Parathyroid glands

The parathyroid glands are small endocrine glands in the neck that produce parathyroid hormone. These glands are usually located behind the thyroid gland and in rare cases are located within the thyroid glands. Most people have four parathyroid glands, but some people have six or even eight.

Anatomy
The parathyroid glands are four or more small glands located on the posterior surface of the thyroid gland. Histologically they are quite easily recognizable from the thyroid as they have densely packed cells in contrast with the follicle structure of the thyroid. However at surgery they are harder to differentiate from the thyroid or fat.

They distinguish themselves from the thyroid gland histologically as they contain two types of cells:

History
The parathyroid glands were discovered by Ivar Viktor Sandström (1852-1889), a Swedish medical student, in 1880. It was the last major organ to be recognized in humans.

Physiology
The sole function of the parathyroid glands is to maintain the body's calcium level within a very narrow range, so that the nervous and muscular systems can function properly.

When blood calcium levels drop below a certain point, calcium-sensing receptors in the parathyroid gland are activated to release hormone into the blood.

Parathyroid hormone (PTH, also known as parathormone) is a small protein that takes part in the control of calcium and phosphate homeostasis, as well as bone physiology. Parathyroid hormone has effects antagonistic to those of calcitonin. It increases blood calcium levels by stimulating osteoclasts to break down bone and release calcium. It also increases gastrointestinal calcium absorption by activating vitamin D, and promotes calcium uptake by the kidneys.

Embryology and evolution
The parathyroid glands originate from the interaction of neural crest mesenchyme and third and fourth branchial pouch endoderm.

Genetically, Eya-1 (transcripitonal co-activator), Six-1 (a homeobox transcription factor), and Gcm-2 (a transcription factor) have been associated with the development of the parathyroid gland, and alterations in these genes alters parathyroid gland development.

The conserved homology of genes and calcium sensing receptors in fish gills with those in the parathryroid glands of birds and mammals is recognized by evolutionary developmental biology as evolution using genes and gene networks in novel ways to generate new structures with some similar functions and novel functions.

The superior parathyroids arise from the fourth pharyngeal pouch and the inferior parathyroids arise from the third pharyngeal pouch. They are vertically transposed during embyrogenesis. This is significant in function-preserving parathyroidectomy because the superior parathyroids are supplied by the inferior thyroid artery and the inferior parathyroids are supplied by the superior thyroid artery. If the surgeon is to leave a single functional parathyroid for the patient, they must preserve the appropriate blood supply.