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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 hypophyseal portal system, 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). The granules of acidophils will specifically stain with azocarmine . 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) in response to TRH.
- gonadotropes secrete luteinizing hormone (LH) and follicle-stimulating hormone (FSH) in response to GnRH.
- corticotropes secrete adrenocorticotropic hormone (ACTH) in response to CRH.
- Acidophils:
- lactotropes secrete prolactin in response to PRH.
- somatotropes secrete growth hormone in response to SRH.
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 (PRL) | 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 medial preoptic and arcuate nuclei and stimulates the secretion of LH and FSH.
- Thyrotropin-releasing hormone (TRH) is expressed in parvocellular neuroendocrine neurons in the paraventricular and anterior hypothalamic nuclei, and stimulates the secretion of TSH.
- Corticotropin-releasing hormone (CRH, also known as corticotropin-releasing factor, CRF) is expressed in parvocellular neuroendocrine neurons in the paraventricular nucleus. CRH 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..
"Anterior pituitary"