Morphological observations on the parathyroid of the opossum (Didelphis virginiana)

Parathyroid III of the opossum, Didelphis virginiana, lies on the medial surface of the internal carotid artery at its origin just above the bifurcation of the common carotid artery. It is limited externally by a capsule and is subdivided into lobule-like structures by connective tissue septae. The parenchyma consists solely of chief (principal) cells, arranged in irregular cords and clumps supported by a delicate vascular connective tissue. Ultrastructurally, the chief cells are polyhedral in shape and have centrally placed nuclei. The cytoplasm contains small, randomly distributed, rod-shaped mitochondria, scattered profiles of granular endoplasmic reticulum, free ribosomes, and small perinuclear Golgi complexes. Secretory granules are small, electron-dense, and appear to be limited by a delicate membrane. The granules are related both to the Golgi complex and the basal cell membrane. Capillaries that lie immediately adjacent to the chief cells show an extensively fenestrated endothelium.