Menstrual Cycle Flashcards
(36 cards)
What are the main aims of the menstrual cycle?
- selection of a single oocyte
- regular spontaneous ovulation
- maintaining haploid chr in gametes
- cyclical changes in vagina, cervix and fallopian tube
- prep. of uterus for potential pregnancy
- support for fertilised zygote
What is the HPG axis control of the menstrual cycle?
hypothalamus -> pulsatile GnRH
anterior pituitary -> pulsatile LH/FSH
Ovaries: oestrogen and progesterone
Feedback on to H and P of HPG (positive/negative depending on concentration of E2 and P)
Why does GnRH have a pulsatile secretion pattern?
needs to be pulsatile to trigger release of LH/FSH
If GnRH has continuous secretion, then the HPG axis is switched off
[LH responds better to GnRH than FSH]
Where is the GnRH-R located?
GnRH-R is a GPCR
located at the PM surface of anterior pituitary cells
What is the menstrual cycle?
2 phases: proliferative/follicular phase and luteal/secretory phase
Separated by ovulation
cycle begins on D1 of bleeding (menses)
What is the follicular phase of the menstrual cycle?
growth of follicles up tot ovulation
Driven by E2 production from the follicles
What is the luteal phase of the menstrual cycle?
formation of a corpus luteum from the remaining empty follicle post-ovulation
Driven by progesterone secretion from the corpus luteum
What is the HPG axis control of the luteal phase of the menstrual cycle?
Progesterone causes negative feedback onto HPG axis
-> reduced GnRH and no LH/FSH
What is the HPG axis control of the follicular phase phase of the menstrual cycle?
[VARIABLE]
- negative feedback brake on HPG axis is released (pseudo-positive feedback, due to lack of inhibition)
- therefore, negative feedback is reinstated initially (low LH/FSH)
- As [E2] > 300pmol then switch to positive feedback (LH surge)
- ovulation
- corpus luteum makes P which causes negative feedback to reduce FSH
Why is the inter-cycle rise in FSH important in the menstrual cycle?
allows selection of a follicle
goes on to become the dominant follicle to ovulate
rise in FSH here is caused by lack of inhibition on the HPG axis, therefore no brake on FSH release
How does follicle selection occur?
raised FSH = window for antral follicle recruitment (for those that are sufficiently developed)
FSH threshold hypothesis
E2 levels rise and restore negative feedback and therefore reduce FSH release
What is the FSH threshold hypothesis?
- one antral follicle from group will is @ the correct stage to survive loss of FSH
- this is the DOMINANT follicle (“selection”) which will be ovulated
- can come from either ovary
How does the dominant follicle survive the fall in FSH?
- increased sensitivity of FSH-R, better able to cope with lower levels
- lower [IGFBPs] in DF allows more free IGFs to sensitise DF to FSH
- increased number of FSH-R
- increased number of granulosa cells
- slower turn over of bound FSH on cells to mediate effects
- acquisition of LH-R
What hormones are at play during dominant follicle selection? How does this aid its survival?
- DF creates E2, which inhibits FSH release causing a fall in FSH
- At the same time, LH increases
- DF acquires LH receptors on its granulosa cells
- other follicles in the selected group of antral follicles DO NOT acquire LH receptors
- this causes them to undergo atresia and die
When does oestrogen synthesis occur in the dominant follicle?
- E2 is produced in the DF (granulosa cells)
- produced from cholesterol
- only starts to occur when LH-R are present
Where does androgen synthesis occur in the female?
in theca cells only
Where does oestrogen synthesis occur in females?
in granulosa cells only
What is the LH surge? What are the mechanisms causing this?
E2 feedback in the follicular phase is negative
at end of follicular phase, E2 reaches 300pmol and switches to positive feedback mechanism
causes LH release/surge from pituitary and therefore increased serum LH
triggers the ovulation cascade
What is the ovulation cascade?
- egg is released
- this causes LUTENISATION of of the follicle cells
- Corpus luteum has both luternised granulosa and theca cells
- E2 production falls, but is still produced
- P is stimulated and predominates the luteal phase
What is lutenisation of the follicle?
change of the (granulosa and theca) cells in follicle to form the corpus luteum
Where does DF selection occur in the ovary?
movement of selected antral follicles from cortex to medulla of ovary
=> better vascular supply
Medulla: where DF selection occurs
What local changes occur before ovulation?
- increased blood flow to follicle
- appearance of apex or stigma on ovary wall
- local release of proteases and inflammatory mediators
- enzymatic breakdown of ovary wall
ovulation = release of cumulus-oocyte complex (COC)
this occurs 12-18hr after LH surge
What happens after the release of the cumulus-oocyte complex from the ovary?
- extrusion of cumulus-ovary complex from ovary
- follicular fluid may fill much of Douglas
- egg is collected by fimbrae of fallopian tube
- Progression down tube by peristalsis and ciliary action
What (unequal) division steps occur post-ovulation?
LH surge -> oocyte in DF completes meiosis I
(50% of nuclear contents are transferred to a polar body)
=> secondary oocyte
- meiosis II is started
- arrests in prophase
- will only complete meiosis II after fertilisation
