parathyroid glands

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parathyroid glands

(pâr'əthī`roid), four small endocrine bodies, located behind the thyroid gland, that govern calcium and phosphorus metabolism. These four masses of tissue (each about the size of a pea) are difficult to distinguish from the thyroid and are often embedded in it. Consequently, before their significance was known they were sometimes accidently removed during thyroid surgery, causing a deficiency in parathormone, the parathyroid hormone. Parathormone increases the concentration of calcium ions in the blood, with accompanying bone absorption and increased reabsorption of calcium ions by the kidneys. The hormone's effect on phosphate ion concentration is the opposite, i.e., phosphate ion concentration in the bloodstream decreases as a result of increased phosphate excretion by the kidneys. Excessive secretion of parathormone, e.g., caused by tumor of the parathyroid glands, is a serious disorder, for excessive blood calcium can cause kidney stones and long-term weakening of the bones. Undersecretion of parathormone, which can be caused by congenital and metabolic disorders, results in too little calcium in the bloodstream, and too much phosphorus. The result is tetany, i.e., violent muscle spasms.

Parathyroid Glands

 

organs of internal secretion in man and vertebrates, excluding fish. A number of mammals, such as mice, rats, moles, shrews, hedgehogs, swine, and seals, have one pair of parathyroid glands, while bats, dogs, rabbits, cats, guinea pigs, camels, sheep, goats, and man have two pairs, located on the surface of the thyroid gland or embedded in its tissue.

The parathyroids consist of glandular epithelial tissue, including chief and oxyphile cells arranged in clusters and bands between the capillaries, covered with a connective-tissue capsule. The chief cells are polygonal in shape; their cytoplasm contains a large number of mitochondria, is weakly basophilic, and stains with difficulty. The cytoplasm of oxyphile cells is readily stained by acid dyes. Both types of cells contain special corpuscles consisting of endoplasmic reticular plates, which are probably the centers of synthesis in the cells.

The parathyroid glands produce parathyroid hormone (parathormone), which helps regulate calcium and phosphorus metabolism. There are reciprocal relations between the calcium and phosphorus concentrations in the blood. Calcium and phosphorus homeostasis is maintained by the influence of parathyroid hormone on bone tissue and on the kidneys. An excess of the hormone causes demineralization of the bone tissue and the excretion of calcium and phosphorus from the body. Excess phosphorus is excreted by the kidneys. Softening of bones and their eventual spontaneous fracture occur in hyperparathyroidism; tooth development is slowed in hypoparathyroidism. The administration of parathyroid hormone relieves the symptoms of parathyroid deficiency. Removal of the glands results in convulsions (tetany), caused by the sharp decrease in the blood’s calcium concentration (from 9–11 to 4.5–5 mg percent). Blood phosphorus increases at the same time. Attacks of tetany may occur in animals with normal parathyroids when their food contains insufficient calcium. The size and functional condition of the parathyroids vary with the blood calcium level.

REFERENCES

Leites, S. M., and N. N. Lapteva. Ocherki po patofiziologii obmena veshchestv i endokrinnoi sistemy. Moscow, 1967.
Eskin, I. A. Osnovy fiziologii endokrinnykh zhelez. Moscow, 1968.
Clegg, P., and A. Clegg. Gormony, kletki, organizm. Moscow, 1971. (Translated from English.)

V. M. SAMSONOVA