Osteoclasts (Figure 22–16) are large, multinucleated cells that originate, like macrophages, from hematopoietic stem cells in the bone marrow (discussed later
Figure 22–14 Deposition of bone matrix by osteoblasts. osteoblasts lining the surface of bone secrete the organic matrix of bone (osteoid) and are converted into osteocytes as they become embedded in this matrix. the matrix calcifies soon after it has been deposited. the osteoblasts themselves are thought to derive from osteogenic stem cells that are closely related to fibroblasts.
Figure 22–15 Trabecular and compact bone. (a) low-magnification scanning electron micrograph of trabecular bone in a vertebra of an adult man. the soft marrow tissue has been dissolved away. (b) a slice through the head of the femur, with bone marrow and other soft tissue likewise dissolved away, reveals the compact bone of the shaft and the trabecular bone in the interior. because of the way in which bone tissue remodels itself in response to mechanical load, the trabeculae become oriented along the principle axes of stress within the bone. (a, courtesy of alan boyde; b, from J.b. kerr, atlas of Functional histology. mosby, 1999.) to matrix matrix of osteoclast
Figure 22–16 Osteoclasts. (a) drawing of an osteoclast in cross section. this giant, multinucleated cell erodes bone matrix. the “ruffled border” is a site of secretion of acids (to dissolve the bone minerals) and hydrolases (to digest the organic components of the matrix). osteoclasts vary in shape, are motile, and often send out processes to resorb bone at multiple sites. they develop from monocytes and can be viewed as specialized macrophages. (b) an osteoclast on bone matrix, seen by scanning electron microscopy. the osteoclast has been crawling over the matrix, eating it away, and leaving a trail of pits where it has done so.  (a, from R.v. krstic,´ ultrastructure of the mammalian Cell: an atlas. berlin: Springer-verlag, 1979; b, courtesy of alan boyde.) in this chapter). The precursor cells are released into the bloodstream and collect at sites of bone resorption, where they fuse to form the multinucleated osteoclasts, which cling to surfaces of the bone matrix and eat it away. Osteoclasts are capable of tunneling deep into the substance of compact bone, forming cavities that are then invaded by other cells. A blood capillary grows down the center of such a tunnel, and the walls of the tunnel become lined with a layer of osteoblasts (Figure 22–17). These osteoblasts lay down concentric layers of new matrix, which gradually fill the cavity, leaving only a narrow canal surrounding the new blood vessel. At the same time as some tunnels are filling up with bone, others are being bored by osteoclasts, cutting through older concentric systems.
Figure 22–17 The remodeling of compact bone. osteoclasts acting together in a small group excavate a tunnel through the old bone, advancing at a rate of about 50 μm per day. osteoblasts enter the tunnel behind them, line its walls, and begin to form new bone, depositing layers of matrix at a rate of 1–2 μm per day. at the same time, a capillary sprouts down the center of the tunnel. the tunnel eventually becomes filled with concentric layers of new bone, with only a narrow central canal remaining. each such canal, besides providing a route of access for osteoclasts and osteoblasts, contains one or more blood vessels that transport the nutrients the bone cells require for survival. typically, about 5–10% of the bone in a healthy adult mammal is replaced in this way each year. (after Z.F.G. Jaworski, b. duck and