Anterior pituitary gland

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The anterior pituitary (also called the adenohypophysis) comprises the anterior lobe of the pituitary gland and is part of the endocrine system. Under the influence of the hypothalamus, the anterior pituitary produces and secretes several peptide hormones that regulate many physiological processes including stress, growth, and reproduction.

Anatomy and development

The adenohypophysis is a pea-size gland anterior to the neurohypophysis, caudal to the hypothalamus, and sits in the medial aspect of the brain. Its blood is supplied by the superior and inferior hypophyseal arteries, and it secretes hormones into the hypophyseal vein.

The anterior pituitary is derived from the ectoderm of the roof of the embryonic mouth. An outpocketing of this ectoderm forms Rathke's pouch, which pinches off from the oral ectoderm and fuses with the developing posterior pituitary. The anterior wall of Rathke's pouch becomes the pars distalis and pars tuberalis, while the posterior wall gives rise to the pars intermedia. Together, the pars distalis, tuberalis, and intermedia comprise the anterior pituitary.

Histology

When stained by PAS, three main types of secretory cell can be seen: chromophobes (which don't take on any stain), basophils (which stain purple, and are not related to the blood cell), and acidophils (stain orange). About half of the cells are chromophobes, 40% are acidophils, and 10% are basophils.

Basophilic staining cells produce the hormones that specifically act on other endocrine organs, such as TSH, ACTH, FSH and LH. Acidophils produce growth hormone and prolactin. Chromophobes are presumed to be "empty" cells, having already released their granules of hormone.

There are also supporting glial like cells called folliculostellate cells.

Physiology

Five different cell types are recognised by the hormone they produce:

• Basophils:
  • thyrotropes secrete thyroid-stimulating hormone (TSH)
  • gonadotropes secrete luteinizing hormone (LH) and follicle-stimulating hormone (FSH)
  • corticotropes secrete adrenocorticotropic hormone (ACTH)
• Acidophils:
  • lactotropes secrete prolactin
  • somatotropes secrete growth hormone.

Electron microscopy and immunohistochemistry allow further identification of the hormone produced by the cells.

Major hormones secreted

Hormone	Target	Effect
Adrenocorticotropic hormone (ACTH)	Adrenal gland	Secretion of glucocorticoids
Follicle-stimulating hormone (FSH)	Ovaries, Testes	Growth of reproductive system
Growth hormone (GH)	Liver, Adipose Tissue	Promotes growth; lipid & carbohydrate metabolism
Luteinizing hormone (LH)	Ovaries, Testes	Sex hormone production
Prolactin	Ovaries, mammary glands	Secretion of estrogens/progesterone; milk production
Thyroid-stimulating hormone (TSH)	Thyroid gland	Secretion of thyroid hormones

Hypothalamic releasing and release-inhibiting factors

Hormone secretion from the anterior pituitary gland is regulated by neurons of the hypothalamus. Neuroendocrine neurons in the hypothalamus project axons to the median eminence, at the base of the brain. At this site, the neurons can release substances into small blood vessels that travel directly to the anterior pituitary gland (the hypothalamo-hypophysial portal vessels).

• Somatostatin is expressed in neuroendocrine neurons of the periventricular nucleus and inhibits the secretion of growth hormone

• Growth hormone-releasing hormone (GHRH) is expressed in arcuate nucleus neuroendocrine neurons, and stimulates the secretion of growth hormone.

• Gonadotropin-releasing hormone (GnRH, also known as luteinising-hormone releasing hormone, LHRH) is expressed in neuroendocrine neurons in the preoptic/septal area, and stimulates the secretion of LH and FSH.

• Thyrotropin-releasing hormone (TRH) is expressed in parvocellular neuroendocrine neurons in the paraventricular nucleus, and stimulates the secretion of TSH.

• Corticotropin-releasing hormone (CRF) is expressed in parvocellular neuroendocrine neurons in the paraventricular nucleus. CRF and vasopressin, from an overlapping population of parvocellular neuroendocrine neurons are stimulators of ACTH secretion. These two factors have synergistic effects on ACTH secretion.

• Dopamine from neuroendocrine neurons of the arcuate nucleus inhibits prolactin secretion.

fr:Anté-hypophyse