Page 279 - Atlas of Histology with Functional Correlations

FIGURE 7.27 ■ Cancellous bone: sternum (decalcified bone, transverse
section). Stain: hematoxylin and eosin. ×64.

FUNCTIONAL CORRELATIONS 7.4 ■ Bone

Bones are dynamic structures. They are continually renewed, or remodeled,
in response to the mineral needs of the body, mechanical stress, thinning as a
result of age or disease, and fracture healing. Calcium and phosphate are

either stored in the bone matrix or released into the blood to maintain proper
levels. Maintenance of normal blood calcium levels is critical to life, because
calcium is essential for muscle contraction, blood coagulation, cell
membrane permeability, transmission of nerve impulses, and numerous other

functions.

Hormones regulate both the calcium release into the bloodstream and its
deposition in the bones. When the calcium level falls below normal,
parathyroid hormone (PTH), released from the parathyroid glands,

indirectly promotes an increase in osteoclast proliferation and osteoclast
activity by stimulating osteoblasts to produce osteoclast-stimulating
(differentiating) factors. This action induces increased breakdown of bone
matrix by the osteoclasts and release of calcium. In addition, parathyroid
hormone also increases calcium reabsorption in the kidneys and small

intestine. These hormonal effects increase and/or maintain the calcium levels
in the blood at normal levels. When the calcium level increases above
normal, a hormone called calcitonin, released by parafollicular cells or C

cells in the thyroid gland, decreases osteoclast activity, bone reabsorption,
and blood calcium levels. In addition, the kidneys increase their excretion of

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