enough insulin.<ref name="Ndisang_2017">{{cite journal | vauthors = Ndisang JF, Vannacci A, Rastogi S | title = Insulin Resistance, Type 1 and Type 2 Diabetes, and Related Complications 2017 | journal = Journal of Diabetes Research | volume = 2017 | issue = | article-number = 1478294 | date = 2017 | pmid = 29279853 | doi = 10.1155/2017/1478294 | doi-access = free | pmc = 5723935 }}</ref> In contrast [[type 2 diabetes]] in which the body creates enough Insulin, but becomes resistant to it over time.<ref name="Ndisang_2017" />

Children makeup approximately 85% of Type 1 Diabetes cases and in America there was an average 22% rise in cases<ref>{{cite journal | vauthors = Kamrath C, Holl RW, Rosenbauer J | title = Elucidating the Underlying Mechanisms of the Marked Increase in Childhood Type 1 Diabetes During the COVID-19 Pandemic-The Diabetes Pandemic | journal = JAMA Network Open | volume = 6 | issue = 6 | pages = e2321231 | date = June 2023 | pmid = 37389881 | doi = 10.1001/jamanetworkopen.2023.21231 | doi-access = free }}</ref> over the first 24 months of the COVID-19 Pandemic. With the increase of developing some form of diabetes across all ranges continually growing the health impacts on bone development and bone health in these populations are still being researched. Most evidence suggests that diabetes, either Type 1 and Type 2, inhibits osteoblastic activity<ref>{{cite journal | vauthors = Loxton P, Narayan K, Munns CF, Craig ME | title = Bone Mineral Density and Type 1 Diabetes in Children and Adolescents: A Meta-analysis | journal = Diabetes Care | volume = 44 | issue = 8 | pages = 1898–1905 | date = August 2021 | pmid = 34285100 | doi = 10.2337/dc20-3128 | pmc = 8385468 }}</ref> and causes both lower BMD and BMC in both adults and children. The weakening of these developmental aspects is thought to lead to an increased risk of developing many diseases such as osteoarthritis, osteoporosis, osteopenia and fractures.<ref>{{cite journal | vauthors = de Araújo IM, Moreira ML, de Paula FJ | title = Diabetes and bone | journal = Archives of Endocrinology and Metabolism | volume = 66 | issue = 5 | pages = 633–641 | date = November 2022 | pmid = 36382752 | doi = 10.20945/2359-3997000000552 | pmc = 10118819 }}</ref> Development of any of these diseases is thought to be correlated with a decrease in ability to perform in athletic environments and activities of daily living.

Focusing on therapies that target molecules like osteocalcin or AGEs could provide new ways to improve bone health and help manage the complications of diabetes more effectively.<ref>{{cite journal | vauthors = Booth SL, Centi A, Smith SR, Gundberg C | title = The role of osteocalcin in human glucose metabolism: marker or mediator? | journal = Nature Reviews. Endocrinology | volume = 9 | issue = 1 | pages = 43–55 | date = January 2013 | pmid = 23147574 | doi = 10.1038/nrendo.2012.201 | pmc = 4441272 }}</ref>

===Other painful conditions===
* [[Osteomyelitis]] is inflammation of the bone or bone marrow due to bacterial infection.<ref>{{cite web |url= https://www.lecturio.com/concepts/osteomyelitis/| title= Osteomyelitis|website=The Lecturio Medical Concept Library |access-date= 26 August 2021}}</ref>
* [[Osteomalacia]] is a painful softening of adult bone caused by severe vitamin D deficiency.<ref>{{cite web |url= https://www.lecturio.com/concepts/osteomalacia-and-rickets/| title= Osteomalacia and Rickets|website=The Lecturio Medical Concept Library |access-date= 26 August 2021}}</ref>
* [[Osteogenesis imperfecta]]<ref>{{cite web |url= https://www.lecturio.com/concepts/osteogenesis-imperfecta/| title= Osteogenesis Imperfecta
|website=The Lecturio Medical Concept Library |access-date= 26 August 2021}}</ref>
* [[Osteochondritis dissecans]]<ref>{{cite web |url=
https://www.lecturio.com/concepts/osteochondritis-dissecans/| title= Osteochondritis Dissecans
|website=The Lecturio Medical Concept Library |access-date= 26 August 2021}}</ref>
* [[Ankylosing spondylitis]]<ref>{{cite web |url= https://www.lecturio.com/concepts/ankylosing-spondylitis/| title= Ankylosing Spondylitis
|website=The Lecturio Medical Concept Library |access-date= 26 August 2021}}</ref>
* [[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.<ref>{{cite journal | vauthors = Whitford GM | title = Intake and metabolism of fluoride | journal = Advances in Dental Research | volume = 8 | issue = 1 | pages = 5–14 | date = June 1994 | pmid = 7993560 | doi = 10.1177/08959374940080011001 | s2cid = 21763028 }}</ref>

===Osteoporosis===
{{Main|Osteoporosis}}
[[File:Osteoporosis in Bones.jpg|thumb|Reduced bone mineral density in Osteoporosis (R), increasing the likelihood of fractures]]
Osteoporosis is a disease of bone where there is reduced [[bone mineral density]], increasing the likelihood of [[bone fracture|fractures]].{{sfn|Davidson|2010|pp=1116–1121}} Osteoporosis is defined in women by the [[World Health Organization]] as a bone mineral density of 2.5 [[standard deviation]]s 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]].<ref name=WHO1994>{{cite journal | vauthors = 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 | pages = 1–129 | year = 1994 | pmid = 7941614 }}</ref> 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 [[glucocorticoid]]s.{{sfn|Davidson|2010|pp=1116–1121}} Osteoporosis usually has no symptoms until a fracture occurs.{{sfn|Davidson|2010|pp=1116–1121}} 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.{{sfn|Davidson|2010|pp=1116–1121}}

One of the most important risk factors for [[osteoporosis]] is [[ageing|advanced age]]. Accumulation of oxidative [[DNA damage (naturally occurring)|DNA damage]] in [[osteocyte|osteoblastic]] and [[osteoclast]]ic cells appears to be a key factor in age-related osteoporosis.<ref>Chen Q, Liu K, Robinson AR, et al. DNA damage drives accelerated bone aging via an NF-κB-dependent mechanism. J Bone Miner Res. 2013;28(5):1214-1228. {{doi|10.1002/jbmr.1851}}</ref>

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 [[bisphosphonate]]s, [[Strontium ranelate]], and [[hormone replacement therapy]].<ref name=DAVIDSONS2010>{{harvnb|Davidson|2010|pages=1116–1121}}</ref>

=== Bone health ===
{{main|Bone health}}
Without strong healthy bones, humans are more at risk for different chronic diseases and fractures, with day-to-day function being more difficult with poor bone health. It is estimated that diet and exercise during childhood can impact peak bone mass as an adult nearly 20–40%.<ref name="pmid38161439">{{cite journal | vauthors = Faienza MF, Urbano F, Chiarito M, Lassandro G, Giordano P | title = Musculoskeletal health in children and adolescents | journal = Frontiers in Pediatrics | volume = 11 | issue = | article-number = 1226524 | date = 2023 | pmid = 38161439 | pmc = 10754974 | doi = 10.3389/fped.2023.1226524 | doi-access = free }}</ref> One study done on children with developmental coordination disorder found an increase in bone mass up to 4% and 5% in the cortical areas of the tibia alone from a 13-week training period.<ref name="pmid33265073">{{cite journal | vauthors = Tan JL, Siafarikas A, Rantalainen T, Hart NH, McIntyre F, Hands B, Chivers P | title = Impact of a multimodal exercise program on tibial bone health in adolescents with Development Coordination Disorder: an examination of feasibility and potential efficacy | journal = Journal of Musculoskeletal & Neuronal Interactions | volume = 20 | issue = 4 | pages = 445–471 | date = December 2020 | pmid = 33265073 | doi = | pmc = 7716678 | url = }}</ref> Peak bone mass occurs between the second and third decade of most people's lives.<ref name="pmid36578952">{{cite journal | vauthors = Baronio F, Baptista F | title = Editorial: Bone health and development in children and adolescents | journal = Frontiers in Endocrinology | volume = 13 | issue = | article-number = 1101403 | date = 2022 | pmid = 36578952 | doi = 10.3389/fendo.2022.1101403 | doi-access = free | pmc = 9791941 | url = }}</ref> Studies have shown that increasing calcium stores in childhood via food intake result in significant improvements in bone-mass density and overall health, even into adulthood.<ref name="Pan_2020">{{cite journal |vauthors=Pan K, Zhang C, Yao X, Zhu Z |date=January 2020 |title=Association between dietary calcium intake and BMD in children and adolescents |journal=Endocrine Connections |volume=9 |issue=3 |pages=194–200 |doi=10.1530/EC-19-0534 |pmc=7040863 |pmid=31990673}}</ref><ref>{{cite journal |vauthors=Closa-Monasterolo R, Zaragoza-Jordana M, Ferré N, Luque V, Grote V, Koletzko B, Verduci E, Vecchi F, Escribano J |date=June 2018 |title=Adequate calcium intake during long periods improves bone mineral density in healthy children. Data from the Childhood Obesity Project |journal=Clinical Nutrition |volume=37 |issue=3 |pages=890–896 |doi=10.1016/j.clnu.2017.03.011 |pmid=28351509}}</ref><ref name="Gordon_2000">{{cite book |title=Endotext |vauthors=Gordon RJ, Misra M, Mitchell DM |date=2000 |publisher=MDText.com, Inc. |veditors=Feingold KR, Anawalt B, Blackman MR, Boyce A |place=South Dartmouth (MA) |chapter=Osteoporosis and Bone Fragility in Children |pmid=37490575 |access-date=2024-11-15 |chapter-url=https://www.ncbi.nlm.nih.gov/books/NBK593436/}}</ref>

== Osteology ==
[[File:Paleopathology; Human femurs from Roman period, Tell Fara Wellcome L0008764.jpg|thumbnail|Human femurs and humerus from Roman period, with evidence of healed [[bone fracture|fractures]]]]
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]].{{Citation needed|date=September 2025}}

Typically anthropologists and archeologists study [[bone tool]]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 [[antler]]s.{{Citation needed|date=September 2025}}
{{Clear}}

==Other animals==
{{Main|Bird anatomy|Exoskeleton}}
[[File:Fluworôze egzostozes1-800h.jpg|thumb|right|180px|alt=knobby hoofed leg|[[Skeletal fluorosis]] in a cow's leg, due to industrial contamination]]
[[File:Bird leg and pelvic girdle skeleton EN.gif|thumb|left|200px|Leg and pelvic girdle bones of bird]]
[[Bird]] skeletons are very lightweight. Their bones are smaller and thinner than those of mammals, to aid flight. Among mammals, [[bat]]s 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.<ref name="RSPB-BirdBoneDensity">{{cite journal | vauthors = Dumont ER | title = Bone density and the lightweight skeletons of birds | journal = Proceedings. Biological Sciences | volume = 277 | issue = 1691 | pages = 2193–2198 | date = July 2010 | pmid = 20236981 | pmc = 2880151 | doi = 10.1098/rspb.2010.0117 }}</ref> A bird's [[beak]] is primarily made of bone as projections of the [[mandible]]s 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]].<ref>{{cite journal | vauthors = Nasoori A | title = Formation, structure, and function of extra-skeletal bones in mammals | journal = Biological Reviews of the Cambridge Philosophical Society | volume = 95 | issue = 4 | pages = 986–1019 | date = August 2020 | pmid = 32338826 | doi = 10.1111/brv.12597 | s2cid = 216556342 }}</ref> A [[deer]]'s [[antler]]s are composed of bone which is an unusual example of bone being outside the skin of the animal once the velvet is shed.<ref name="pmid10321994">{{cite journal | vauthors = Rolf HJ, Enderle A | title = Hard fallow deer antler: a living bone till antler casting? | journal = The Anatomical Record | volume = 255 | issue = 1 | pages = 69–77 | date = May 1999 | pmid = 10321994 | doi = 10.1002/(SICI)1097-0185(19990501)255:1<69::AID-AR8>3.0.CO;2-R | doi-access = free }}</ref>

The extinct predatory fish ''[[Dunkleosteus]]'' had sharp edges of hard exposed bone along its jaws.<ref>{{Cite web|url=https://www.amnh.org/exhibitions/permanent-exhibitions/fossil-halls/hall-of-vertebrate-origins/dunkleosteus|title=Dunkleosteus|website=American Museum of Natural History}}</ref><ref>{{Cite web|url=https://www.cmnh.org/dunkjaws|title=My, What a Big Mouth You Have {{pipe}} Cleveland Museum of Natural History}}</ref>

The proportion of cortical bone that is 80% in the human skeleton may be much lower in other animals, especially in [[marine mammal]]s and [[marine turtles]], or in various [[Mesozoic]] [[marine reptile]]s, such as [[Ichthyosauria|ichthyosaur]]s,<ref name="Buff&Maz90">{{cite journal | vauthors = de Buffrénil V, Mazin JM | year = 1990 | title = Bone histology of the ichthyosaurs: comparative data and functional interpretation | jstor = 2400968 | journal = Paleobiology | volume = 16 | issue = 4| pages = 435–447 |doi=10.1017/S0094837300010174 |bibcode=1990Pbio...16..435D |s2cid=88171648 }}</ref> among others.<ref name=ZL11>{{cite journal| vauthors = Laurin M, Canoville A, Germain D | year=2011 |title=Bone microanatomy and lifestyle: a descriptive approach |journal=Comptes Rendus Palevol |volume=10 |issue=5–6 |pages=381–402 | doi = 10.1016/j.crpv.2011.02.003}}</ref> This proportion can vary quickly in evolution; it often increases in early stages of returns to an aquatic lifestyle, as seen in early [[whale]]s and [[pinniped]]s, 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, [[Pachyostosis|pachyostotic]],<ref>{{cite journal | vauthors = Houssaye A, De Buffrenil V, Rage JC, Bardet N |title=An analysis of vertebral 'pachyostosis' in Carentonosaurus mineaui (Mosasauroidea, Squamata) from the Cenomanian (early Late Cretaceous) of France, with comments on its phylogenetic and functional significance |journal=Journal of Vertebrate Paleontology |date=12 September 2008 |volume=28 |issue=3 |pages=685–691 |doi=10.1671/0272-4634(2008)28[685:AAOVPI]2.0.CO;2 |s2cid=129670238  |issn=0272-4634}}</ref> [[Osteosclerosis|osteosclerotic]], or pachyosteosclerotic<ref>{{cite journal | vauthors = de Buffrénil V, Canoville A, D'Anastasio R, Domning DP  |title=Evolution of Sirenian Pachyosteosclerosis, a Model-case for the Study of Bone Structure in Aquatic Tetrapods |journal=Journal of Mammalian Evolution |date=June 2010 |volume=17 |issue=2 |pages=101–120 |doi=10.1007/s10914-010-9130-1|s2cid=39169019 }}</ref> bones, especially if they move slowly, like [[Sirenia|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 lake|hypersaline]] (denser) water.<ref>{{cite journal | vauthors = Dewaele L, Lambert O, Laurin M, De Kock T, Louwye S, de Buffrénil V |title=Generalized Osteosclerotic Condition in the Skeleton of Nanophoca vitulinoides, a Dwarf Seal from the Miocene of Belgium |journal=Journal of Mammalian Evolution |date=December 2019 |volume=26 |issue=4 |pages=517–543 |doi=10.1007/s10914-018-9438-9|s2cid=20885865 |url=https://hal.sorbonne-universite.fr/hal-02550689/file/Dewaele%20et%20al.%20-%202019%20-%20Generalized%20Osteosclerotic%20Condition%20in%20the%20Skelet.pdf }}</ref><ref>{{cite journal | vauthors = Dewaele L, Gol'din P, Marx FG, Lambert O, Laurin M, Obadă T, de Buffrénil V | title = Hypersalinity drives convergent bone mass increases in Miocene marine mammals from the Paratethys | language = English | journal = Current Biology | volume = 32 | issue = 1 | pages = 248–255.e2 | date = January 2022 | pmid = 34813730 | doi = 10.1016/j.cub.2021.10.065 | s2cid = 244485732 | doi-access = free | bibcode = 2022CBio...32E.248D }}</ref><ref>{{cite journal | vauthors = Houssaye A | title = Evolution: Back to heavy bones in salty seas | journal = Current Biology | volume = 32 | issue = 1 | pages = R42–R44 | date = January 2022 | pmid = 35015995 | doi = 10.1016/j.cub.2021.11.049 | bibcode = 2022CBio...32..R42H | url = https://hal.archives-ouvertes.fr/hal-03820094/file/Houssaye_et_al_Current_Biology.pdf | url-status = live | s2cid = 245879886 | archive-url = https://web.archive.org/web/20221123162936/https://hal.archives-ouvertes.fr/hal-03820094/file/Houssaye_et_al_Current_Biology.pdf | archive-date = 2022-11-23 }}</ref>

Many animals, particularly [[herbivore]]s, 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==
[[File:Bones of cattle on a farm in Namibia.jpg|thumb|Bones of slaughtered [[cattle]] on a [[farm]] in [[Namibia]]]]
Bones from slaughtered animals have a number of uses:

* In [[prehistoric times]], they have been used for making [[bone tool]]s.<ref>{{Cite book| vauthors = Laszlovszky J, Szabo P |url=https://books.google.com/books?id=ft2d-zrlLWcC&q=Bones+from+slaughtered+animals+have+a+number+of+uses.+In+prehistoric+times%2C+they+have+been+used+for+making+bone+tools&pg=PA142|title=People and Nature in Historical Perspective |date= January 2003 |publisher=Central European University Press|isbn=978-963-9241-86-2|language=en}}</ref> They have further been used in [[bone carving]], already important in [[prehistoric art]], and also in [[modern time]] as crafting materials for [[button]]s, [[bead]]s, [[handle]]s, [[bobbin]]s, [[Napier's bones|calculation aids]], [[head nut]]s, [[dice]], [[poker chip]]s, [[pick-up sticks]], [[arrow]]s, [[scrimshaw]], and ornaments.
* [[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.
* The [[Furcula|wishbone]]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.

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]].

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==
<gallery>
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
</gallery>

== See also ==
* [[Artificial bone]]
* [[Calcareous]]
* [[Cuttlebone]]
* [[Distraction osteogenesis]]
* [[National Bone Health Campaign]]
* [[Skeleton]]
* [[Ossicle (echinoderm)]]
* [[Ossification#Evolution|Ossification § Evolution]]

== References ==
{{reflist}}

== Sources ==
{{refbegin}}
* {{cite book | vauthors = Davidson S | veditors = Colledge NR, Walker BR, Ralston SH |others=Illustrated by Robert Britton|title=Davidson's Principles and Practice of Medicine.|date=2010|publisher=Churchill Livingstone/Elsevier |location=Edinburgh|isbn=978-0-7020-3085-7|edition=21st }}
* {{cite book | vauthors = Hall AC, Guyton JE | title = Textbook of Medical Physiology | date = 2005 | publisher = W.B. Saunders | location = Philadelphia | isbn = 978-0-7216-0240-0 | edition = 11th | ref = {{harvid|Hall|2005}} }}
* {{cite book | vauthors = Young B, Lowe JS, Stevens A, Heath JW | title = Wheater's Functional Histology: a text and colour atlas | date = 2006 | publisher = Churchill Livingstone/Elsevier | location = London | isbn = 978-0-443-068-508 | edition = 5th | ref = {{harvid|Young|2006}} }}
{{refend}}

== Further reading ==
{{refbegin}}
* {{cite book | vauthors = Derrickson BH, Tortora GJ |title=Principles of anatomy and physiology |publisher=Wiley |location=New York |year=2005|isbn=978-0-471-68934-8 }}
* {{cite book | vauthors = Fauci AS, Braunwald E, Kasper DL, Hauser SL, Longo DL, Jameson JL, Loscalzo J | title = Harrison's principles of internal medicine | date = 2008 | publisher = McGraw-Hill Medical | location = New York | isbn = 978-0-07-147692-8 | edition = 17th }}
* {{cite book | vauthors = Hoehn K, Marieb EN |title=Human Anatomy & Physiology |publisher=Benjamin Cummings |location=San Francisco |year= 2007|isbn=978-0-8053-5909-1 |edition=7th }}
* {{cite book | vauthors = Kini U, Nandeesh BN |chapter=Ch 2: Physiology of Bone Formation, Remodeling, and Metabolism | veditors = Fogelman I, Gnanasegaran G, van der Wall H |title=Radionuclide and hybrid bone imaging|publisher=Springer|location=Berlin|isbn=978-3-642-02399-6|pages=29–57|chapter-url=https://www.springer.com/cda/content/document/cda_downloaddocument/9783642023996-c1.pdf?SGWID=0-0-45-1356540-p173959977 |date=2013-01-03|access-date=28 August 2017|archive-date=6 November 2020|archive-url=https://web.archive.org/web/20201106152855/https://www.springer.com/cda/content/document/cda_downloaddocument/9783642023996-c1.pdf?SGWID=0-0-45-1356540-p173959977}}
{{refend}}

== External links ==
{{commons category|Bones}}
{{Wikiquote}}
* [https://web.archive.org/web/20170509104452/http://depts.washington.edu/bonebio/ASBMRed/ASBMRed.html Educational resource materials (including animations) by the American Society for Bone and Mineral Research]
* [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://www.histology-world.com/photoalbum/thumbnails.php?album=8 Bone histology photomicrographs]

{{Bone and cartilage}}
{{Human bones}}
{{Fractures}}

{{Authority control}}

[[Category:Bones| ]]
[[Category:Skeletal system]]
[[Category:Connective tissue]]