The Menstrual Cycle Flashcards
What are the aims of the menstrual cycle?
- Selection of a single oocyte
- Correct number of chromosomes in eggs
- Regular spontaneous ovulation
- Cyclical changes in the vagina, cervix and Fallopian tube
- Preparation of the uterus
What are the different factors that control the menstrual cycle?
- Hypothalamus
- Anterior pituitary
- Ovary
- Oestrogen
- Progesterone
In what condition does the GnRH has to be for the menstrual cycle?
GnRH has to be in pulsatile
What are the different stages of the menstrual cycle?
- Day 1: first day of menses
- Follicular phase
- Ovulation
- Luteal phase
What happens in the Follicular phase?
- Growth of the Antral follicles up to ovulation ->
- dominated by oestradiol productions from Antral follicles
When does Ovulation occur during the cycle?
During day 14 of cycle
What happens in the Luteal phase of the menstrual cycle?
- Formatiom of the corpus luteum from remaining cells of follicles ->
- Dominated by progesterone production from corpus luteum
What occurs at the end of the 28 day cycle?
Menstruation occurs at the end if there’s no pregnancy
State what the corpus luteum is?
Remnant of the follicle that becomes the corpus luteum
What type of response is the Luteal phase?
Negative feedback by progesterone
What type of response is the Follicular phase?
Variable response
1. Release/removal of negative feedback
2. Negative feedback is reinstated
3. Switch from negative to positive feedback
What happens at the late Luteal early follicular stage?
- Progesterone declines
- Selectively raises FSH
- Leads to an inter cycle rise
What happens at the mid follicular stage?
- E2 increases
- Negative feedback occurs
- FSH falls
What happens at the mid cycle phase?
- 2 days of E2 >300 pmol
- Positive feedback occurs
- Causes an LH surge
What happens at the mid Luteal stage?
- High progesterone
- Causes negative feedback to occur
- Low LH/FSH
- Progesterone overcomes E2
Describe the inter cycle rise in FSH
The inter cycle rise and fall in FSH is very important because it allows selection of a single follicle which will go onto become the dominant follicle that will ovulate
How is Follicle selection achieved?
- Raised FSH present a “window” of opportunity to recruit Antral follicles that are at the right stage to continue growth
Follicle Selection (PART 2)
What is the FSH threshold hypothesis for DF selection?
- One follicle from the group of antral follicles in ovary is just at the right stage at the right time to survive declining FSH
- This becomes the dominant follicle which goes into ovulate
- Known as Selection
- Can be in either ovary
How is Follicle selection achieved? (PART 3)
Oestradiol levels rise reinstating negative feedback at pituitary causing FSH levels to fall prevents further follicle growth
How does the dominant follicle survive the fall in FSH?
- As FSH falls, LH increases.
- Dominant follicle acquires LH receptors on Granulosa cells
- Other follicles do not so they lose their stimulant and die
What happens to the dominant follicle?
The dominant follicle survives fall in FSH by
- Increased sensitivity to FSH, Increased FSH receptors
- 2-5 million GC in EFP and 50-100 million in ovulation
- Increases E2 production because of increase aromatise lvl
- 200x more E2 in DF than in others
Acquisition of LH receptors
- The LHR (LH receptor) gene is switched on by FSH
Describe the LH surge (PART 1)
- Throughout early and mid follicular phase E2 feedback was negative
- At the end of follicular phase, If E2 levels raised for long enough (48 Hours) and high enough (>300pM) enough -> feedback switches from negative to positive
- Causes massive release of LH from pituitary
Describe the LH surge (PART 2)
- Exponential rise in LH in serum
- Triggers ovulation cascade
- Egg is released
- Above result in changes in follicle cells = Luteinisation (Formation of corpus luteum)
- Corpus Luteum has both luteinised Granulosa and Theca cells
- E2 production falls but still produced and P is stimulated & dominates
Describe Ovulation in more detail (PART 1)
- Ovulation occurs via a cascade of events
1. Blood flow to the follicle increases dramatically
2. Appearance of the Alex or stigma on ovary wall
3. Local release of proteases and inflammatory mediators
4. Enzymatic breakdown of protein of the ovary wall - 12-18 hours after peak of LH, the follicle wall is digested and ovulation occurs with release of cumulus-oocyte complex (COC)
