Function
Once activated, osteoclasts move to areas of microfracture in the bone by chemotaxis. Osteoclasts lie in small cavities called Howships lacunae, formed from the digestion of the underlying bone. The sealing zone is the attachment of the osteoclasts plasma membrane to the underlying bone. Sealing zones are bounded by belts of specialized adhesion structures called podosomes. 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 carbonic anhydrase (H2O + CO2 → HCO3− + H+) through the ruffled border into the resorptive cavity, acidifying and aiding dissolution of the mineralized bone matrix into Ca2+, H3PO4, H2CO3, 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 proton pumps, specifically a unique vacuolar-ATPase. This enzyme has been targeted in the prevention of osteoporosis. In addition, several hydrolytic enzymes, such as members of the cathepsin and matrix metalloprotease (MMP) groups, are released to digest the organic components of the matrix. These enzymes are released into the compartment by lysosomes. Of these hydrolytic enzymes, cathepsin K is of most importance. Cathepsin K and other cathepsins
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 protease involved in the degradation of type I collagen and other noncollagenous proteins.
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. 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 Howships 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 lysosomes filled with acid phosphatase. This permits characterization of osteoclasts by their staining for high expression of tartrate resistant acid phosphatase (TRAP) and cathepsin K. Osteoclast rough endoplasmic reticulum is sparse, and the Golgi complex is extensive.At a site of active bone resorption, the osteoclast forms a specialized 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 osteoblasts are of the same lineage, and osteoblasts fuse together to form osteoclasts.