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.
Clinical significance
Giant osteoclasts can occur in some diseases, including Pagets disease of bone and bisphosphonate toxicity. In cats, abnormal odontoclast activity can cause feline odontoclastic resorptive lesions, necessitating extraction of the affected teeth. History
Osteoclasts were discovered by Kolliker in 1873. See also
List of human cell types derived from the germ layers

References
External links
MedicineNet
Osteoclasts at the US National Library of Medicine Medical Subject Headings (MeSH)
The Life of Osteoclast
Random42: Animation by Random42 Scientific Communication on the role of osteoclasts in bone remodeling