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PARATHYROID
The parathyroid glands are small endocrine glands in the neck, usually located behind the thyroid gland, which produce parathyroid hormone. In rare cases the parathyroid glands are located within the thyroid glands. Most often there are four parathyroid glands but some people have six or even eight.
Anatomy
The parathyroid glands are four small glands located on the posterior (closer to the back) surface of the thyroid gland. They distinguish themselves from the thyroid gland histologically as they contain two types of cells- parathyroid chief cells and oxyphil cells. Quite easily recognizable from the thyroid as it has densly packed cell as contrasted by the follicle structure of the thyroid.
Physiology
Parathyroid hormone is a small protein that takes part in the control of calcium and phosphorus homeostasis, as well as bone physiology. When blood calcium levels drop below a certain point, calcium-sensing receptors in the parathyroid gland are activated to release hormone into the blood. It then stimulates osteoclasts to break down bone and release calcium into the blood. The sole purpose of the parathyroid glands is to regulate the calcium level in our bodies within a very narrow range so that the nervous and muscular systems can function properly.
Role in disease
Disorders of the parathyroid hormone receptor have been associated with Jansen's metaphyseal chondroplasia and Blomstrand's chondroplasia.
The single major disease of parathyroid glands is overactivity of one or more of the parathyroid lobes which make too much parathyroid hormone causing a potentially serious calcium imbalance. This is called hyperparathyroidism; it leads to hypercalcemia and osteitis fibrosa cystica. Since hyperparathyroidism was first described in 1925, the symptoms have become known as "moans, groans, stones, and bones." The primary treatment for this disease is the surgical removal of the faulty gland.
A Sestamibi scan is often used to determine which parathyroid gland(s) are responsible for overproduction of parathyroid hormone.
Embryology and Evolution
The parathyroid glands originate from endoderm of the third and fourth pharyngeal pouches.
A study published in the Proceedings of the National Academy of Sciences linked a regulatory gene to the formation of both the parathyroid glands in terrestrial vertebrates and gills in fish. Furthermore, two genes involved in calcium ion regulation were identified to be active in both structures. Thus both structures are thought to be homologous; derived from a common ancestor. Their location in the body thus becomes corroborating evidence.
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