Figure 811

Photomicrograph of an osteoclast on a bone spicule. This Mallory-stained specimen shows a spicule made of calcified cartilage (stained light blue) and a covering of bone tissue (stained dark blue). An osteoclast on the left side of the spicule has resorbed bone tissue and lies in a depression (Howship's lacuna) in the spicule. The light band between the osteoclast and the bone spicule corresponds to the ruffled border of the osteoclast. The arrows on the nongrowing surface indicate cytoplasm of inactive bone lining cells (osteoprogenitor cells). In contrast, bone is being deposited on the opposite side of the spicule, as evidenced by the presence of osteoblasts on this surface and newly formed osteocytes just below the surface of the spicule. x550.

fects on osteocytes, described above. In contrast, calcitonin, secreted by parafollicular cells of the thyroid gland, has a counterbalancing effect, reducing osteoclast activity. Little is known, however, about the role of endocrine activity in normal bone remodeling during growth.

Contrary to what was once thought, osteoclasts are not related to osteoblasts. They are derived from mononuclear hemopoietic progenitor cells, namely, CFU-GM, a cell that gives rise to the neutrophilic granulocyte and monocyte lineages, and CFU-M, a cell that gives rise to monocytes. It

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