Parathyroid glands: The parathyroid glands are small, yellowish-brown, ovoid or lentiform structures, usually lies between the posterior lobar borders of the thyroid gland and its capsule. They are commonly 6 mm long, 3-4 mm across, and 1-2 mm from back to front, each weighing about 50 mg. Usually, there are two on each side, superior and inferior. Variations: There may be only three or many minute parathyroid islands scattered in connective tissue near the usual sites. Normally the inferior parathyroids migrate only to the inferior thyroid poles, but they may descend with the thymus into the thorax or not descend at all, remaining above their normal level near the carotid bifurcation. To help identification, the anastomotic connection between the superior and inferior thyroid arteries along the posterior border of the thyroid gland usually passes very close to the parathyroids. The inferior pair are more variably situated, and may be within the fascial thyroid sheath, below the inferior thyroid arteries and near the inferior lobar poles; or outside the sheath, immediately above an inferior thyroid artery; or in the thyroid gland near its inferior pole. The superior parathyroids are usually dorsal, the inferior parathyroids ventral, to the recurrent laryngeal nerves. Vasculature: The parathyroid glands have a rich blood supply from the inferior thyroid arteries or from anastomoses between the superior and inferior vessels. Approximately one-third of human parathyroid glands have two or more parathyroid arteries. Lymph vessels are numerous and associated with those of the thyroid and thymus glands. Nerve supply:The nerve supply is sympathetic, either direct from the superior or middle cervical ganglia or via a plexus in the fascia on the posterior lobar aspects. Parathyroid activity is controlled by variations in blood calcium level: it is inhibited by a rise and stimulated by a fall. The nerves are believed to be vasomotor but not secretomotor Histology : Contains two types of cells Active cells or chief cells which produce parathormone. Chief cells differ ultrastructurally according to their level of activity: active chief cells have large Golgi complexes with numerous vesicles and small membrane-bound granules. Glycogen granules are most abundant in inactive cells, which appear histologically as ‘clear' cells. In normal human parathyroid glands, inactive chief cells outnumber active cells in a ratio of 3-5:1 Second cell type, the oxyphil (eosinophil) cell, appears just before puberty and increases in number with age. Oxyphil cells are larger than chief cells and contain more cytoplasm, which stains deeply with eosin. Their nuclei are smaller and more darkly staining than those of chief cells, and their cytoplasm is unusually rich in mitochondria. The functional significance of oxyphil cells and their relationship to chief cells are uncertain