Flashcards in Female Physiology Deck (28)
What are the two main functions of the ovaries?
- 1) oogenesis
- 2) secretion of female sex hormones (estrogen and progesterone)
What are the three zones of the ovary? What does each contain?
- cortex: the outer zone, largest; contains all the oocytes, lined by germinal epithelium
- medulla: the middle zone; has a mixture of cell types
- hilum: the inner zone; contains blood vessels and lymphatics
What steps are involved in oogenesis?
- during gestation, primordial germ cells undergo mitosis to yield oogonia (this process stops by week 24 of gestation)
- the oogonia undergo meiosis, stopping during prophase I, to yield primordial follicles containing primary oocytes (all oogonia become primordial follicles by 6 months of age; this development is independent of gonadotropins)
- developing primordial follicles are primary follicles (reproductive years)
- primary follicles develop into secondary follicles (occurs in several follicles during each menstrual cycle)
- only one secondary follicle will mature enough into the graafian/mature follicle and becomes the dominant follicle, which is released during ovulation (at the time of release, the primary oocyte completes meiosis I and becomes suspended at metaphase of meiosis II, becoming a secondary oocyte)
- (maturation is triggered mainly by FSH, ovulation by LH)
What is the corpus luteum and what is its role? How long does it last for?
- the corpus luteum is made up of the residual follicle elements that remain in the ovary after the dominant follicle is released during ovulation (so it is a part of the luteal phase of the menstrual cycle)
- its major role is to act as a temporary endocrine gland necessary for implantation and initial zygote maintenance; it mainly secretes progesterone (but also secretes estrogen to a lesser degree)
- if fertilization does not take place, the corpus luteum degenerates into the corpus albicans within 14 days
- if fertilization does occur, it will remain active until the placenta develops to take over the endocrine function (so it remains active for about 13 weeks)
Which cells of the ovarian follicle produce the female sex hormones? Where is each located in relation to the others?
- the granulosa cells and the theca cells
- theca cells (specifically the theca interna cells) produce progesterone and androgens
- the androstenedione (from the theca cells) enters the granulosa cells to be made into testosterone and then into estradiol
- (theca cells are external to the granulosa cells, which are external to the antrum and to the oocyte)
What enzymes are found in the theca and granulosa cells of the ovarian follicle? What stimulates each?
- the theca cells have the same machinery as found in the adrenal cortex needed to make progesterone and androstenedione
- the granulosa cells have 17beta-hydroxysteroid dehydrogenase and aromatase to convert androstenedione into testosterone and testosterone into estradiol, respectively
- LH stimulates the conversion of cholesterol into pregnenolone (which will then become progesterone) by stimulating cholesterol desmolase in the theca cells
- FSH stimulates aromatase in the granulosa cells
What are the major effects of estrogen?
- (secreted by the granulosa cells)
- dominates the follicular/proliferative phase of the menstrual cycle
- maturation and maintenance of uterus, fallopian tubes, cervix, vagina
- makes cervical mucous more water (promotes sperm entry into uterus)
- in smaller doses, negative feedback on anterior pituitary's FSH and LH release; in large doses, positive feedback
- development of female secondary sex characteristics
- essential for breast development (progesterone helps)
- maintenance of pregnancy (with progesterone)
- stimulates prolactin secretion, but blocks it from acting on the breast
- lowers LDL levels
- decreases bone resorption (anti-osteoporosis)
- lowers uterine threshold to contractile stimuli (pro-contraction)
Why are the major effects of progesterone?
- (secreted by the theca cella)
- dominates the luteal/secretory phase of the menstrual cycle
- maintenance of secretory activity of uterus
- thickens cervical mucus (inhibits sperm entry into uterus)
- negative feedback on anterior pituitary's FSH and LH release
- assists in breast development (estrogen is essential)
- maintenance of pregnancy (with estrogen)
- raises uterine threshold to contractile stimuli (anti-contraction)
What are the two main phases of the menstrual cycle? What happens during each? What occurs between these two phases?
- (average cycle is 28 days)
- 1st 14 days: follicular/proliferative phase; dominated by estrogen; endometrium proliferates and prepares for possible implantation
- 2nd 14 days: luteal/secretory phase; dominated by progesterone; uterine glands increase secretions to increase nutrition for possible implantation
- in-between these two phases is ovulation
What is the normal range of length for a menstrual cycle? Which phase does the variability occur in?
- average cycle is 28 days
- can range from 21 to 35 days and still be normal
- the variability occurs in the follicular/proliferative phase; the luteal/secretory phase is constant and is always 14 days long following ovulation
What happens during the follicular/proliferative phase in the ovaries? In the uterus?
- (day 0 - 14); can be longer or shorter
- dominated by estradiol
- FSH and LH are inhibited from the (relatively low levels of) estradiol
- ovaries: primordial/primary follicles become graafian (mature) follicles, one becomes the dominant follicle and all others become atretic
- uterus: endometrial proliferation
What happens during ovulation?
- (day 14; will always be 14 days before menses)
- accumulated estradiol from the follicular phase results in a sudden positive feedback on LH (and FSH) secretion
- the sudden large spike in LH secretion triggers the dominant follicle to rupture out of the ovary
- *at this time, the ovum moves from meiosis I prophase into meiosis II metaphase*
What happens during the luteal/secretory phase in the ovaries? In the uterus?
- (day 14 - 28); will always be 14 days long
- dominated by progesterone* (although estradiol levels are also quite high)
- ovaries: corpus luteum develops and regresses into the corpus albicans by the end of the phase if no fertilization takes place (if it regresses, estrogen and progesterone levels drop abruptly)
- uterus: endometrial secretory activity and vascularity increase
- *note that progesterone actually raises the hypothalamic set-point for basal body temperature, so temperature actually increases during this phase
What happens during menses?
- (day 0-4/5); triggers the start of a new cycle, some overlap with the beginning of the follicular/proliferative phase
- the corpus albicans results in the abrupt loss of estrogen and progesterone; without this hormonal support, endometrial sloughing occurs
- at this time, primordial/primary follicles of the next cycle are beginning to develop
What are the relative hormone levels at each phase of the menstrual cycle?
- follicular: very low P; low, but increasing E; moderate FSH and LH (FSH slightly greater than LH)
- ovulation: very low P; very high E; very high (spikes) FSH and LH (LH is much greater than FSH) (the high level of unopposed E triggers positive feedback)
- luteal: very high P; high E; moderate FSH and LH (LH slightly greater than FSH) (the high level of E does NOT trigger positive feedback because of the even higher P)
- menses: very low E and P; FSH becomes slightly greater than LH again
How long is gestation? What happens to estrogen and progesterone during gestation?
- gestation AKA pregnancy lasts about 40 weeks (3 trimesters of about 13 weeks each)
- throughout pregnancy, levels of E and P increase steadily to maintain the endometrium, develop the breasts for lactation, and to suppress new ovarian follicles from developing
- the increase in E and P is initially done by the corpus luteum (for the 1st trimester) and then by the placenta (2nd and 3rd)
How soon after ovulation does fertilization take place? When does the blastocyte enter the uterus? When does it implant? When does beta-hCG secretion occur?
- fertilization occurs within 24 hours after ovulation
- the blastocyte (fertilized ovum) enters the uterus about 3 days later (4 days after ovulation)
- implantation occurs 1 day later (5 days after ovulation)
- beta-hCG secretion begins 3 days after implantation begins (8 days after ovulation)
What is needed for implantation to occur?
- the receptivity of the endometrium towards implantation requires a low estrogen:progesterone ratio (AKA it requires a high progesterone level)
What secretes the beta-hCG? What does this hormone do?
- beta-hCG = beta human chorionic gonadotropin
- secreted by the trophoblast (develops into the fetal part of the placenta), specifically the syncytiotrophoblast
- beta-hCG acts like LH
- it informs the corpus luteum that fertilization and implantation have occurred, thus "saving" it from degrading into the corpus albicans
What is the major estrogen of pregnancy? How is it made?
- estriol is the major estrogen of pregnancy
- it is made by the placenta (which also makes progesterone)
- pregnenolone (made from cholesterol via cholesterol desmolase) is made into progesterone, but some also enters the fetal circulation
- in the fetal adrenal cortex, this pregnenolone is made into dehydroepiandrosterone-sulfate (DHEA-sulfate/DHEAS)
- DHEA-sulfate gets hydroxylated in the fetal liver into 16-OH-DHEA-sulfate, which re-enters the placenta and gets converted into estriol by aromatase
What is parturition? How does it occur?
- parturition is delivery/birth
- distention of the uterus triggers contractions (these start off as uncoordinated contractions about 1 month before birth; called Braxton-Hicks contractions)
- the E:P ratio increases (more E) which sensitizes the uterus to contractile stimuli
- prostaglandins increase contractility
- oxytocin stimulates uterine contractions (positive feedback with cervical dilation)
What prevents lactation from occurring during pregnancy?
- the high levels of E and P actually inhibit lactation from occurring during pregnancy (even though they are needed for the development of breasts to lactate)
- after birth, E and P drop, allowing prolactin to act on the breasts
- (note that prolactin suppresses ovulation by inhibiting GnRH secretion)
What happens in menopause?
- as the number of follicles drops each year, the amount of estradiol drops as well; several years before menopause, anovulatory menstrual cycles become more common
- eventually, secretion of estradiol stops and FSH and LH levels rise because of the absent negative feedback (so post-menopausal women will have low estrogen and high FSH and LH; *FSH:LH is high*)
- low estrogen levels: decreased vaginal secretions, vaginal atrophy, decreased breast mass, accelerated bone loss, vascular instability (hot flashes), emotional lability
How many oocytes are present at birth? At puberty? How many of these will mature?
- at birth: 1-2 million (this number is actually final by the 5th month of gestation)
- at puberty: 300,000
- of these 300,000 only about 400-500 will develop enough to trigger ovulation (one each month)
- at menopause: 0
What are the relative strengths of each estrogen hormone? How much is each increased during pregnancy?
- estradiol is by far the strongest (50x increase in pregnancy)
- estrone has a medium potency (50x increase)
- estriol is by far the weakest (1000x increase!)
What is inhibin?
- inhibin is another hormone secreted by the corpus luteum that acts to further inhibit the anterior pituitary's release of FSH and LH
What are estrogens effects on the hypothalamus and the anterior pituitary?
- in low doses, estrogen inhibits the hypothalamic release of GnRH (resulting in lowered levels of LH and FSH); in large doses, estrogen promotes release of GnRH
- in low doses, estrogen inhibits the anterior pituitary release of MAINLY FSH (thus, LH levels are higher than FSH); in large doses, estrogen promotes release of MAINLY LH
- estrogen acts on the hypothalamus via kiss 1 neurons; in small doses, it acts on the neurons in the arcuate (ARC) nucleus; but in large doses it acts on neurons in the anteroventral periventricular (AVPV nucleus); *only females have the AVPV nucleus - so estrogen can not have this positive feedback effect in males*