Reproductive System: Regulation of the Ovarian Cycle

positive feedback stimulates gonadtropin release

Estrogen levels continue to rise as a result of continued release by dominant follicle When levels reach a critical high value, a brief positive feedback occurs on brain and anterior pituitary Triggers LH surge

FSH and LH stimulate follicles to grow, mature, and secrete sex hormones

FSH stimulates granulosa cells to release estrogen, and LH prods thecal cells to produce androgens, which granulosa converts to estrogens

LH surge triggers ovulation and formation of the corpus luteum

High estrogen levels trigger release of stored LH, and some FSH, by anterior pituitary at midcycle

negative feedback inhibits gonadtropin release

Increasing levels of plasma estrogen levels exert negative feedback inhibition on FSH and LH release Inhibin from granulosa cells also inhibits FSH release Only dominant follicle can withstand this dip in FSH Other developing follicles deteriorate

negative feedback inhibits LH and FSH release

Negative feedback from rising plasma progesterone and estrogen levels inhibits LH and FSH release Inhibin, from corpus luteum and granulosa cells, enhances inhibitory effect Declining LH ends luteal activity and inhibits follicle development

Day 1-5 of uterine cycle

Ovarian hormones are at lowest levels Gonadotropin levels are beginning to rise Stratum functionalis detaches from uterine wall and is shed Menstrual flow of blood and tissue lasts 3-5 days By day 5, growing ovarian follicles start to produce more estrogen

Days 15-28: Secretory Phase

Phase that is most consistent in duration Endometrium prepares for embryo to implant Rising progesterone levels from corpus luteum prompt: Functional layer to become a secretory mucosa Endometrial glands to enlarge and secrete nutrients into uterine cavity Thickened mucus to form cervical mucus plug that blocks entry of more sperm, pathogens, or debris

Days 6-14: proliferative phase

Rising estrogen levels prompt generation of new stratum functionalis layer As layer thickens, glands enlarge, and spiral arteries increase in number Estrogen also increases synthesis of progesterone receptors in endometrium Thins out normally thick, sticky cervical mucus to facilitate sperm passage Ovulation occurs at end of proliferative phase on day 14

follicular phase

The first phase of the ovarian cycle, during which a follicle (an oocyte and its surroudning cells) enlarges and matures. This phase is under the control of FSH from the anterior pituitary, and typically lasts from day 1 to day 14 of the menstrual cycle. The follicle secretes estrogen during this time period.

Why does only one egg, rather than four eggs, develop during oogenesis, given that spermatogenesis results in four sperm formed from one stem cell?

Unequal cytoplasmic division ensures that a fertilized egg has ample nutrients for its journey to the uterus.

A surge in __________ directly triggers ovulation.

When estrogen reaches a certain level in the body, it triggers a positive feedback loop that triggers an LH surge. This triggers ovulation.

What ovarian structure forms at the beginning of the secretory phase, encouraging its progress, and then degenerates just before menses?

corpus luteum

Which is NOT a part of the proliferative phase of the female menstrual cycle?

corpus luteum secretion

Formation of a secondary oocyte occurs during __________.

follicular phase

three phases of the uterine cycle

menstrual phase, proliferative phase, secretory phase

What event coincides with the transition from the proliferative phase to the secretory phase?

ovulation occurs

luteal phase of ovarian cycle

period of corpus luteum activity (days 14-28)

Amenorrhea can occur in women engaged in extremely strenuous physical activity. It also occurs in women who ______.

pregnant

During the secretory phase of the menstrual cycle, ________ reaches its highest levels.

progesterone

During the secretory phase of the uterine cycle, __________.

the endometrium prepares for implantation

What structure is the target for gonadotropin stimulation occurring during the events that lead to ovulation?

vesicular follicle Gonadotropins (FSH and LH) released by the anterior pituitary stimulate vesicular follicles to grow, mature, and secrete sex hormones.