REPRO: 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 tube
- preparation of the uterus
- support of the fertilised dividing egg
Menstrual cycle phases?
Follicular Phase- first 14 days (variable)
Ovulation-mid cycle
Luteal Phase-last 14 days (constant)
Menstruation
What is the follicular phase of the menstrual cycle?
Which hormone dominates the follicular phase?
First 14 days of the menstrual cycle
- growth of follicles up to follicles
- selection of dominant follicle
- dominated by oestradiol production (oestrogen) from the dominant follicle.
- feedback is variable on the HPO axis
What is the luteal phase of the menstrual cycle?
Which hormone dominates the luteal phase?
Last 14 days of the menstrual cycle
- remainder of the empty follicle after ovulation is luteinised and becomes corpus luteum
- ovulation to the end of the cycle.
Progesterone (from corpus luteum)
-exerts negative feedback on the HPO axis
Why do GnRH releases have to be pulsatile rather than continuous?
When GnRH release is continuous it leads to a fall in LH release. Whereas when it is pulsatile, LH release remains consistently high.
Describe how the feedback in the follicular phase is variable.
- We get the release of negative feedback
- Negative feedback is then reinstated
- Then, we switch from negative feedback to positive feedback
first two stages of the menstrual cycle
Late Luteal/Early Follicular
- no fertilisation, corpus luteum dies, no more progesterone production, no more -ve feedback on HPO axis, causing inter-cycle rise in FSH
- increase in FSH stimulates recruitment of antral follicles into menstrual cycle and further growth, producing oestrogen from granulosa cells
Mid-Follicular
- oestrogen exerts negative feedback on HPO axis
- drop in FSH causes antral follicle to die except dominant follicle
- dominant follicle grows in size and contains many granulosa cells which produce tons of oestrogen
last 3 stages of the menstrual cycle
Late Follicular
-when oestrogen levels sustained for 2 days (>300pmol), feedback switches to positive
Mid-cycle
- massive LH surge due to positive feedback from oestrogen
- LH surge induces completion of meiosis I and ovulation of cumulus-oocyte complex (day 14)
Mid-luteal
- remainder of follicle is luteinised by LH to become corpus luteum for progesterone production
- progesterone exerts negative feedback, reducing FSH+LH
Describe follicle selection.
The raised FSH presents a ‘window’ of opportunity.
The FSH threshold hypothesis:
- one follicle from the group of antral follicles in the ovary is just at the right stage at the right time
- this becomes the dominant follicle which goes on to ovulate
- this is known as ‘selection’
- it can happen in either ovary
Oestradiol levels rise, reinstating negative feedback at the pituitary, causing FSH levels to drop, preventing further follicle growth.
What does the LH surge trigger?
completion of meiosis I & ovulation
Why is the inter-cycle rise and fall in FSH important?
The intercycle rise and fall of FSH is very important because it allows for the selection of a single follicle.
How does the dominant follicle survive a drop in FSH?
As FSH falls, LH increases.
Increases FSH receptors
-to pick up declining levels of FSH (increase sensitivity to FSH)
Increases number of granulosa cells
-more aromatase for oestrogen production
Acquiring LH receptors on granulosa cells
-LH receptor gene is switched on by FSH and causes LH surge
Other follicles do not, so they lose their stimulant and die
What are some rules of receptors on follicles?
Theca cells will always have LHr, never FSHr.
- remember that LH drives androgen and progesterone production from theca cells
Granulosa cells will have FSHr, then LHr are required from the mid follicular phase onwards
- FSH and then LH drive oestrogen production in the follicular phase
What happens when the dominant follicle is selected?
It grows rapidly, doubling in diameter in seven days, from 7mm to 14mm. It needs masses of growth factors, nutrients and steroids. There is also rapid neoangiogenesis (formation of new blood vessels).
Oestrogen is released from the follicle into the circulation.
Describe the LH surge during the menstrual cycle.
Throughout the follicular phase, oestrogen feedback was negative. At the end of the follicular phase, oestrogen levels are raised for long enough that feedback switches from negative to positive.
This causes a massive release of LH from the pituitary. There is an exponential rise of LH in the serum. This triggers the ovulation cascade.
How does the oocyte move from primary to secondary?
Half of the chromosomes are put into an egg called the ‘first polar body’. The egg is now a secondary oocyte. The first polar body plays no further part in the process and does not divide again.
The oocyte begins the second meiotic division, but arrests again.
What is the result of meiosis I completion in primary oocyte?
Unequal division of cytoplasm:
-small polar body extruded
-secondary oocyte containing majority of cytoplasm because egg needs mitochondria, proteins etc. to support fertilised embryo through early developmental stages until it attaches to placenta
>secondary oocyte enters meiosis II and arrests
