The menstrual cycle Flashcards
What are the aims of the menstrual cycle?
- Selection of a single oocyte
- Regular spontaneous ovulation
- Correct number of chromosomes in eggs
- Cyclical changes in the vagina, cervix and fallopian tubes
- Preparation of the uterus
- Support of the fertilised dividing egg
What is the first day known as in the menstrual cycle>
First day is known as menses
Brief overview of the menstrual cycle
- First phase is follicular phase which is oestrogen dominating
- The next step is ovulation
- Then we have the luteal phase where the corpus luteum forms from the empty follicle
- Menstruation occurs at the end
What does progesterone exert during the luteal phase?
During the luteal phase, progesterone exerts a negative feedback
How is feedback during the follicular phase?
During the follicular phase, feedback is variable
- In the first half of the follicular phase, there’s the release of negative feedback from progesterone
- Then negative feedback is reinstated from oestrogen
- Then we switch from negative to positive feedback
Steps involved in the menstrual cycle
- Late luteal early follicular
- Progesterone levels decline removing negative feedback
- This allows selective rise in FSH levels and this is known as an intercycle ruse - Mid follicular
- Oestrogen levels increase
- Negative feedback exerted again
- FSH levels drop - Mid cycle
- Dominant follicle grows exponentially
- There’s 2 days of oestrogen release >300pmol
- We switch to positive feedback and there’s a surge in LH
- LH triggers ovulation of the dominating follicle and then triggers lutenisation
- The follicle then becomes the corpus luteum - Mid luteal
- The corpus luteum produces a lot of progesterone which exerts a negative feedback
Why is the intercycle rise and fall in FSH important?
Intercycle rise and fall in FSH is very important because it allows selection of a single follicle
In follicle growth, what does a raised level of FSH present?
Raised FSH present a window of opportunity
What is the FSH threshold hypothesis?
- One follicle from the group of antral follicles in ovary is just at the right stage at the right time
- This becomes the dominant follicle
- This is known as selection
- Can occur in either ovary
What happens when oestradiol levels rise?
Oestradiol levels rise reinstating negative feedback at pituitary causing FSH levels to fall, preventing 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
- Dominant follicle also increases sensitivity to FSH by increasing FSH receptors and increases number of granulosa cells
- Other follicles don’t, so they lose their stimulant and die
What happens once the dominant follicle is selected?
It:
- Grows rapidly
- Needs masses of growth factors, nutrients and steroids
- Rapid neoangiogenesis(Formation and recruitment of blood vessels)
- Oestrogen released from follicle into circulation
How does ovulation occur?
Occurs via a cascade of events:
-Blood flow to follicle increases dramatically
-Increase in vascular permeability increases intra
follicular pressure
-Appearance of apex or stigma on the ovary wall
-Local release of proteases
-Enzymatic breakdown of protein of the ovary wall
-12-18 hours after the peak of LH, hole appears in follicle wall and ovulation occurs
-Oocyte with cumulus cells is extruded from ovary under pressure
-Follicular fluid may pour into pouch of douglas
-Egg is collected by fimbria of fallopian tube
-Egg progresses down tube by peristalsis
and action of cilia
Between when is the oocyte arrested in the first meiotic division and what does this allow?
From its formation as a primary oocyte in the fetal ovary up until ovulation, the oocyte has been arrested in the first meiotic division
-This allows it to retain all of the DNA and remain as large as possible during its long wait
What does the nucleus of the oocyte in the dominant follicle do in response to the LH surge?
In response to the LH surge, the nucleus of the oocyte in the dominant follicle completes the first meiotic division but doesn’t divide
