Osteoclast

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Osteoclast

Osteoblasts and osteoclasts on trabecula of lower jaw of calf embryo.
Gray's	subject #18 88
Dorlands/Elsevier	o_08/12600400

An osteoclast (from the Greek words for "bone" and "broken") is a type of bone cell that removes bone tissue by removing the bone's mineralized matrix. This process is known as bone resorption. Osteoclasts and osteoblasts are instrumental in controlling the amount of bone tissue. Osteoblasts form bone; osteoclasts resorb bone. Osteoclasts are formed by the fusion of cells of the monocyte-macrophage cell line.<ref>Netter, p. 169</ref>

1 Morphology
2 Formation
3 Regulation
4 Use in orthodonture
5 Alternate use of term
6 See also
7 Notes
8 References
9 External link

[edit] Morphology

Image:Osteoclast.jpg

An osteoclast is a large cell that is characterized by multiple nuclei and a cytoplasm with a homogenous, "foamy" appearance. This appearance is due to a high concentration of vesicles and vacuoles.<ref>Holtrop</ref><ref>Vaananen, p. 378</ref> At a site of active bone resorption, the osteoclast forms a specialized cell membrane, the "ruffled border", which touches the bony surface.<ref>Netter, p. 169</ref> The ruffled border facilitates removal of the bone matrix. The mineral portion of the matrix (called hydroxyapatite) includes calcium and phosphate ions. These ions are absorbed into small vesicles (see endocytosis) which move across the cell and eventually are released into the extracellular fluid, thus increasing levels of the ions in the blood.

[edit] Formation

Osteoclasts are formed when RANK on osteoclast precursors are activated by RANK Ligand on the surface of a stromal cell. Once activated, they move to areas of microfracture in the bone by chemotaxis. Osteoclasts lie in a small cavity called Howship's lacuna, formed from the digestion of the underlying bone. The sealing zone is the attachment of the osteoclast's plasmalemma to the underlying bone. The osteoclast releases hydrogen ions (H₂O + CO₂ → HCO₃^- + H⁺) into the cavity, acidifying and dissolving the mineralized bone matrix into Ca²⁺, H₃PO₄, H₂CO₃ and water. Hydrogen ions are pumped against a high concentration gradient by proton pumps, specifically a unique vacuolar-ATPase. This enzyme has been targeted in the prevention of osteoporosis. In addition, several hydrolytic enzymes are released to digest the organic components of the matrix, such as Collagenase and Cathepsin K. These enzymes are released into the compartment by lysosomes.

[edit] Regulation

Osteoclasts are regulated by several hormones, including parathyroid hormone (PTH) from the Parathyroid gland, calcitonin from the Thyroid gland, and growth factor interleukin 6 (IL-6). This last hormone, IL-6, is one of the factors in the disease osteoporosis, which is an imbalance between bone resorption and bone formation.

[edit] Use in orthodonture

Osteoclastic activity is also the basis for straightening of the teeth with dental braces. The pressure exerted on the periodontal ligament of the tooth (by the braces) causes osteoclasts to absorb the alveolar bone. Osteoblasts then relay the bone to coincide with less pressure on the tooth, thus manipulating of the orientation of the tooth.

[edit] 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.

[edit] See also

Odontoclast

[edit] Notes

[edit] References

Hankermeyer, C.R., et al., Dissolution rates of carbonated hydroxyapatite in hydrochloric acid. Biomaterials, 2002. 23(3): p. 743-750. PMID 11771694

Holtrop, M. E. and G. J. King (1977), "The ultrastructure of the osteoclast and its functional implications", Clinical Orthopaedics and Related Research, 123: 177-196