Parathyroid glands of amphibians develop from the pharyngeal pouches and are located adjacent to the external jugular vein and hypoglossal nerve. They are composed of whorls of compact epithelial cells covered by a vascular capsule of connective tissue. Possibly two cell types are present: (a) peripheral clear cells, which are implicated in elaboration of hormone and contain secretory granules, and (b) dense reserve cells. During winter, the parathyroids undergo degeneration, characterized by morphological details which vary with the species; this is followed by regeneration in the spring. Parathyroidectomy may cause hyperexcitability, but rarely does tetanic death occur. Parathyroidectomized amphibians, like mammals, become hypocalcemic and hyperphosphatemic. In contrast to higher vertebrates, however, parathyroprivic anurans show an elevated urine phosphorus, which is presently interpreted as a reflection of the increased plasma load due to absence of a hormonally-induced, soft-tissue accumulation of phosphate. Further, parathyroidectomized frogs are hypercalciuric, possibly because of the lack of hormone-mediated mesonephric reabsorption. In both mammals and amphibians, treatment with exogenous parathormone induces hypercalcemia, hypercalciuria, and hypophosphatemia. The absence of change in the urine phosphorus content of hormone-treated frogs, associated with its decline in plasma, may be a consequence of metabolic variations based upon the heterothermic nature of the Amphibia.

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Parathyroid glands of amphibians. I. Parathyroid structure and function in the amphibian, with emphasis on regulation of mineral ions in body fluids.

The ultimobranchial body of Rana pipiens. 8. Effects of extirpation upon calcium distribution and bone cell types.

The comparative physiology of calcium regulation in submammalian vertebrates.

Ultrastructural evaluation of parathyroid and ultimobranchial glands in iguanas with experimental nutritional osteodystrophy.

Garter snakes, Thamnophis sirtalis, have four parathyroid glands that are similar histologically to those of other reptiles. A high percentage of the animals also possess accessory parathyroid tissue. Complete parathyroidectomy of the snakes produced severe depression of serum calcium values, tetanic convulsions, and significant elevation of serum phosphate within several weeks of the operation. Administration of mammalian parathyroid extract to normal snakes had the opposite effects, elevating serum calcium and depressing serum phosphate concentrations. In contrast, mammalian calcitonin preparations did not significantly affect serum calcium or phosphate values.



The parathyroid glands of snakes thus function to maintain normal neuromuscular coordination by regulating blood calcium and phosphate concentrations.

D.J. McWhinnie

Papers

Parathyroid extract effects on phosphorus metabolism in Rana pipiens.

Influence of parathyroid extract on blood and urine mineral levels in iguanid lizards