parathyroid gland

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An endocrine organ usually associated with the thyroid gland and possessed by all vertebrates except the fishes. In response to lowered serum calcium concentration, a hormone is produced which promotes bone destruction and inhibits the phosphorus-conserving activity of the kidneys.

In humans, there are typically four glands, however, the number varies between three and six, with four appearing about 80% of the time. Variations in the positioning of the glands along the craniocaudal axis occur but, excepting parathyroid III which may occasionally be found upon the anterior surface of the trachea, the relation to the posterior surface of the thyroid is rarely lost.

The parathyroid glands are essential for the regulation of calcium and phosphate concentrations in the extracellular fluids of amphibians and higher vertebrates. Parathyroid hormone has two major target organs, bone and kidney. It acts on bone in several ways. Short-term changes include a rapid uptake of bone fluid calcium into osteoblast cells, which in turn pump the calcium into the extracellular fluids. Long-term effects include increased activity and number of osteoclasts, bone cells which act to break down bone matrix and release calcium from bone. All of these effects result in increased blood calcium values. See also Bone; Calcium metabolism.

Parathyroid hormone inhibits the renal reabsorption of phosphate, thus increasing the urinary output of phosphate. Phosphate reabsorption across the renal tubule is dependent upon sodium transport, and parathyroid hormone interferes with this sodium-dependent phosphate transport in the proximal tubule. Another important effect of parathyroid hormone on the kidney is to increase the renal reabsorption of calcium, thus reducing the loss of calcium in the urine and conserving calcium in the body. See also Kidney.

Finally, there are reports that parathyroid hormone indirectly stimulates calcium uptake into the body across the intestine. Parathyroid hormone stimulates the production of the most active metabolite of vitamin D, 1,25-dihydroxycholecalciferol, during vitamin D synthesis. This metabolite of vitamin D directly stimulates the intestinal absorption of calcium. See also Endocrine system (vertebrate); Parathyroid hormone; Vitamin D.

These small but vitally necessary groups of cells, usually four of them, lie on the back of the thyroid gland in the neck. They are endocrine glands — meaning that they deliver their secretion directly into the passing blood. The peptide parathyroid hormone (or parathormone) will therefore reach the whole of the rest of the body, but attaches only to those cells which have receptors for it. The activities which it then modifies in these ‘target’ cells all work towards an increase in the concentration of calcium ions in the extracellular fluid compartment of the body — the blood plasma and the tissue (interstitial) fluid. It is necessary for a great variety of physiological functions — indeed for virtually all of them — that this concentration remains within quite narrow limits.

Where and how, then, would a hormone need to act in order to promote the addition of calcium into the body pool?

(i) Calcium from food is absorbed from the intestine: parathyroid hormone promotes conversion of Vitamin D to its active form; this in turn enhances intestinal absorption of calcium.
(ii) Calcium is stored in bone mineral: parathyroid hormone stimulates resorption from bone into the blood by activating the osteo-clasts, which break down bone and mobilize the minerals from it.
(iii) When the blood passes through the kidneys some of its calcium ions escape into the urine: parathyroid hormone enhances reabsorption by acting on the renal tubule cells.

An appropriate concentration of calcium ions in body fluids is necessary for every transfer of a stimulus from nerve to muscle, and for the contraction of muscle of all types, including the beating of the heart and the regulation of the diameter of blood vessels. The translocation of calcium in and out of cells, and in and out of storage by chemical binding within cells, is also crucial for the secretory activity of glandular cells, producing both external and internal secretions — sweat, milk, saliva, insulin, cortisol, and all the rest, including parathyroid hormone itself.

The parathyroid glands ‘know’ how much hormone to secrete, because their activity is regulated by the concentration of ionized calcium in the blood that flows through them: a rise in calcium inhibits secretion of the hormone and vice versa (a negative feedback mechanism).

Parathyroid hormone thus keeps calcium concentration up — otherwise known as a hypercalcaemic effect. There is also another hormone keeping calcium down: calcitonin, secreted by the ‘C-cells’ within the thyroid gland.

Parathyroid deficiency is rare; it can occur acutely if the glands are inadvertently removed along with the thyroid or with cancerous neck glands. Rarely also, a parathyroid tumour can cause excess of the hormone, resulting in decalcification and cysts in the bones, and kidney problems due to the high concentration of calcium in the filtered blood.

— Sheila Jennett

See also calcium; hormones.

One of four small endocrine glands on the hack of the thyroid gland. It secretes parathormone (PTH or parathyroid hormone), which affects calcium and phosphate levels in extracellular fluid. High levels of PTH cause calcium to be transferred from bones to blood; abnormally low levels result in a lowering of blood calcium levels, which can lead to tetany. An overproduction of PTH has been linked to the development of fragile bones in a group of female long-distance runners.

A set of four small glands on the undersurface of the thyroid gland that function in the endocrine system. The parathyroid glands secrete a hormone that regulates the metabolism of calcium and phosphorus.

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Parathyroid gland
Endocrine system. (Parathyroid gland not pictured, but are present on surface of thyroid gland, as shown below.)
Thyroid and parathyroid.
Latin	glandula parathyroidea inferior, glandula parathyroidea superior
Gray's	subject #273 1271
Artery	superior thyroid artery, inferior thyroid artery,
Vein	superior thyroid vein, middle thyroid vein, inferior thyroid vein,
Nerve	middle cervical ganglion, inferior cervical ganglion
Precursor	neural crest mesenchyme and third and fourth pharyngeal pouch endoderm

The parathyroid glands are small endocrine glands in the neck that produce parathyroid hormone. Humans have four parathyroid glands, which are usually located behind the thyroid gland, and, in rare cases, within the thyroid gland or in the chest. Parathyroid glands control the amount of calcium in the blood and within the bones.

Contents 1 Anatomy 2 History 3 Physiology 4 Role in disease 5 Embryology and evolution 6 Additional images 7 References 8 External links

Anatomy

Micrograph of a parathyroid gland. H&E stain.

The parathyroid glands are four or more small glands located on the posterior surface (back side) of the thyroid gland. The parathyroid glands are named for their proximity to the thyroid but serve a completely different role than the thyroid gland. They are quite easily recognizable from the thyroid as they have densely packed cells, in contrast with the follicle structure of the thyroid. ^[1] However, at surgery, they are harder to differentiate from the thyroid or fat.

In the histological sense, they distinguish themselves from the thyroid gland, as they contain two types of cells:^[2]

Name	Staining	Quantity	Size	Function
parathyroid chief cells	darker	many	smaller	manufacture PTH (see below).
oxyphil cells	lighter	few	larger	function unknown.[3]

History

The parathyroid glands were first discovered in the Indian Rhinoceros by Richard Owen in 1850.^[4] The glands were first discovered in humans by Ivar Viktor Sandström (1852-1889), a Swedish medical student, in 1880.^[5] It was the last major organ to be recognized in humans.

Physiology

The major 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. PTH increases blood calcium levels by stimulating osteoclasts to break down bone and release calcium. PTH also increases gastrointestinal calcium absorption by activating vitamin D, and promotes calcium uptake by the kidneys. PTH affects the perception of well being and absence of PTH can be associated with feeling of fatigue and anxiety.

Role in disease

Main article: Parathyroid disease

Many conditions are associated with disorders of parathyroid function. These can be divided into those causing hyperparathyroidism, and those causing hypoparathyroidism.

Embryology and evolution

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

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 both the superior and the inferior parathyroids are supplied by the inferior thyroid artery. If the surgeon is to leave a single functional parathyroid for the patient, he/she must preserve the appropriate blood supply.

Additional images

High magnification micrograph of a parathyroid gland. H&E stain.	Intermediate magnification micrograph of a parathyroid gland. H&E stain.	Low magnification micrograph of a parathyroid gland and parathyroid adenoma (bottom left). H&E stain.	Scheme showing development of branchial epithelial bodies. I, II, III, IV. Branchial pouches.
Human parathyroid glands

References

1. ^ Histology at BU 15001ooa
2. ^ Histology at BU 15002loa
3. ^ Histology at OU 40_06
4. ^ Cave, A.J.E. (1953). "Richard Owen and the discovery of the parathyroid glands". in E. Ashworth Underwood. Science, Medicine and History. Essays on the Evolution of Scientific Thought and Medical Practice. 2. Oxford University Press. pp. 217-222. 
5. ^ Eknoyan G (November 1995). "A history of the parathyroid glands". Am. J. Kidney Dis. 26 (5): 801–7. doi:10.1016/0272-6386(95)90447-6. PMID 7485136. http://linkinghub.elsevier.com/retrieve/pii/0272638695904476. 

External links

• Parathyroid disease and treatments discussed in layman's terms at Parathyroid.com
• Parathyroid Surgery and minimal access parathyroid surgery by a UK surgeon
• Endocrine Web at endocrineweb.com
• The origin of the parathyroid gland at pnas.org
• Parathyroid+gland at eMedicine Dictionary
• Virtual Slidebox at Univ. Iowa Slide 149

v • d • e Human anatomy, endocrine system: endocrine glands (TA A11, GA 11.1269) Islets of pancreas Alpha cell · Beta cell · Delta cell · PP cell · Epsilon cell Hypothalamic/ pituitary axes +parathyroid Pituitary Posterior pituitary Pars nervosa · Median eminence · Infundibular stalk · Pituicyte · Herring bodies Anterior pituitary Pars intermedia · Pars tuberalis · Pars distalis Acidophils (Somatotropes, Lactotropes) · Basophils (Corticotropes, Gonadotropes, Thyrotropes) · Chromophobe Thyroid axis Thyroid gland (Parafollicular cell, Thyroid epithelial cell, Thyroid isthmus, Lobes of thyroid gland, Pyramid of thyroid) Parathyroid gland (Oxyphil cell, Chief cell) Adrenal axis: Adrenal gland Medulla Chromaffin cells Cortex Zona glomerulosa · Zona fasciculata · Zona reticularis Paraganglia Organ of Zuckerkandl · Aortic body · Carotid body Gonadal axis Gonad: Testes · Ovaries · Corpus luteum Pineal gland Pinealocyte · Corpora arenacea endocrine navs: anat/physio/dev/hormones, noncongen/congen/neoplasia, symptoms+signs/eponymous, proc

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