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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 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.
The parathyroid glands are two pairs of glands usually positioned behind the left and right lobes of the thyroid. 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. 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 vessels from the parathyroid glands drain into deep cervical lymph nodes and paratracheal lymph nodes. 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.
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. Intermediate magnification micrograph.
The white round structures are fat cells. High magnification micrograph. The small, dark cells are chief cells , which are responsible for secreting parathyroid hormone. 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 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.
Parathyroid development is regulated by a number of genes , including those coding for several transcription factors. The major function of the parathyroid glands is to maintain the body's calcium and phosphate levels within a very narrow range, so that the nervous and muscular systems can function properly. The parathyroid glands do this by secreting parathyroid hormone PTH. Parathyroid hormone 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. Parathyroid disease is conventionally divided into states where the parathyroid is overactive hyperparathyroidism , and states where the parathyroid is under- or hypoactive hypoparathyroidism. Both states are characterised by their symptoms, which relate to the excess or deficiency of parathyroid hormone in the blood. Hyperparathyroidism is the state in which there is excess parathyroid hormone circulating.
This may cause bone pain and tenderness, due to increased bone resorption. Due to increased circulating calcium, there may be other symptoms associated with hypercalcemia , most commonly dehydration. Hyperparathyroidism is most commonly caused by a benign proliferation of chief cells in single gland, and rarely MEN syndrome. This is known as primary hyperparathyroidism ,  which is generally managed by surgical removal of the abnormal parathyroid gland.
Renal disease may lead to hyperparathyroidism. When too much calcium is lost, there is a compensation by the parathyroid, and parathyroid hormone is released. The glands hypertrophy to synthesise more parathyroid hormone. This is known as secondary hyperparathyroidism. If this situation exists for a prolonged period of time, the parathyroid tissue may become unresponsive to the blood calcium levels, and begin to autonomously release parathyroid hormone.
This is known as tertiary hyperparathyroidism. The state of decreased parathyroid activity is known as hypoparathyroidism. This is most commonly associated with damage to the glands or their blood supply during thyroid surgery — it may be associated with rarer genetic syndromes such as DiGeorge syndrome , which is inherited as an autosomal dominant syndrome. Hypoparathyroidism will occur after surgical removal of the parathyroid glands.
Occasionally, an individual's tissues are resistant to the effects of parathyroid hormone. This is known as pseudohypoparathyroidism. In this case the parathyroid glands are fully functional, and the hormone itself is not able to function, resulting in a decrease in blood calcium levels. Pseudohypoparathyroidism is often associated with the genetic condition Albright's hereditary osteodystrophy. Pseudo pseudohypoparathyroidism, one of the longest words in the English language , is used to describe an individual with Albright's hereditary osteodystrophy; with normal parathyroid hormone and serum calcium levels.
Hypoparathyroidism may present with symptoms associated with decreased calcium , and is generally treated with Vitamin D analogues. Parathyroid glands are found in all adult tetrapods ; they vary in their number and position.
Mammals typically have four parathyroid glands, while other types of animals typically have six. The removal of parathyroid glands in animals produces a condition resembling acute poisoning with irregular muscle contractions.
Fish do not possess parathyroid glands; several species have been found to express parathyroid hormone. Developmental genes and calcium-sensing receptors in fish gills are similar to those within the parathyroid glands of birds and mammals.
It has been suggested that the tetrapod glands may have been evolutionarily derived from these fish gills. Scheme showing development of branchial epithelial bodies. Branchial pouches. From Wikipedia, the free encyclopedia. Parathyroid glands Diagram 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.
See also: Parathyroid hormone. Main article: Parathyroid disease. Main article: Hyperparathyroidism. Main article: Hypoparathyroidism. Williams, Peter L; Warwick, Roger eds. Gray's Anatomy 36th ed. Churchill Livingstone. Journal of Clinical Pathology. Mitchell; illustrations by Richard; Richardson, Paul Gray's anatomy for students. Jacob; dissections by David J. Hinchcliffe; photography by Mick A. Turton; illustrated by Amanda Human anatomy: a clinically-orientated approach New ed. Edinburgh: Churchill Livingstone.
Anat Sci Int. George Wheater's functional histology: a text and colour atlas 5th ed. August Human embryology 3rd ed. Philadelphia, Pa. Current Opinion in Nephrology and Hypertension. Guyton, John E. Textbook of medical physiology 11th ed. Philadelphia: W. Davidson's principles and practice of medicine 21st ed. Endocrine surgery. Georgetown TX: Landes Bioscience. Ashworth Underwood ed. Science, Medicine and History. Oxford University Press. Retrieved American Journal of Kidney Diseases.
Journal of the American Medical Association. Current Surgery.
Parathyroid Glands: Facts, Function & Disease
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 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.
Extirpación de las glándulas paratiroideas
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Though they are small, the parathyroid glands are big players in the endocrine system. They have a very important purpose: keeping bones strong, the nervous system running and the muscles pumping. The parathyroids are four mustard-colored glands that function as one. They are located behind the thyroid, which is a gland that stretches across the front of the neck, below the voice box. Each parathyroid gland is around the size of a pea, or 3 to 5 millimeters in diameter and 30 to 60 milligrams in weight, according to the University of Michigan.