Hormones Of The Adenohypophysis:
Pars Distalis: Chromophils
Acidophils produce the following hormones:
SOMATOTROPIN, also called "growth hormone," and abbreviated GH or STH in some references. This is a peptide hormone, synthesized by somatotropes. It has a general growth enhancing effect on tissues. It promotes fat mobilization, protein synthesis in muscles, and inhibition of glucose utilization. It has a specific effect on epiphyseal plate cartilage in bones, and an excess of somatotropin causes giantism in growing animals. After closure of the epiphyses occurs, production of somatotropin can lead to acromegaly, with progressive enlargement of the head, face, hands and feet. A deficiency in early years leads to dwarfism.
PROLACTIN, sometimes abbreviated PRL, is another peptide. It's synthesized by the mammatropes (which are also called prolactin cells and/or luteotropes in some references). In mammals, this hormone stimulates the production of milk, and it may, in pregnancy, cause enlargement of the breasts. Ultrastructural changes in the RER and Golgi apparatus of the mammatropes have been observed to coincide with the cycling of pregnancy. After an infant has been weaned, the mammatropes undergo involution, and the excess prolactin remaining in them is destroyed by the lysosomal enzymes of the mammatropes themselves in a process called crinophagy (literally, "secretion eating").
Basophils produce the following hormones:
THYROTROPIN, abbreviated TSH or TRH in some references, also called thyrotropic hormone or thyroid stimulating hormone. This hormone is a glycoprotein, rather than a simple peptide, which accounts for the PAS+ reaction in the cells that make it. These cells are called thyrotropes. Thyrotropin acts to stimulate the production of thyroxin in the thyroid gland, and to increase the size of the gland itself. Animals under severe long term cold stress (for example, sled dogs or wolves in the Arctic) may show increased activity of the thyrotropes and the gland itself, since thyroxin has a general effect of increasing metabolic rate. Deficiency of thyrotropin leads to primary hypothyroidism.
FOLLICLE-STIMULATING HORMONE and LUTEINIZING HORMONE are glycoproteins produced by the gonadotropes of the pars distalis. They are usually abbreviated FSH and LH. Some authorities believe that there are two subcategories of gonadotropes, one for each hormone; but labeled antibody studies have localized both hormones in the same cell, and the question of whether there are two populations is not settled. FSH stimulates Graafian follicles in the ovary, and assists in follicular maturation and the secretion of estradiol, in females. In males, it stimulates the epithelium of the seminiferous tubules and is partially responsible for initiating spermatogenesis. LH is also sometimes called interstitial cell stimulating hormone (ICSH). It stimulates the final ripening of follicles, their rupture to release an egg, their secretion of progesterone, and conversion of the ruptured follicle into a corpus luteum. In males, it stimulates the production of testosterone by the interstitial cells of the testis.
Pars Distalis: Chromophobes
The hormone ACTH, adrenocorticotropic hormone, is secreted by the corticotropes of the pars distalis. This is a large peptide, which governs the nutrition and growth of the adrenal cortex. It stimulates the production of the glucocorticoids by the adrenal gland. These cells also produce LIPOTROPHIC HORMONE, LPH, which is further processed to yield beta endorphin in some species.
The pars intermedia secretes MELANOCYTE STIMULATING HORMONE, abbreviated MSH, as its principal product. This is sometimes called intermedin. It's a large peptide with some structural similarities to ACTH, and there are in fact some cells in the pars intermedia which are structurally identical to corticotropes. This part of the pituitary also produces ACTH and corticotropin-like intermediate lobe peptide, mercifully abbreviated as CLIP and beta endorphin. The main hormone, MSH, causes the increased production and distribution of melanin by melanocytes, and excess production results in a darkening of the skin. It's thought that the increased skin pigmentation in pregnancy and in Addison's disease results from an excess of MSH and ACTH.
The pars tuberalis is present in all vertebrates that have been studied, but no hormone product has been localized to it.
The cells of the pars nervosa are associated with, but don't actually produce, two hormones: OXYTOCIN and VASOPRESSIN. These are the only hormones of the neurohypophysis which have specific target organs outside the pituitary gland. Oxytocin is a peptide with nine amino acids. It causes contraction of the smooth muscle of the uterus at the commencement of labor, and is sometimes therapeutically administered for this purpose. It also causes contraction of the myoepithelial cells of the mammary gland to eject milk.
Vasopressin is also a nona-peptide, and differs from oxytocin only at positions 3 and 8 in the chain. It's the same hormone as antidiuretic hormone, called ADH. Vasopressin also causes the contraction of smooth muscle (to cut off flow through capillary beds) and increased permeability in the walls of the collecting tubules of the kidney. This causes increased absorption of water and a consequent decrease in the volume of urine that is produced. Both of these hormones are actually produced by neurons, and transported to the neurohypophysis for release. Oxytocin is produced in the paraventricular nucleus, and vasopressin in the supraoptic nucleus of the brain. There is now evidence that oxytocin is also produced by the interstitial cells of the testis. Its function in males is unknown.
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