Prenatal development of parathyroid gland in Sheep (Ovis aries)

At 25 days of gestation, the external parathyroid gland was observed at the floor of the pharynx while internal parathyroid was yet to be differentiated. At 44 days of gestation, the external parathyroid was surrounded by a delicate capsule, while the internal parathyroid was embedded within the thyroid gland without a separate capsule. Two types of chief cells were observed in the parenchyma of both external and internal parathyroid glands. At 85 days of gestation, the external parathyroid parenchyma of the gland consisted of cords of chief cells. Type I chief cells had compact nucleus and type II chief cells showed vesicular nucleus which were arranged to form semi circles. These cords of cells were surrounded by numerous sinusoidal capillaries. By 128 days of gestation, the external parathyroid gland was surrounded by a thicker connective tissue capsule, capillaries were numerous and spaces surrounding the cords of parenchymal cells were much reduced compared to group I and II.