The parathyroid glands of two species of dolphin--Risso's dolphin, Grampus griseus, and bottlenose dolphin, Tursiops truncatus.

Although there have been many reports regarding the structure of the parathyroid glands of various terrestrial mammals, little is known about the parathyroid glands of marine mammals including Cetacea. The morphology of the parathyroid glands of three Risso's dolphins, Grampus griseus (about 3 m in length and 300 kg in weight), and three bottlenose dolphins, Tursiops truncatus (about 3 m in length and 300 kg in weight), was examined macroscopically and microscopically. The dolphins examined in the present study had two or four parathyroid glands that varied in size and location on the thyroid gland. Each parathyroid gland was encapsulated by fibrous tissue on the dorsal surface of the thyroid gland, and was divided into several lobules by interlobular connective tissue which contained numerous capillaries. The parenchymal cells consisted of pale staining chief cells. Each cell was polygonal and about 15 microm in diameter, and had one round or oval nucleus. Oxyphil cells were not found. Considering their greater body size, the parathyroid glands were rather small. By electron microscopic observation, the parathyroid gland of the bottlenose dolphin had sparse granular endoplasmic reticulum, poorly developed Golgi complexes, and abundant secretory granules in the cytoplasm of the chief cells. These results support a possibility that the activity of the parathyroid gland is suppressed to adapt to a sea habitat.