Ultrastructure of the parathyroid gland.

Since the first detailed description of the parathyroid glands, reported in human beings by Sandstrom in 1880, and Lever first described the ultrastructure of the parathyroid chief cells of rat in 1957, a large number of histological and ultrastructural studies have been done on the parathyroid glands of numerous animal species under normal and experimental conditions. The biological function of the parathyroid glands is to raise serum calcium concentrations by multiple actions on the bone, kidney and gut. The parathyroid glands recognized in al1 vertebrate animals higher than fishes arise from the third and fourth branchial pouches. Severa1 animals, such as the newt. lizard, gecko, mouse, rat, hamster and gerbil, have only two parathyroid glands, but most animals have four (Shoumura and Isono, 1985). In mammals, most of the parathyroid glands are closely associated with the thyroid gland, but in amphibians, reptiles and birds, the glands separate from the thyroid gland (Shoumura and Isono, 1985). In the love birds. the parathyroid glands are situated close to the carotid body (Shoumura, 1974), and in rabbits having two pairs of the parathyroid glands, two parathyroid glands (one pair) separate from the thyroid gland (Shournura and Isono, 1985). In some mammals, the parenchymal cells composing the parathyroid gland are classified under a light microscope into two main types of cells: chief cells and oxyphil cells (Bargmann, 1939), and examinations under an electron microscope also show chief cells as having many cell organelles and oxyphil cells filled with