Cells

Like any other connective tissue, bone consists of cells, fibers, and ground substance. There are four principal types of bone cells (fig. 7.3):

  1. Osteogenic10 cells occur in the endosteum, the inner layer of the periosteum, and in the central canals. They are stem cells that arise from embryonic fibroblasts. Osteogenic cells multiply continually and some of them differentiate into the osteoblasts described next. Osteoblasts are nonmitotic, so the only source of new osteoblasts is mitosis and differentiation of the osteogenic cells.
  2. Osteoblasts11 are bone-forming cells that synthesize the organic matter of the bone matrix and help to mineralize the bone. They line up in rows in the endosteum and inner layer of periosteum and resemble a cuboidal epithelium on the bone surface. Stress and fractures stimulate accelerated mitosis and differentiation of osteogenic cells, and therefore a rapid rise in the number of osteoblasts.
  3. Osteocytes are former osteoblasts that have become trapped in the matrix they deposited. They reside in tiny cavities called lacunae,12 which are connected to each other by slender channels called

Chapter 7 Bone Tissue 221

canaliculi13 (CAN-uh-LIC-you-lye). Each osteocyte has delicate cytoplasmic processes that reach into the canaliculi to meet the processes of neighboring osteocytes. The processes of neighboring osteocytes are joined by gap junctions, which allow osteocytes to pass nutrients and chemical signals to each other and to transfer wastes to the nearest blood vessels for disposal. Osteocytes also communicate by gap junctions with the osteoblasts on the bone surface. Osteocytes play no significant role in depositing or resorbing bone. They are strain sensors; when they detect strain in a bone (deformation in response to stress), they communicate this information to osteoblasts at the surface. Osteoblasts then deposit bone where needed—for example, building up bone in response to weight-bearing exercise—and they chemically signal osteoclasts (the cells discussed next) to remove bone elsewhere.

4. Osteoclasts14 are bone-dissolving cells found on the bone surface. They develop from the same bone marrow cells that produce monocytes of the blood. Several of these marrow cells fuse with each other to form an osteoclast; thus osteoclasts are unusually large (up to 150 ^m in diameter) and typically have 3 or 4 nuclei, but sometimes up to 50. The side of the osteoclast facing the bone has a ruffled border with many deep infoldings of the plasma membrane, increasing its surface area. Osteoclasts often reside in little pits called resorption bays (Howship15 lacunae) that they have etched into the bone surface.

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Essentials of Human Physiology

Essentials of Human Physiology

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