Page 313 - Color_Atlas_of_Physiology_5th_Ed._-_A._Despopoulos_2003

Hormonal Control of the Menstrual restrict FSH and LH secretion (via negative
Cycle feedback control and with the aid of inhibin;
! A2) but later stimulate LH receptor produc-
In sexually mature women, gonadoliberin or tion in granulosa cells. These cells now also
gonadotropin-releasing hormone (Gn-RH) is start to produce progesterone (start of luteini-
secreted in one-minute pulses every zation), which is absorbed by the theca cells
60–90 min in response to signals from various (! A3) and used as precursor for further in-
neurotransmitters. This, in turn, induces the crease in androgen synthesis (! p. 295 A, steps
pulsatile secretion of FSH and LH from the f and l).
anterior pituitary. If the rhythm of Gn-RH Inhibin and estrogens secreted by the dominant fol-
Hormones and Reproduction changes during the course of the menstrual This leads to an androgen build-up in and apoptosis
secretion is much faster or continuous, less
licle increasingly inhibit FSH secretion, thereby
FSH and LH will be secreted, which can result
decreasing the estrogen production in other follicles.
in infertility. The LH : FSH secretion ratio
of the unselected follicles.
cycle. Their release must be therefore subject
Increasing quantities of LH and FSH are re-
to additional factors besides Gn-RH.
leased in the late follicular phase (! A3), caus-
ing a sharp rise in their plasma concentrations.
The secretion of LH and FSH is, for example, subject
to central nervous effects (psychogenic factors,
The FSH peak occurring around day 13 of the
stress) mediated by various transmitters circulating
ovum. Estrogens increase the LH secretion
norepinephrine (NE) and neuropeptide Y (NPY) as
(mainly via the hypothalamus), resulting in
well as by ovarian hormones, i.e., by estrogens
the increased production of androgens and
11 in the portal blood in the hypothalamic region, e.g., cycle induces the first meiotic division of the
(estrone, estradiol, estriol, etc.), progesterone and
inhibin. Ovarian hormones affect Gn-RH secretion in- estrogens (positive feedback) and a rapid rise in
directly by stimulating central nerve cells that acti- the LH conc. (LH surge). The LH peak occurs
vate Gn-RH-secreting neurons by way of neu- around day 14 (! A2). The follicle ruptures
rotransmitters such as norepinephrine and NPY and and discharges its ovum about 10 hours later
inhibit Gn-RH secretion by way of GABA and opioids. (ovulation). Ovulation does not take place if
FSH production again increases toward the the LH surge does not occur or is too slow.
end of the luteal phase (! p. 299, A1). In the Pregnancy is not possible in the absence of
early follicular phase (! A1), FSH induces the ovulation.
proliferation of the stratum granulosum in Luteal phase (! A4). LH, FSH and estrogens
about 20 follicles and stimulates the secretion transform the ovarian follicle into a corpus lu-
of aromatase in their granulosa cells. Aro- teum. It actively secretes large quantities of
matase catalyzes the conversion of the andro- progesterone (progestational hormone),
gens testosterone and androstenedione to marking the beginning of the luteal phase
estradiol (E 2) and estrone (E 1) (! p. 295 A, (! A). Estrogens and progesterone now inhibit
steps r and o). Estrogens are synthesized in the secretion of FSH and LH directly and in-
theca cells and absorbed by granulosa cells. Al- directly (e.g., through inhibition of Gn-RH; see
though relatively small amounts of LH are above), causing a rapid drop in their plasma
secreted (! A1 and p. 299 A1), this is enough conc. This negative feedback leads to a marked
to activate theca cell-based enzymes (17!-hy- drop in the plasma conc. of estrogens and pro-
droxysteroid dehydrogenase and C17/C20- gesterone towards the end of the menstrual
lyase) that help to produce the androgens cycle (approx. day 26), thereby triggering the
needed for estrogen synthesis. The follicle- menses (! p. 299, A2). FSH secretion starts to
based estrogens increase their own FSH recep- rise just before the start of menstruation
tor density. The follicle with the highest estro- (! A4).
gen content is therefore the most sensitive to Combined administration of estrogens and ge-
FSH. This loop has a self-amplifying effect, and stagens during the first half of the menstrual cycle
the follicle in question is selected as the domi- prevents ovulation. Since ovulation does not occur,
300 nant follicle around the 6th day of the cycle pregnancy cannot take place. Most contraceptives
(! A2). In the mid-follicular phase, estrogens work according to this principle.
Despopoulos, Color Atlas of Physiology © 2003 Thieme
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