Amphibians living partially or totally in a terrestrial environment ar
e the first tetrapods to possess parathyroid glands. Purely aquatic am
phibians and amphibian larvae lack these endocrine glands. The parathy
roids develop at the time of metamorphosis. The parathyroid glands in
caecilians consist of a single cell type, that of urodeles may be comp
osed of basal (supporting) cells and suprabasal (chief) cells, and tha
t of anurans of small and large chief cella Parathyroid glands of caec
ilians and anurans lack connective tissue, blood vessels, and nerves.
The parathyroid cells become activated in response to decreased blood
calcium concentration and undergo changes indicating increased parathy
roid hormone secretion. Increased blood calcium concentration suppress
es secretory activity. Usually, parathyroidectomy elicits hypocalcemia
in most amphibians. Such operations have no effect in lower urodeles.
Parathyroid hormone administration provokes hypercalcemia in most amp
hibians. The parathyroids of caecilians have not been studied in detai
l. The urodeles and anurans exhibit seasonal changes in the parathyroi
d glands. These changes may be initiated by environmental stimuli such
as light, temperature, or alterations in blood calcium levels caused
by natural hibernation. (C) 1995 Wiley-Liss, Inc.