Pituitary Gland/Hypophysis

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Pituitary Gland/Hypophysis

Small gland located in the sella turcica of sphenoid bone, just below third ventricle, 1.3cm in diametre. Suspended by a stalk (the infundibulum) to the hypothalamus, which is located immediately above the pituitary. Has two lobes of separate embryological origin – posterior/neurohypophysis (mostly neural tissue; releases neurohormones) and anterior/adenohypophysis (makes up 75% of gland; mostly glandular tissue).

Anterior pituitary hormones:
Growth hormone (GH):
• polypeptide protein (191 amino acids)
• stimulated by growth hormone releasing hormone (GHRH)  triggered by release of low serum levels of GH. Secondary triggers – hypoglycaemia, increases in serum levels of amino acids, low levels of fatty acids  initiate secretion of growth hormone inhibiting hormone (GHIH)
• inhibited by negative feedback from levels of GH and IGF-1; also by hyperglycaemia, hyperlipidaemia, obesity
• acts on liver and other tissues  release insulin like growth factor-1 (IGF-1)
• IGF-1 stimulates growth of most tissues, promotes protein synthesis and reduces glucose utilisation

Thyroid stimulating hormone (TSH)/thyrotropin:
• glycoprotein
• stimulated by thyrotropin releasing hormone, pregnancy and cold temperature
• inhibited by increased levels of thyroid hormone
• effect is to stimulate thyroid gland to increase iodine uptake, synthesis and release thyroid hormone – also stimulates growth of thyroid gland

Adrenocorticotropic hormone (ACTH)/corticotropin:
• polypeptide of 38 amino acids
• stimulated by corticotropin releasing hormone (CRH)  release stimulated by fever, hypoglycaemia
• inhibited by feedback from levels of glucocorticoids
• promotes adrenal cortex release of glucocorticoids and androgens

Follicle stimulating hormone (FSH):
glycoprotein
stimulated by gonadotropin releasing hormone (GnRH)
inhibited by levels of oestrogen (females) and testosterone and inhibin (males)
stimulates ovarian follicle maturation and oestrogen production (females) or sperm production (males)

Luteinising hormone (LH):
• glycoprotein
• stimulated by GnRH
• inhibited by oestrogen and progesterone (females) and testosterone (males)
• stimulates ovulation and ovarian release of oestrogen and progesterone (females) and testosterone production (males)

Prolactin (PRL):
• polypeptide protein
• stimulated by prolactin releasing hormone (PRH)  stimulated by oestrogens, opiates, breast feeding
• inhibited by prolactin inhibiting hormone (PIH)
• promotes lactation after parturition

Posterior pituitary hormones:
Oxytocin:
• peptide
• stimulated from impulses from hypothalamic neurons  respond to uterine stretching and suckling of infant at breast
• inhibited by lack of neural impulse
• stimulates uterine contractions, initiates labour, initiates milk secretion

Antidiuretic hormone (ADH)/vasopressin:
• nine peptide protein
• produced in the neurosecretory cells of hypothalamus  transported down axons  stored in terminals until released
• stimulated from impulses from hypothalamic neurons  respond to increased osmolarity of blood or decreased blood volume
• inhibited by adequate hydration, alcohol
• stimulate kidney tubule cells to reabsorb/conserve water

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