2.1 - hypothalamic-pituitary-gonadal axis Flashcards
what does successful reproduction require?
each sex to produce the right number of gametes at the right time
an effective apparatus to transfer sperm from male to female
creation of opportunities to use apparatus
sperm transport and fertilisation
support of the conceptus, embryo and foetus
successful parturition
support of the neonate
what controls successful reproduction?
hormones
where are major controlling structures located?
in the brain:
hypothalamus
pituitary gland (esp. anterior)
gonads
in a pregnant female, important control is exerted by what?
the placenta
how do structures interact with each other?
primarily by secretion of hormones
where does the pituitary gland arise from embryologically?
Rathke’s pouch
describe Rathke’s pouch
has a number of cell types secreting different hormones, most of which are involved in, or affected by, reproduction
what type of hormone are most pituitary hormones?
trophic hormones
what are trophic hormones?
hormones which act on other tissues to promote the secretion of further hormones
what is each secretory cell type in the anterior pituitary controlled by? produced by? passes to?
controlled by releasing (sometimes inhibitory) hormones
produced by nerve cells in the hypothalamus
which pass to the anterior pituitary
where do the hormones travel through from the nerve cells in hypothalamus (production) to anterior pituitary?
hypophyseal portal vessels
what is the significance of the hypophyseal portal vessels?
carries blood from median eminence of the hypothalamus (hypothalamic releasing and inhibiting hormones are secreted) to the anterior pituitary (hormones act here, resulting in further secretion - trophic hormones)
what are the main reproductive anterior pituitary hormones?
FSH and LH
which cell type produces FSH and LH?
gonadotrophs
which hormone controls gonadotrophs? (release of FSH + LH)
gonadotrophin releasing hormone (GnRH)
where is GnRH released from? under the influence of?
several groups of hypothalamic neurones secrete GnRH
under influence both of other structures in brain and circulating hormones
what does ‘environment via sensory input’ for release of GnRH mean?
under influence of other structures in brain (from sensory input)
how is GnRH secreted?
in pulsatile fashion
a burst of secretion about once an hour
what is secreted and from where in response to pulses of GnRH from hypothalamic neurones?
gonadotrophs in the anterior pituitary secrete gonadotrophins
what happens in the absence of GnRH?
no LH or FSH is secreted (gonadotrophins)
secreted by gonadotrophs in ant. pit. by GnRH
where do FSH and LH primarily act?
on the gonads (sex organs - testis, ovaries)
what do FSH and LH do?
influence the production of gametes (ova / sperm) and to promote the secretion of gonadal steroids
what are the gonadal steroids?
females: oestradiol and progesterone
males: testosterone
what do gonadal steroids (oestradiol, testosterone) influence?
GnRH-secreting neurones (which secrete GnRH - direct feedback control)
what is GnRH also affected by aside from gonadal steroids?
affected by GnRH’s connection to other parts of the brain influenced by factors e.g. body weight + changes in environment (environment via sensory input?)
in males, how does testosterone affect secretion of GnRH?
testosterone reduces secretion of GnRH
in females, how does INTERMEDIATE levels of oestrogen affect secretion of GnRH?
oestrogen (principally oestradiol) at intermediate concentration REDUCES secretion of GnRH
in females, how does progesterone affect secretion of GnRH?
increase the INHIBITORY effect of oestrogen
enhance inhibition on GnRH with oestrogen - amplify
in females, how does MODERATE levels of oestrogen affect secretion of GnRH?
REDUCES the amount of GnRH secreted per pulse, progesterone…?
how does oestrogen ALONE at HIGH concentration affect GnRH?
promotes the release of GnRH - producing a surge
how does progesterone affect oestrogen? further affecting?
prevents high levels of oestrogen producing a GnRH surge (releases LH + FSH)
what are gonadotrophs secreted by? in response to?
secretes FSH + LH
in response to pulsatile secretion of GnRH by the hypothalamus
the amount of FSH + LH secreted in response to each pulse of GnRH by hypothalamus is affected by what?
- oestrogen at intermediate concentration which reduces it
2. oestrogen at high concentration which promotes it
what reduces the amount of FSH secreted in response to GnRH?
inhibin
what is inhibin?
a protein hormone produced by ovarian follicles (granulosa cells) and in seminiferous tubules (Sertoli cells)
in the testis, LH stimulates the secretion of what? by?
LH stimulates secretion of testosterone by Leydig cells
how much testosterone is secreted by Leydig cells each day? (stimulated by LH)
4-10mg