The parathyroid glands are small endocrine glands closely associated with the thyroid. They are ovoid, a few millimeters in diameter, and arranged in two pairs, constituting the superior and inferior parathyroid glands. They are usually located in the connective tissue on the posterior surface of the lateral lobes of the thyroid gland. However, the number and location may vary. In 2 to 10% of individuals, additional glands are associated with the thymus.
Structurally, each parathyroid gland is surrounded by a thin connective tissue capsule that separates it from the thyroid. Septa extend from the capsule into the gland to divide it into poorly defined lobules and to separate the densely packed cords of cells. The connective tissue is more evident in the adult, with the development of fat cells that increase with age and ultimately constitute as much as 60 to 70% of the glandular mass.
The glands receive their blood supply from the inferior thyroid arteries or from anastomoses between the superior and inferior thyroid arteries. Typical of endocrine glands, rich networks of fenestrated blood capillaries and lymphatic capillaries surround the parenchyma of the parathyroids.
Parathyroid glands develop from the endodermal cells derived from the third and fourth branchial pouches
Embryologically, the inferior parathyroid glands (and the thymus) are derived from the third branchial pouch; the superior glands, from the fourth branchial pouch. Normally, the inferior parathyroids separate from the thymus and come to lie below the superior parathyroids. Failure of these structures to separate results in the atypical association of the parathyroids with the thymus in the adult. The principal (chief) cells differentiate during embryonic development and are functionally active in regulating fetal calcium metabolism. The oxyphil cells differentiate later at puberty.
Principal cells and oxyphil cells constitute the epithelial cells of the parathyroid gland
• Principal (chief) cells, the more numerous of the parenchymal cells of the parathyroid (Fig. 20.15), are re-
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