Electron micrograph of an osteoclast. This micrograph shows a segment of bone surface (B) and a portion of an osteoclast that is in apposition to the partially digested bone. The resorption front (RF) of the osteoclast possesses numerous infoldings of the plasma membrane. When viewed in the light microscope, these infoldings are evident as the ruffled border. When the plane of section is parallel to the infoldings (asterisks), a broad, nonspecialized expanse of cytoplasm is seen. The cytoplasm of the osteoclast contains numerous mitochondria (M), lysosomes, and Golgi apparatus, all of which are functionally linked with the resorption and degradation of the bone matrix. In the upper part of the figure, some collagen fibrils are evident; the arrows indicate where 68-nm cross-banding is visible, x 10,000.
is thought that osteoclasts arise by fusion of either CFU-GM cells or CFU-M cells. In both origin and function, they are closely related to macrophages. Morphologically, osteoclasts resemble Langhans' giant cells formed by fusion of tissue macrophages (histiocytes).
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