parathyroid gland parathyroid gland parathyroid glands are small endocrine glands in the neck of humans and other tetrapods. humans usually have four parathyroid glands, located on the back of the thyroid gland in variable locations. the parathyroid gland produces and secretes parathyroid hormone in response to a low blood calcium, which plays a key role in regulating the amount of calcium in the blood and within the bones. parathyroid glandsdiagram showing structures in the human neck. the four green shaded areas represent the most common position of the parathyroid glands, which are generally four in number and situated behind the lateral lobes of the thyroid gland (shaded orange).thyroid and parathyroids as viewed from the front of the neckdetailsprecursorneural crest mesenchyme and third and fourth pharyngeal pouch endodermsystemendocrinearterysuperior thyroid artery, inferior thyroid artery,veinsuperior thyroid vein, middle thyroid vein, inferior thyroid vein,nervemiddle cervical ganglion, inferior cervical ganglionlymphpretracheal, prelaryngeal, jugulodigastric lymph nodesidentifierslatinglandula parathyreoidea inferior, glandula parathyreoidea superiormeshd010280ta98a11.4.00.001ta23870fma13890anatomical terminology parathyroid glands share a similar blood supply, venous drainage, and lymphatic drainage to the thyroid glands. parathyroid glands are derived from the epithelial lining of the third and fourth pharyngeal pouches, with the superior glands arising from the fourth pouch and the inferior glands arising from the higher third pouch. the relative position of the inferior and superior glands, which are named according to their final location, changes because of the migration of embryological tissues. hyperparathyroidism and hypoparathyroidism, characterized by alterations in the blood calcium levels and bone metabolism, are states of either surplus or deficient parathyroid function. structure the parathyroid glands are two pairs of glands usually positioned behind the left and right lobes of the thyroid. each gland is a yellowish-brown flat ovoid that resembles a lentil seed, usually about 6 mm long and 3 to 4 mm wide, and 1 to 2 mm anteroposteriorly. there are typically four parathyroid glands. the two parathyroid glands on each side which are positioned higher are called the superior parathyroid glands, while the lower two are called the inferior parathyroid glands. healthy parathyroid glands generally weigh about 30 mg in men and 35 mg in women. these glands are not visible or able to be felt during examination of the neck. each parathyroid vein drains into the superior, middle and inferior thyroid veins. the superior and middle thyroid veins drain into the internal jugular vein, and the inferior thyroid vein drains into the brachiocephalic vein. lymphatic drainage lymphatic vessels from the parathyroid glands drain into deep cervical lymph nodes and paratracheal lymph nodes. variation the parathyroid glands are variable in number: three or more small glands, and can usually be located on the posterior surface of the thyroid gland. occasionally, some individuals may have six, eight, or even more parathyroid glands. rarely, the parathyroid glands may be within the thyroid gland itself, the chest, or even the thymus. microanatomy the parathyroid glands are named for their proximity to the thyroid—and serve a completely different role than the thyroid gland. the parathyroid glands are quite easily recognizable from the thyroid as they have densely packed cells, in contrast with the follicular structure of the thyroid. two unique types of cells are present in the parathyroid gland: chief cells, which synthesize and release parathyroid hormone. these cells are small, and appear dark when loaded with parathyroid hormone, and clear when the hormone has been secreted, or in their resting state. oxyphil cells, which are lighter in appearance and increase in number with age, have an unknown function. intermediate magnification micrograph. h&e stain. the white round structures are fat cells. adipose tissue comprises 25–40% of normal parathyroid gland tissue. high magnification micrograph. h&e stain. the small, dark cells are chief cells, which are responsible for secreting parathyroid hormone. the cells with orange/pink staining cytoplasm are oxyphil cells development in the early development of the human embryo, a series of five pharyngeal arches and four pharyngeal pouches form that give rise to the face, neck, and surrounding structures. the pouches are numbered such that the first pouch is the closest to the top of the embryo's head and the fourth is the furthest from it. the parathyroid glands originate from the interaction of the endoderm of the third and fourth pouch and neural crest mesenchyme. the position of the glands reverses during embryological development. the pair of glands which is ultimately inferior develops from the third pouch with the thymus, whereas the pair of glands which is ultimately superior develops from the fourth pouch. during embryological development, the thymus migrates downwards, dragging the inferior glands with it. the superior pair are not dragged downwards by the fourth pouch to the same degree. the glands are named after their final, not embryological, positions. since the thymus's ultimate destination is in the mediastinum of the chest, it is occasionally possible to have ectopic parathyroids derived from the third pouch within the chest cavity if they fail to detach in the neck. parathyroid development is regulated by a number of genes, including those coding for several transcription factors. function