MORPHOLOGY AND PHYSIOLOGICAL SIGNIFICANCE OF PARATHYROID-GLANDS IN REPTILIA

Adult reptiles possess one or two pairs of parathyroid glands that hav e been shown in many species to derive from the third and fourth phary ngeal pouches, respectively. Up to five pairs may develop during early embryonic life. Excess glands may involute during late embryogenesis. The location of the parathyroid glands differs in the various species . As a general rule, they lie just anterior to the heart, the anterior pair (parathyroid III) being associated with the carotid artery, the posterior pair (parathyroid IV) with the aortic arch. In snakes, howev er, the anterior pair (parathyroid III) is associated with the carotid artery near the angle of the jaw. As shown by light microscopy and, t o a lesser extent, by electron microscopy, the parathyroid parenchyma comprises secretory cells which may form dark and light variants, occa sional oxyphil cells, and stellate cells. They are arrangend in cords separated by connective tissue containing a capillary network. Parathy roid secretory cells often form follicles which might be the result of degeneration. Degeneration may occur as a form of involution during w inter in species undergoing seasonal changes. The product of parathyro id cells, the parathyroid hormone, is responsible for the maintenance of blood calcium concentration. The sites of action-bones, kidneys, in testine, endolymphatics, and dermal. skeleton-are not well understood or not investigated. In some turtles, parathyroid hormone is not the ( main) factor for the regulation of calcium homeostasis. (C) 1995 Wiley -Liss, Inc.