Hypothalamo-pituitary-ovarian axis Flashcards
general functions of hypothalamus (2)
- produces hormones that control secretion of anterior pituitary hormones (promote = GnRH, inhibit)
- produces other peptide hormones in cell bodies of another type of its neurons (oxytocin, vasopressin)
release of hypothalamic hormones
- peptides produced in small quantities in precisely timed bursts
- hypothalamo -hypophyseal portal system: capillary plexus at both ends –> median eminence picks up hormones, anterior pituitary transports them
GnRH info
- stimulates synthesis of gonadotropins FSH and LH from anterior pituitary
- transported via hypothalamo-hypophyseal portal system
- pulsatile secretion
hormones used in GnRH feedback control
- progesterone (inhibitory)
- estrogen (inhibitory in luteal phase, inhibitory in early follicular phase, stimulatory in late follicular phase)
function of anterior pituitary gland
produces FSH and LH (gonadotropes) –> ovarian function
what is the maintenance of LH and FSH synthesis and secretion dependent on
- GnRH –> LH can only follow GnRH pulse, FSH is less under its control but still stimulated by it
- high frequency GnRH pulses favors LH, low frequency pulses favor FSH
progesterone and GnRH
acts solely on the hypothalamus (no direct pituitary action) –> only reduces gonadotropin secretion by its action on GnRH
inhibin and GnRH
- product of granulosa cells of ovarian follicles
- acts on pituitary to decrease FSH secretion
FSH
- granulosa cell differentiation
- increases levels of rate-limiting enzymes (aromatase) –> increases estrogen production
- stimulates inhibin production
- increases receptors for FSH, LH, prolactin
LH
- main controller of follicular steroidogenesis
- increase substrate availability for steroidogenesis
- pulsatile secretion
granulosa cells
site of estrogen synthesis but can’t convert progesterone to androgens
theca cells
androgen synthesis from progesterone but can’t aromatize androgens to estrogens
2-cell 2-gonadotropin model
- LH stimulates synthesis of androgens in theca interna
- androgens diffuse into granulosa cells –> aromatized to form estradiol under FSH
inhibin
- produced in granulosa cells of follicle under influence of FSH and estrogen
- acts on pituitary gland to decrease FSH
luteal phase (diestrus)
- hypothalamus: high progesterone from CL and small amounts of estrogen decease GnRH
- anterior pituitary: low GnRH favor FSH > LH, low levels of estradiol suppress FSH and LH, inhibin inhibits FSH
early follicular phase
- hypothalamus: no progesterone, estrogen provides negative feedback for GnRH, some LH and FSH, no surge release of GnRH
- anterior pituitary: estrogen negatively feedback to reduce LH/FSH, inhibin reduces FSH
- overall: some estrogen to continue follicle growth
late estrus
- hypothalamus: high levels of estradiol switches feedback from negative to positive –> activates GnRH surge center, increased GnRH
- anterior pituitary: increases LH in response to GnRH
- overall: increased GnRH, LH, FSH
immediately post-ovulation (early metestrus)
- hypothalamus: GnRH depleted
- anterior pituitary: LH secretion abolished, surge released of FSH in a couple of days
- overall: FSH recruits new follicles
