Like any other connective tissue, bone consists of cells, fibers, and ground substance. There are four principal types of bone cells (fig. 7.3):
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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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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.