Anterior pituitary
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.
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.
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..
