View source for Biology:Osteoclast

{{Short description|Cell that breaks down bone tissue}}
{{Infobox cell
| Name        = Osteoclast
| Latin       = osteoclastus
| Greek       = 
| Image       = Osteoclast.jpg
| Caption     = [[Micrograph|Light micrograph]] of an osteoclast displaying typical distinguishing characteristics: a large cell with multiple nuclei and a "foamy" cytosol.
| Width       = 300 px
| Image2      = Bone_degrading_cells_-_Osteoclasts_3_--_Smart-Servier.png
| Width2 = 100 px
| Caption2    = Illustration showing a single osteoclast
| Precursor   = osteoclast progenitors
| System      = 
| Location = [[Biology:Bone|Bone]]
| Function = Breakdown of bone tissue
}}
An '''osteoclast''' ({{ety|grc|''ὀστέον'' (osteon)|bone||''κλαστός'' (clastos)|broken}}) is a type of bone cell that breaks down bone tissue.  This function is critical in the maintenance, repair, and [[Biology:Bone remodeling|remodeling]] of [[Biology:Bone|bone]]s of the [[Biology:Vertebrate|vertebral]] [[Biology:Skeleton|skeleton]]. The osteoclast disassembles and digests the composite of hydrated protein and [[Earth:Mineral|mineral]] at a molecular level by secreting acid and a [[Biology:Collagenase|collagenase]], a process known as ''[[Medicine:Bone resorption|bone resorption]]''. This process also helps regulate the level of blood [[Chemistry:Calcium|calcium]].

Osteoclasts are found on those surfaces of bone that are undergoing resorption. On such surfaces, the osteoclasts are seen to be located in shallow depressions called ''resorption bays (Howship's lacunae)''. The resorption bays are created by the erosive action of osteoclasts on the underlying bone. The border of the lower part of an osteoclast exhibits finger-like processes due to the presence of deep infoldings of the [[Biology:Cell membrane|cell membrane]]; this border is called ''ruffled border''. The ruffled border lies in contact with the bone surface within a resorption bay. The periphery of the ruffled border is surrounded by a ring-like zone of [[Biology:Cytoplasm|cytoplasm]] which is devoid of cell [[Biology:Organelle|organelles]] but is rich in [[Biology:Microfilament|actin filaments]]. This zone is called ''clear zone'' or ''sealing zone''. The actin filaments enable the cell membrane surrounding the sealing zone to be anchored firmly to the bony wall of Howship's lacunae. In this way, a closed subosteoclastic compartment is created between the ruffled border and the bone that is undergoing resorption. The osteoclasts secrete [[Physics:Hydrogen ion|hydrogen ions]], [[Biology:Collagenase|collagenase]], [[Biology:Cathepsin K|cathepsin K]] and hydrolytic enzymes into this compartment. Resorption of bone matrix by the osteoclasts involves two steps: (1) dissolution of inorganic components (minerals), and (2) digestion of organic component of the bone matrix. The osteoclasts pump hydrogen ions into ''subosteoclastic compartment'' and thus create an acidic microenvironment, which increases solubility of bone mineral, resulting in the release and re-entry of bone minerals into the cytoplasm of osteoclasts to be delivered to nearby capillaries. After the removal of minerals, collagenase and gelatinase are secreted into the subosteoclastic compartment. These enzymes digest and degrade collagen and other organic components of decalcified bone matrix. The degradation products are phagocytosed by osteoclasts at the ruffled border. Because of their phagocytic properties, osteoclasts are considered to be a component of the [[Biology:Mononuclear phagocyte system|mononuclear phagocyte system]] (MPS). The activity of osteoclasts is controlled by hormones and cytokines. Calcitonin, a hormone of thyroid gland, suppresses the osteoclastic activity. The osteoclasts do not have receptors for parathyroid hormone (PTH). However, PTH stimulates the osteoblasts to secrete  the cytokine called osteoclast-stimulating factor, which is a potent stimulator of the osteoclastic activity.<ref>Medical Histology by Laiq Hussain Siddiqui (6th Edition)</ref>

An odontoclast (/odon·to·clast/; o-don´to-klast) is an osteoclast associated with absorption of the roots of deciduous teeth.<ref name="The Free Dictionary">{{cite web|url=http://medical-dictionary.thefreedictionary.com/odontoclast |title=Odontoclast |publisher=Farlex, The Free Dictionary |year=2007 |access-date=2013-11-06 }}</ref><ref name=":0">{{cite journal | vauthors = Wang Z, McCauley LK | title = Osteoclasts and odontoclasts: signaling pathways to development and disease | journal = Oral Diseases | volume = 17 | issue = 2 | pages = 129–42 | date = March 2011 | pmid = 20659257 | doi = 10.1111/j.1601-0825.2010.01718.x | doi-access = free }}</ref><ref name=":1">{{cite book|vauthors=Chatterjee K|title=Essentials of Oral Histology|url=https://books.google.com/books?id=1_ge-OtrElQC&q=Odontoclast&pg=PA155|date=1 December 2006|publisher=Jaypee Brothers Publishers|isbn=978-81-8061-865-9|page=155}}{{Dead link|date=September 2023 |bot=InternetArchiveBot |fix-attempted=yes }}</ref>

==Structure==
[[File:TRAPosteoclastculture.jpg|350px|right|thumb|Tartrate resistant acid phosphatase positive osteoclast in cell culture]]
[[File:osteoclast1.jpg|350px|right|thumb|Illustrated cross-section of an activated osteoclast]]
An osteoclast is a large [[Biology:Multinucleate|multinucleated cell]] and human osteoclasts on bone typically have four nuclei<ref>{{cite journal |vauthors=Piper K, Boyde A, Jones SJ | year = 1992 | title = The relationship between the number of nuclei of an osteoclast and its resorptive capability in vitro | journal = Anatomy and Embryology | volume = 186 | issue = 4| pages = 291–9 | doi = 10.1007/BF00185977 | pmid = 1416078| s2cid = 7570118 }}</ref> and are 150–200&nbsp;µm in diameter. When osteoclast-inducing cytokines are used to convert [[Biology:Macrophage|macrophage]]s to osteoclasts, very large cells that may reach 100&nbsp;µm in diameter occur. These may have dozens of nuclei, and typically express major osteoclast proteins but have significant differences from cells in living bone because of the not-natural substrate.<ref>{{cite journal |vauthors=Basle MF, Mazaud P, Malkani K, Chretien MF, Moreau MF, Rebel A | year = 1988 | title = Isolation of osteoclasts from Pagetic bone tissue: morphometry and cytochemistry on isolated cells | journal = Bone | volume = 9 | issue = 1| pages = 1–6 | pmid = 2837260 | doi=10.1016/8756-3282(88)90020-8}}</ref><ref>{{cite journal |vauthors=Jain N, Weinstein RS | year = 2009 | title = Giant osteoclasts after long-term bisphosphonate therapy: diagnostic challenges | journal = Nat Rev Rheumatol | volume = 5 | issue = 6| pages = 341–6 | doi = 10.1038/nrrheum.2009.87 | pmid = 19491914 | pmc = 2860596 }}</ref>  The size of the multinucleated assembled osteoclast allows it to focus the ion transport, protein secretory and vesicular transport capabilities of many macrophages on a localized area of bone.

=== Location ===
In bone, osteoclasts are found in pits in the bone surface which are called resorption bays, or Howship's lacunae. Osteoclasts are characterized by a cytoplasm with a homogeneous, "foamy" appearance. This appearance is due to a high concentration of vesicles and vacuoles. These vacuoles include [[Biology:Lysosome|lysosome]]s filled with [[Biology:Acid phosphatase|acid phosphatase]]. This permits characterization of osteoclasts by their staining for high [[Biology:Gene expression|expression]] of tartrate resistant acid phosphatase (TRAP) and [[Biology:Cathepsin K|cathepsin K]]. Osteoclast rough endoplasmic reticulum is sparse, and the Golgi complex is extensive.<ref>{{cite book | veditors = Standring S | title = Gray's Anatomy | url = https://archive.org/details/graysanatomyanat0000unse | url-access = registration | edition = 39th | date = 2005 | publisher =  Elsevier }}</ref><ref>{{cite journal | vauthors = Holtrop ME, King GJ | title = The ultrastructure of the osteoclast and its functional implications | journal = Clinical Orthopaedics and Related Research | volume = 123 | issue = 123 | pages = 177–96 | year = 1977 | pmid = 856515 | doi = 10.1097/00003086-197703000-00062 | s2cid = 30010574 }}</ref><ref>{{cite journal | vauthors = Väänänen HK, Zhao H, Mulari M, Halleen JM | title = The cell biology of osteoclast function | journal = Journal of Cell Science | volume = 113 | issue = 3 | pages = 377–81 | date = February 2000 | doi = 10.1242/jcs.113.3.377 | pmid = 10639325 }}</ref>

At a site of active bone resorption, the osteoclast forms a specialized [[Biology:Cell membrane|cell membrane]], the "ruffled border", that opposes the surface of the bone tissue.  This extensively folded or ruffled border facilitates bone removal by dramatically increasing the cell surface for secretion and uptake of the resorption compartment contents and is a morphologic characteristic of an osteoclast that is actively resorbing bone.

==Development==
Since their discovery in 1873 there has been considerable debate about their origin. Three theories were dominant: from 1949 to 1970 the connective tissue origin was popular, which stated that osteoclasts and [[Biology:Osteoblast|osteoblast]]s are of the same lineage, and osteoblasts fuse together to form osteoclasts. After years of controversy it is now clear that these cells develop from the self fusion of macrophages.<ref name="Teitelbaum">{{Citation | title=Bone Resorption by Osteoclasts | journal = Science | author = S. L. Teitelbaum | year = 2000 | volume = 289 | issue = 5484 | pages = 1504–1508 |doi=10.1126/science.289.5484.1504 | pmid=10968780| bibcode = 2000Sci...289.1504T }}</ref> It was in the beginning of 1980 that the [[Biology:Mononuclear phagocyte system|monocyte phagocytic system]] was recognized as precursor of osteoclasts.<ref name="cells of bone" /> Osteoclast formation requires the presence of [[Biology:RANKL|RANKL]] (receptor activator of nuclear factor κβ ligand) and [[Biology:Macrophage colony-stimulating factor|M-CSF (Macrophage colony-stimulating factor)]]. These membrane-bound proteins are produced by neighbouring stromal cells and osteoblasts, thus requiring direct contact between these cells and osteoclast [[Biology:Precursor|precursors]].

M-CSF acts through its receptor on the osteoclast, c-fms ([[Biology:Colony stimulating factor 1 receptor|colony-stimulating factor 1 receptor]]), a transmembrane [[Biology:Tyrosine kinase|tyrosine kinase]]-receptor, leading to secondary messenger activation of tyrosine kinase Src. Both of these molecules are necessary for osteoclastogenesis and are widely involved in the differentiation of monocyte/macrophage derived cells.

RANKL is a member of the tumour necrosis family (TNF), and is essential in osteoclastogenesis. RANKL knockout mice exhibit a phenotype of [[Medicine:Osteopetrosis|osteopetrosis]] and defects of tooth eruption, along with an absence or deficiency of osteoclasts. RANKL activates NF-κβ (nuclear factor-κβ) and NFATc1 (nuclear factor of activated t cells, cytoplasmic, calcineurin-dependent 1) through [[Biology:RANK|RANK]]. NF-κβ activation is stimulated almost immediately after RANKL-RANK interaction occurs and is not upregulated. NFATc1 stimulation, however, begins ~24–48 hours after binding occurs and its expression has been shown to be RANKL dependent.

Osteoclast differentiation is inhibited by [[Biology:Osteoprotegerin|osteoprotegerin]] (OPG), which is produced by osteoblasts and binds to RANKL thereby preventing interaction with RANK. While osteoclasts are derived from the hematopoietic lineage, osteoblasts are derived from mesenchymal stem cells.<ref>{{Cite journal | doi=10.2147/DDDT.S88845| pmid=26316708| pmc=4544727| title=Paget's disease of bone: An osteoimmunological disorder?| journal=Drug Design, Development and Therapy| volume=9| pages=4695–707| year=2015| last1=Michou| first1=Laëtitia| last2=Numan| first2=Mohamed| last3=Amiable| first3=Nathalie| last4=Brown| first4=Jacques P.| doi-access=free}}</ref><ref>{{Cite journal |doi = 10.3390/ijerph120202289|pmid = 25690002|pmc = 4344726|title = Impact of Air Pollutants on Oxidative Stress in Common Autophagy-Mediated Aging Diseases|journal = International Journal of Environmental Research and Public Health|volume = 12|issue = 2|pages = 2289–2305|year = 2015|last1 = Numan|first1 = Mohamed|last2 = Brown|first2 = Jacques|last3 = Michou|first3 = Laëtitia|doi-access = free}}</ref>

==Function==

Once activated, osteoclasts move to areas of microfracture in the bone by [[Physics:Chemotaxis|chemotaxis]]. Osteoclasts lie in small cavities called Howship's lacunae, formed from the digestion of the underlying bone. The sealing zone is the attachment of the osteoclast's [[Biology:Cell membrane|plasma membrane]] to the underlying bone. Sealing zones are bounded by belts of specialized adhesion structures called [[Biology:Podosome|podosome]]s. Attachment to the bone matrix is facilitated by integrin receptors, such as αvβ3, via the specific amino acid motif Arg-Gly-Asp in bone matrix proteins, such as osteopontin. The osteoclast releases hydrogen ions through the action of [[Biology:Carbonic anhydrase|carbonic anhydrase]] ([[Chemistry:Water (molecule)|H<sub>2</sub>O]] + [[Chemistry:Carbon dioxide|CO<sub>2</sub>]] → [[Chemistry:Bicarbonate|HCO<sub>3</sub>]]<sup>−</sup> + [[Physics:Hydrogen ion|H]]<sup>+</sup>) through the ''ruffled border'' into the resorptive cavity, acidifying and aiding dissolution of the mineralized bone [[Biology:Matrix|matrix]] into [[Chemistry:Calcium|Ca<sup>2+</sup>]], H<sub>3</sub>PO<sub>4</sub>, H<sub>2</sub>CO<sub>3</sub>, water and other substances. Dysfunction of the carbonic anhydrase has been documented to cause some forms of osteopetrosis.  Hydrogen ions are pumped against a high concentration gradient by [[Physics:Proton pump|proton pump]]s, specifically a unique [[Biology:V-ATPase|vacuolar-ATPase]]. This enzyme has been targeted in the prevention of [[Medicine:Osteoporosis|osteoporosis]]. In addition, several hydrolytic enzymes, such as members of the [[Biology:Cathepsin|cathepsin]] and matrix metalloprotease (MMP) groups, are released to digest the organic components of the matrix. These enzymes are released into the compartment by [[Biology:Lysosome|lysosome]]s. Of these hydrolytic enzymes, cathepsin K is of most importance.

===Cathepsin K and other cathepsins===
[[Biology:Cathepsin K|Cathepsin K]] is a collagenolytic papain-like cysteine protease that is mainly expressed in osteoclasts, and is secreted into the resorptive pit. Cathepsin K is the major [[Biology:Protease|protease]] involved in the degradation of type I collagen and other noncollagenous proteins. Mutations in the cathepsin K gene are associated with [[Medicine:Pycnodysostosis|pycnodysostosis]], a hereditary [[Medicine:Osteopetrosis|osteopetrotic]] disease, characterised by a lack of functional cathepsin K expression. Knockout studies of cathepsin K in mice lead to an osteopetrotic phenotype, which, is partially compensated by increased expression of proteases other that cathepsin K and enhanced osteoclastogenesis.

Cathepsin K has an optimal enzymatic activity in acidic conditions. It is synthesized as a proenzyme with a molecular weight of 37kDa, and upon activation by autocatalytic cleavage, is transformed into the mature, active form with a molecular weight of ~27kDa.

Upon polarization of the osteoclast over the site of resorption, cathepsin K is secreted from the ruffled border into the resorptive pit.  Cathepsin K transmigrates across the ruffled border by intercellular vesicles and is then released by the functional '''secretory domain'''. Within these intercellular vesicles, cathepsin K, along with [[Biology:Reactive oxygen species|reactive oxygen species]] generated by TRAP, further degrades the bone extracellular matrix.

Several other cathepsins are expressed in osteoclasts including [[Biology:Cathepsin B|cathepsins B]], [[Biology:Cathepsin C|C]], [[Biology:Cathepsin D|D]], [[Biology:Cathepsin E|E]], [[Biology:Cathepsin G|G]], and [[Biology:Cathepsin L2|L]].  The function of these [[Biology:Cysteine protease|cysteine]] and aspartic proteases is generally unknown within bone, and they are expressed at much lower levels than cathepsin K.

Studies on cathepsin L knockout mice have been mixed, with a report of reduced trabecular bone in homozygous and heterozygous cathepsin L knockout mice compared to wild-type and another report finding no skeletal abnormalities.

===Matrix metalloproteinases===
The matrix metalloproteinases (MMPs) comprise a family of more than 20 zinc-dependent endopeptidases. The role of matrix metalloproteinases (MMPs) in osteoclast biology is ill-defined, but in other tissue they have been linked with tumor promoting activities, such as activation of growth factors and are required for tumor metastasis and angiogenesis.

[[Biology:MMP9|MMP9]] is associated with the bone microenvironment. It is expressed by osteoclasts, and is known to be required for osteoclast [[Biology:Cell migration|migration]] and is a powerful gelatinase. Transgenic mice lacking MMP-9 develop defects in bone development, intraosseous [[Biology:Angiogenesis|angiogenesis]], and fracture repair.

MMP-13 is believed to be involved in bone resorption and in osteoclast differentiation, as knockout mice revealed decreased osteoclast numbers, osteopetrosis, and decreased bone resorption.

MMPs expressed by the osteoclast include MMP-9, -10, -12, and -14. apart from MMP-9, little is known about their relevance to the osteoclast, however, high levels of MMP-14 are found at the sealing zone.

=== Osteoclast physiology ===
In the 1980s and 90s the physiology of typical osteoclasts was studied in detail.  With the isolation of the ruffled border, ion transport across it was studied directly in biochemical detail. Energy-dependent acid transport was verified and the postulated proton pump purified.<ref>{{cite journal | vauthors = Blair HC, Teitelbaum SL, Ghiselli R, Gluck S | title = Osteoclastic bone resorption by a polarized vacuolar proton pump | journal = Science | volume = 245 | issue = 4920 | pages = 855–7 | date = August 1989 | pmid = 2528207 | doi = 10.1126/science.2528207 | bibcode = 1989Sci...245..855B }}</ref><ref name="pmid7929181">{{cite journal | vauthors = Mattsson JP, Schlesinger PH, Keeling DJ, Teitelbaum SL, Stone DK, Xie XS | title = Isolation and reconstitution of a vacuolar-type proton pump of osteoclast membranes | journal = The Journal of Biological Chemistry | volume = 269 | issue = 40 | pages = 24979–82 | date = October 1994 | doi = 10.1016/S0021-9258(17)31485-0 | pmid = 7929181 | doi-access = free }}</ref> With the successful culture of osteoclasts, it became apparent that they are organized to support the massive transport of protons for acidification of the resorption compartment and solubilization of the bone mineral.  This includes ruffled border Cl<sup>−</sup> permeability to control membrane potential and basolateral Cl<sup>−</sup>/HCO<sub>3</sub><sup>−</sup> exchange to maintain cytosolic pH in physiologically acceptable ranges.<ref>{{cite journal | vauthors = Teti A, Blair HC, Teitelbaum SL, Kahn AJ, Koziol C, Konsek J, Zambonin-Zallone A, Schlesinger PH | title = Cytoplasmic pH regulation and chloride/bicarbonate exchange in avian osteoclasts | journal = The Journal of Clinical Investigation | volume = 83 | issue = 1 | pages = 227–33 | date = January 1989 | pmid = 2910910 | pmc = 303666 | doi = 10.1172/jci113863 }}</ref><ref name="pmid1829326">{{cite journal | vauthors = Blair HC, Teitelbaum SL, Tan HL, Koziol CM, Schlesinger PH | title = Passive chloride permeability charge coupled to H(+)-ATPase of avian osteoclast ruffled membrane | journal = The American Journal of Physiology | volume = 260 | issue = 6 Pt 1 | pages = C1315-24 | date = June 1991 | pmid = 1829326 | doi = 10.1152/ajpcell.1991.260.6.C1315 }}</ref><ref>{{cite journal | vauthors = Schlesinger PH, Blair HC, Teitelbaum SL, Edwards JC | title = Characterization of the osteoclast ruffled border chloride channel and its role in bone resorption | journal = The Journal of Biological Chemistry | volume = 272 | issue = 30 | pages = 18636–43 | date = July 1997 | pmid = 9228032 | doi = 10.1074/jbc.272.30.18636 | doi-access = free }}</ref>

The effectiveness of its ion secretion depends upon the osteoclast forming an effective seal around the resorption compartment.  The positioning of this "sealing zone" appears to be mediated by integrins expressed on the osteoclast surface.<ref>{{cite journal | vauthors = Väänänen HK, Zhao H, Mulari M, Halleen JM | title = The cell biology of osteoclast function | journal = Journal of Cell Science | volume = 113 ( Pt 3) | pages = 377–81 | date = February 2000 | issue = 3 | doi = 10.1242/jcs.113.3.377 | pmid = 10639325 }}</ref> With the sealing zone in place, the multinucleated osteoclast reorganizes itself.  Developing the highly invaginated ruffled membrane apposing the resorption compartment allows massive secretory activity.  In addition, it permits the vesicular [[Biology:Transcytosis|transcytosis]] of the mineral and degraded collagen from the ruffled border to the free membrane of the cell, and its release into the extracellular compartment.<ref>{{cite journal | vauthors = Salo J, Lehenkari P, Mulari M, Metsikkö K, Väänänen HK | title = Removal of osteoclast bone resorption products by transcytosis | journal = Science | volume = 276 | issue = 5310 | pages = 270–3 | date = April 1997 | pmid = 9092479 | doi = 10.1126/science.276.5310.270 }}</ref><ref>{{cite journal | vauthors = Nesbitt SA, Horton MA | title = Trafficking of matrix collagens through bone-resorbing osteoclasts | journal = Science | volume = 276 | issue = 5310 | pages = 266–9 | date = April 1997 | pmid = 9092478 | doi = 10.1126/science.276.5310.266 }}</ref> This activity completes the bone resorption, and both the mineral components and collagen fragments are released to the general circulation.

=== Regulation ===
Osteoclasts are regulated by several hormones, including [[Biology:Parathyroid hormone|parathyroid hormone]] (PTH) from the parathyroid gland, [[Biology:Calcitonin|calcitonin]] from the thyroid gland,  and [[Biology:Interleukin|growth factor interleukin 6]] (IL-6). This last hormone, IL-6, is one of the factors in the disease [[Medicine:Osteoporosis|osteoporosis]].  Osteoporosis occurs when there is an imbalance between the bone resorption activities of osteoclasts and the bone formation activities of osteoblasts.<ref name="Dance">{{cite journal |last1=Dance |first1=Amber |title=Fun facts about bones: More than just scaffolding |journal=Knowable Magazine |date=23 February 2022 |doi=10.1146/knowable-022222-1|doi-access=free |url=https://knowablemagazine.org/article/health-disease/2022/fun-facts-about-bones-more-just-scaffolding |access-date=8 March 2022}}</ref>

Osteoclast activity is also mediated by the interaction of two molecules produced by osteoblasts, namely [[Biology:Osteoprotegerin|osteoprotegerin]] and [[Biology:RANKL|RANK ligand]]. These molecules also regulate differentiation of the osteoclast.<ref>{{cite journal | vauthors = Schoppet M, Preissner KT, Hofbauer LC | title = RANK ligand and osteoprotegerin: paracrine regulators of bone metabolism and vascular function | journal = Arteriosclerosis, Thrombosis, and Vascular Biology | volume = 22 | issue = 4 | pages = 549–53 | date = April 2002 | pmid = 11950689 | doi = 10.1161/01.ATV.0000012303.37971.DA | doi-access = free }}</ref><ref>{{Cite journal |last1=Shirazi |first1=Sajjad |last2=Ravindran |first2=Sriram |last3=Cooper |first3=Lyndon F. |date=2022 |title=Topography-mediated immunomodulation in osseointegration; Ally or Enemy |journal=Biomaterials |volume=291 |pages=121903 |doi=10.1016/j.biomaterials.2022.121903 |issn=1878-5905 |pmid=36410109|pmc=10148651 }}</ref>

==Odontoclast==
An odontoclast (/odon·to·clast/; o-don´to-klast) is an osteoclast associated with absorption of the roots of deciduous teeth.<ref name="The Free Dictionary"/><ref name=":0"/><ref name=":1"/>

==Alternate use of term==
An osteoclast can also be an instrument used to fracture and reset bones (the origin is Greek ''osteon'': bone and ''klastos'': broken). To avoid confusion, the cell was originally termed osotoclast. When the surgical instrument went out of use, the cell became known by its present name.

== Clinical significance ==
Giant osteoclasts can occur in some diseases, including [[Medicine:Paget's disease of bone|Paget's disease of bone]] and [[Chemistry:Bisphosphonate|bisphosphonate]] toxicity.

In cats, abnormal odontoclast activity can cause feline odontoclastic resorptive lesions, necessitating extraction of the affected teeth.

Osteoclasts play a major role in orthodontic tooth movement and pathologic migration of periodontally compromised teeth.

== History ==
Osteoclasts were discovered by Kölliker in 1873.<ref name="cells of bone">{{cite journal | vauthors = Nijweide PJ, Burger EH, Feyen JH | title = Cells of bone: proliferation, differentiation, and hormonal regulation | journal = Physiological Reviews | volume = 66 | issue = 4 | pages = 855–86 | date = October 1986 | pmid = 3532144 | doi = 10.1152/physrev.1986.66.4.855 }}</ref>

== See also ==
*List of human cell types derived from the germ layers
*List of distinct cell types in the adult human body

== References ==
{{Reflist|2}}

==External links==
* [http://www.medterms.com/script/main/art.asp?articlekey=11794 MedicineNet] {{Webarchive|url=https://web.archive.org/web/20130525195120/http://www.medterms.com/script/main/art.asp?articlekey=11794 |date=2013-05-25 }}
* {{MeSH name|Osteoclasts}}
* [https://www.youtube.com/watch?v=6Cn4uusbGk8 The Life of Osteoclast]
* Random42: [https://random42.com/scientific_animation/bone-biology-osteoblasts-and-osteoclasts Animation by ''Random42 Scientific Communication'' on the role of osteoclasts in bone remodeling]

{{Myeloid blood cells and plasma}}
{{Bone and cartilage}}

[[Category:Macrophages]]
[[Category:Human cells]]
[[Category:Skeletal system]]

{{Sourceattribution|Osteoclast}}