Anterior pituitary gland ( or adenohypophysis )
The anterior pituitary is stimulated to release hormones due to the action of ‘ releaser ’ hormones , which are excreted by the hypothalamus and travel via the blood to the anterior pituitary .
Six hormones are released from the anterior pituitary ( Figure 2 ):
FOLLICLE STIMULATING HORMONE ( FSH ) In females , FSH secretion is stimulated by gonadotropin-releasing hormone ( GRH ) from the hypothalamus . FSH stimulates germ cells in the ovary to develop into mature follicular cells , each of which contains an ovum . This leads to secretion of oestrogen , and this increased level of oestrogen in the blood inhibits secretion of further FSH by negative feedback .
Follicle stimulating hormone ( FSH )
Luteinising hormone ( LH )
Adrenocorticotropic hormone ( ACTH )
Thyroid stimulating hormone ( TSH )
Prolactin
Interstitial cell stimulating hormone ( ICSH )
Growth hormone ( aka somatotropin )
LUTEINISING HORMONE ( LH ) Secretion of this hormone is stimulated by the presence of oestrogen . LH causes ovulation ( release of the ovum ) before causing the remaining follicular tissue to change into corpus luteum which , in turn , secretes progesterone ( Davidson & Stabenfeldt , 2020a ).
ADRENOCORTICOTROPIC HORMONE ( ACTH ) stimulates the adrenal cortex to secrete corticosteroids . Over-secretion of ACTH leads to the endocrine disorder hyperadrenocorticism ( Cushing ’ s disease ) while under-production results in hypoadrenocorticism ( Addison ’ s disease ).
THYROID STIMULATING HORMONE ( TSH ) This hormone stimulates the release of thyroxine from the thyroid glands .
PROLACTIN This hormone acts on the mammary glands during the second half of pregnancy . Prolactin stimulates the development of glandular tissues and production of milk in preparation for feeding neonates . Levels of prolactin remain high post-partum to ensure the supply of milk is maintained .
INTERSTITIAL CELL STIMULATING HORMONE ( ICSH ) In males , secretion of this hormone is in response to GRH . ICSH stimulates the interstitial cells in the testes to secrete male sex hormones , most importantly testosterone .
GROWTH HORMONE ( AKA SOMATOTROPIN ) This hormone is of particular importance in growing animals and acts on a variety of tissues . Growth hormone affects the rate of epiphyseal growth ( in bones ), inhibits the formation of fat and increases the rate of amino acid uptake for protein synthesis . Under-secretion of growth hormone in young animals results in dwarfism , while over-secretion leads to gigantism . Over-secretion in adults results in acromegaly .
Figure 2 . The FLAT PIG mnemonic used by the author to remember the hormones released from the anterior pituitary gland .
Posterior pituitary gland ( or neurohypophysis )
The posterior pituitary stores and releases hormones secreted by the hypothalamus . The two hormones released by the posterior pituitary are :
OXYTOCIN This hormone acts during parturition and in the post-partum period . Release of oxytocin is controlled by a reflex arc as a result of the foetus passing through the cervix . Oxytocin acts on the smooth muscle in the uterus , strengthening the uterine contractions to expel the foetus . This hormone is also responsible for stimulating milk let-down ; it acts on the smooth muscles of the mammary glands to contract and release the milk ( Davidson & Stabenfeldt , 2020b ), after prolactin has stimulated the mammary glands to produce this milk . Injections of oxytocin can be administered both in cases of dystocia to assist with parturition and post-partum to release milk in cases where the neonates are having problems feeding .
‘ Oxytocin acts on the smooth muscle in the uterus , strengthening the uterine contractions to expel the fetus . This hormone is also responsible for stimulating milk let-down ’
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