15-The Hypothalamic Pituitary Axis & Growth Hormone

Question 1

• what do the hypothalamus and pituitary gland form & what is their function? What part of the pit. gland is physically connected to the hypo?
• what are the parts of the pituitary gland, what are they also known as and what are their embryological origins?
• what is the function of the posterior & anterior pituitary gland? How many pathways do hormones produced by the hypo act via?

Answer 1

-form a functional unit that serves as the major link between nervous & endocrine systems. Involved in body growth, milk secretion, lactation, reproduction, water homeostasis. -the posterior pituitary gland is connected to the hypo via the infundibulum (pit. stalk)
-1)anterior/adenohypophysis=from evagination of oral ectoderm (primitive gut tissue). 2)posterior/neurohypophysis=from neuroectoderm (primitive brain tissue)
-posterior=oxytocin & ADH produced by neurosecretory cells in supraoptic and paraventricular nuclei of the hypo. Transported down nerve axons to the post. pit, stored and released into blood to act on distal areas (neurocrine func). DOES NOT SYNTHESISE THESE HORMONES, ONLY RELEASE
anterior=hormones made in hypo transported down axons, stored in median eminence then released into hypophyseal portal system, hormones stimulate or inhibit target endocrine cells in ant.pit (neurocrine function), endocrine cells secrete hormones into blood (endocrine func), pit. cells also affect neighbouring cells (auto + paracrine func)
-2 pathways= posterior pit w ADH and oxytocin to act on distal targets, anterior pit where endocrine cells affected by hormones secreted into hypophyseal portal system.

Question 2

• what are the 2 hormones produced by the hypothalamus for release by the posterior pit gland & functions?
• name the 6 tropic hormones made by the hypothalamus and what do they affect?
• name the 6 hormones released by the anterior pituitary gland & give their functions.

Answer 2

• oxytocin (milk let down & uterine contractions), ADH (reg of water vol)
• TRH (thyrotropin releasing hormone), PIH (prolactin release-inhibiting hormone), CRH (corticotropin releasing hormone), GnRH (gonadotropin releasing hormone), GHRH (growth hormone releasing hormone) and GHIH (growth hormone-inhibiting hormone). All affect release of other hormones in the anterior pit gland!!
• TSH (thyroid stimulating hormone-secreted TH), ACTH (adrenocorticotropic hormone-secretes hormones from adrenal cortex), LH (luteinising hormone-ovulation), FSH (follicle stimulating hormone-development of sperm & egg), Prolactin (mammary gland development), GH (growth hormone).

Question 3

• define negative feedback. give an example.
• give examples of factors influencing growth.
• where is growth hormone produced, what is it inhibited & stimulated by and what are the main promotors of growth?
• where is GH secretion controlled and at what points is it high or low?
• what do GH and IGFs do to bones?

Answer 3

• reversal of a change away from normal. eg the hypothalamic pituitary axis: hypothalamus-> CRH -> anterior pit gland -> ACTH -> adrenal cortex -> release of cortisol * then neg feedback to hypo*
• genetics, nutrition, environment, Growth hormone.
• GH produced in the anterior pituitary gland, stimulated by hypothalamic GHRH & inhibited by hypothalamic somatostatin (GHIH) . Growth promotion mainly via insulin-like growth factors (IGFs)
• controlled via the hypothalamus, GHRH stimulates (eg in deep sleep, exercise) and somatostatin inhibits (eg in REM sleep, stress, decreased glucose, obesity)
• GH stimulates long bone growth (length and width before epiphyseal growth plate closes, then after only width), IGFs stimulate both bone and cartilage growth, in adults maintains muscle and bone mass.

Question 4

• GH secretion is regulated by long loop and short loop negative feedback. Describe.
• what can a GH deficiency cause?
• what can a GH excess cause?
• how does a GH exert its effects on cells?

Answer 4

• long loop=mediated by IGFs, they inhibit release of GHRH from hypo, stimulate release of somatostatin from hypo, inhibit release of GH from anterior pit. Short loop=mediated by GH itself via stimulation of somatostatin release.
• in childhood results in pituitary dwarfism= a proportionate dwarfism where the whole body is smaller, shorter, delayed or no sexual development in teenage years, can be complete or partial & both respond well to GH therapy.
• in childhood causes gigantism, rare and often caused by pituitary adenoma, in adulthood causes acromegaly (Large extremities ie hands, feet, lower jaw)
• GH receptors activate Janus Kinases (JAKs)- phosphorylation causes activation of signalling pathways then TF and IGFs formed.

Question 5

• how many IGFs are in mammals and what are they called & do?
• what sort of actions can they do?
• name 5 other hormones that also influence growth.

Answer 5

• 2 IGFs in mammals, IGF1-fetal growth, IGF2- adult growth, binding proteins modulate their activity
• actions can be parachute or autocrine, act via IGF receptors to modulate cell growth, cell number etc
• 1)insulin (enhances somatic growth), 2)TH (promote CNS development), 3) androgens (accelerate pubertal growth spurt), 4)oestrogens (decrease somatic growth), 5) glucocorticoids (inhibit somatic growth).