Hypothalamo-pituitary-ovarian axis

Question 1

general functions of hypothalamus (2)

Answer 1

• 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)

Question 2

release of hypothalamic hormones

Answer 2

• 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

Question 3

GnRH info

Answer 3

• stimulates synthesis of gonadotropins FSH and LH from anterior pituitary
• transported via hypothalamo-hypophyseal portal system
• pulsatile secretion

Question 4

hormones used in GnRH feedback control

Answer 4

• progesterone (inhibitory)

- estrogen (inhibitory in luteal phase, inhibitory in early follicular phase, stimulatory in late follicular phase)

Question 5

function of anterior pituitary gland

Answer 5

produces FSH and LH (gonadotropes) –> ovarian function

Question 6

what is the maintenance of LH and FSH synthesis and secretion dependent on

Answer 6

• 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

Question 7

progesterone and GnRH

Answer 7

acts solely on the hypothalamus (no direct pituitary action) –> only reduces gonadotropin secretion by its action on GnRH

Question 8

inhibin and GnRH

Answer 8

• product of granulosa cells of ovarian follicles

- acts on pituitary to decrease FSH secretion

Question 9

FSH

Answer 9

• granulosa cell differentiation
• increases levels of rate-limiting enzymes (aromatase) –> increases estrogen production
• stimulates inhibin production
• increases receptors for FSH, LH, prolactin

Question 10

LH

Answer 10

• main controller of follicular steroidogenesis
• increase substrate availability for steroidogenesis
• pulsatile secretion

Question 11

granulosa cells

Answer 11

site of estrogen synthesis but can’t convert progesterone to androgens

Question 12

theca cells

Answer 12

androgen synthesis from progesterone but can’t aromatize androgens to estrogens

Question 13

2-cell 2-gonadotropin model

Answer 13

• LH stimulates synthesis of androgens in theca interna

- androgens diffuse into granulosa cells –> aromatized to form estradiol under FSH

Question 14

inhibin

Answer 14

• produced in granulosa cells of follicle under influence of FSH and estrogen
• acts on pituitary gland to decrease FSH

Question 15

luteal phase (diestrus)

Answer 15

• 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

Question 16

early follicular phase

Answer 16

• 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

Question 17

late estrus

Answer 17

• 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

Question 18

immediately post-ovulation (early metestrus)

Answer 18

• hypothalamus: GnRH depleted
• anterior pituitary: LH secretion abolished, surge released of FSH in a couple of days
• overall: FSH recruits new follicles