The Menstrual Cycle Flashcards
what regulates the menstrual cycle?
interactions between the hypothalamic-pituitary-ovarian axis and the uterus
what does the hypothalamus secreting gonadotrophin releasing hormone result in?
stimulates anterior pituitary gland to release follicle stimulating hormone and leutinising hormone
how does the ovaries respond to levels of gonadotrophin?
by secreting steroid sex hormones
what does FSH do?
-initiates recruitment of follicles
-supports growth of the follicle, especially the granulose cells
what does LH do?
-supports theca cells
-receptors expressed on maturing follicle
-LH surge triggers ovulation
what are the stages of the menstrual cycle?
preovulatory follicular phase
postovulatory luteal phase
what are the phases of the menstrual system termed in the endometrium?
proliferative and secretory phases
what is the follicular phase?
-varies in length (10-14d)
-characterised by growth of dominant follicle
-progesterone production is low
-oestrogen is rising due to conversion of androgens to oestrogens via aromatase
how does the development of the secondary follicle occur?
Recruited follicles increase production of oestradiol via conversion of androgens produced in the theca interna into oestrogens by the granulosa cells – aromatase
Stimulates LH and FSH synthesis but inhibits their secretion
FSH levels decrease. FSH and LH levels diverge partly becauseoestradiol inhibits FSH secretion more than LH secretion.
Developing follicles produce the hormone inhibin, which inhibits FSH secretion but not LH secretion.
Levels of oestrogen, particularly oestradiol, increase exponentially.
how does development of the secondary follicle occur?
-FSH secretion increases slightly, stimulating further growth of recruited follicles
-circulating LH levels increase slowly, beginning 1 to 2 days after the increase in FSH
-theca develops- follicle gains an independent blood supply
-granulosa cells develop FSH, oestrogen and androgen receptors
what effect does oestrogen have on the endometrium?
-thickening of storm
-elongation of uterine glands
-growth of spiral arteries
how does ovulation occur?
Towards the end of the proliferative phase, rising oestrogens
Increase responsiveness of pituitary to GnRH
Surge in hypothalamic secretion of GnRH
Oestradiol peaks and progesterone levels begin to increase.
High levels ofoestradioltrigger LH secretion by gonadotropes (positive feedback).
Stored LH is released in massive amounts (LH surge), usually over 36 to 48 h, with a smaller increase in FSH.
Ovulation occurs around day 14
Oestradiol decreases
how does the LH surge lead to ovulation?
The LH surge stimulates enzymes that initiate breakdown of the follicle wall and release of mature oocyte within about 16 to 32 h.
The LH surge also triggers completion of the first meiotic division of the oocyte within about 36h of ovulation.
what is the hormonal control of the luteal/secretory phase?
Because levels of circulatingoestradiol, progesterone, and inhibin are high during most of the luteal phase, LH and FSH levels decrease.
Oestradiol and progesterone levels decrease late in this phase.
If implantation occurs, the corpus luteum does not degenerate but remains, supported by human chorionic gonadotropin that is produced by the developing embryo.
what is the luteal/secretory phase?
The length of this phase is the most constant, averaging 14 days
Formation of the corpus luteum from the follicle
The corpus luteum secretes primarily progesterone in increasing quantities, peaking at about 6 to 8 days after ovulation
Progesterone stimulates development of the secretory endometrium
