Figure 2015

Photomicrograph of human parathyroid gland. This H&E-stained specimen shows the gland with part of its connective tissue capsule (Cap). The blood vessels (BV) are located in the connective tissue septum between lobes of the gland. The principal cells are arranged in two masses (top and bottom) and are separated by a large cluster of oxyphil cells (center). The oxyphil cells are the larger cell type with prominent eosinophilic cytoplasm. They may occur in small groups or in larger masses, as seen here. The principal cells are more numerous. They are smaller, having less cytoplasm, and consequently exhibit closer proximity of their nuclei. Adipose cells (AC) are present in variable, though limited, numbers. x175.

sponsible for the secretion of PTH. They are small, polygonal cells, with a diameter of 7 to 10 pan and a centrally located nucleus. The pale-staining, slightly acidophilic cytoplasm contains lipofuscin-containing vesicles, large accumulations of glycogen, and lipid droplets. Small, dense, membrane-limited vesicles seen with the TEM or after using special stains with the light microscope are thought to be the storage form of PTH.

• Oxyphil cells constitute a minor portion of the parenchymal cells and are not known to have a secretory role. They are found singly or in clusters; the cells are more rounded, considerably larger than the principal cells, and have a distinctly acidophilic cytoplasm (see Fig. 20.15). Mitochondria, often with bizarre shapes and sizes, almost fill the cytoplasm and are responsible for the strong acidophilia of these cells. No secretory vesicles and little if any rER are present. Cytoplasmic inclusion bodies consist of occasional lysosomes, lipid droplets, and glycogen distributed among the mitochondria.

PTH regulates calcium and phosphate levels in the blood

The parathyroids function in the regulation of calcium and phosphate levels. PTH, or parathormone, is essential for life. Therefore, care must be taken during thyroidectomy to leave some functioning parathyroid tissue. If the glands are totally removed, death will ensue because muscles, including the laryngeal and other respiratory muscles, go into tetanic contraction as the blood calcium level falls.

PTH is an 84-amino acid linear peptide (Table 20.8). It binds to a specific PTH receptor on target cells that interacts with G protein to activate a second-messenger system. PTH release causes the level of calcium in the blood to increase. Simultaneously, it reduces the concentration of serum phosphate. Secretion of PTH is regulated by the serum calcium level through a simple feedback system. Low levels of serum calcium stimulate secretion of PTH; high levels of serum calcium inhibit its secretion. PTH functions at several sites:

• Bone resorption is stimulated by PTH. The hormone activates osteolysis by osteoclasts during which calcium

TABLE 20.8. Parathyroid Hormone

Major Functions

Parathyroid hormone Polypeptide con- Principal (chief Increases blood calcium level in three ways: (1) promotes calcium (parathormone, PTH) taining 84 amino cells)* release from bone (increases relative number of osteoclasts), (2) acts acids on kidney to stimulate calcium reabsorption by distal tubule while inhibiting phosphate reabsorption in the proximal tubule, (3) increases formation of hormonally active 1,25-dihydroxychoIecalciferoI (1,25-(OH)2 vitamin D3)in the kidney, which promotes tubular reabsorption of calcium

^Some evidence suggests that oxyphil cells, which first appear in the parathyroid gland at about 4 to 7 years of age and increase in number after puberty, may also produce PTH.

Hormone Composition Source and phosphate are both released from calcified bone matrix into the extracellular fluid.

• Kidney excretion of calcium is decreased by PTH stimulation of tubular reabsorption, thus conserving calcium.

• Urinaiy phosphate excretion is increased by PTH secretion, thus lowering phosphate concentration in the blood and extracellular fluids.

• Kidney conversion of25-OH vitamin D3 to hormonally active l,2S-(OH)2 vitamin D, is regulated primarily by PTH, which stimulates activity of 1-«-hydroxylase and increases the production of active hormone.

• Intestinal absorption of calcium is increased under the influence of PTH. Vitamin D,, however, has a greater effect than PTH on intestinal absorption of calcium.

PTH and calcitonin have reciprocal effects in the regulation of blood calcium levels

Although PTH increases blood calcium levels, the peak increase following its release is not reached for several hours. PTH appears to have a rather slow, long-term homeostatic action. Calcitonin, however, rapidly lowers blood calcium levels and has its peak effect in about 1 hour; therefore, it has a rapid, acute homeostatic action.

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