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
What is the follicular phase of the menstrual cycle?
It is the first half of the menstrual cycle, up to ovulation (Day 14).
It encompassed the growth of follicles up to ovulation, and is dominated by oestradiol production from the dominant follicle.
What is the luteal phase of the menstrual cycle?
It is the second half of the menstrual cycle, from ovulation to the end of the cycle.
It is the formation of the corpus luteum from the empty follicle, and dominates by progesterone production from the corpus luteum.
Describe how the feedback in the follicular phase is variable.
- We get the release of negative feedback induced by progesterone from the corpus luteum
- Negative feedback is then reinstated
- Then, we switch from negative feedback to positive feedback
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.
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.
How does the dominant follicle survive the fall in FSH?
As FSH falls, LH increases.
The dominant follicle acquires LH receptors on granulosa cells. The dominant follicle also increases sensitivity to FSH by increasing FSH receptors and increases the number of granulosa cells
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 midfollicular phase onwards
- FSH and then LH drive oestrogen production in the follicular phase
What happens to the dominant follicle when it 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 due to the dominant follicle growing (greater than 300pmol for 48 hours), so 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.
What is the ovulation cascade?
- blood flow to the follicle increases dramatically
- an appearance of an apex or stigma in the ovary wall
- the local release of proteases (various types of enzymes will digest proteins and break down ovarian wall) and inflammatory mediators
- the enzymatic breakdown of the ovary wall
- 12-18 hours after the peak of LH, a hole appears in the follicle wall and ovulation occurs
- the oocyte with cumulus cells is extruded from the ovary under pressure
- follicular fluid may pour into the Pouch of Douglas
- the egg is ‘collected’ by the fimbrae of the fallopian tube
- the egg progresses down the tube by peristalsis and the action of the cilia
How is the oocyte prepared for division?
For its formation as a primary oocyte in the foetal ovary up until ovulation, the oocyte has been arrested in the first meiotic division.
This permits the oocyte to retain all of the DNA and remain as large as possible during its long wait.
In response to the LH surge, the nucleus of the oocyte in the dominant follicle completes the first meiotic division, but it doesn’t divide!
How does the oocyte move from primary to secondary?
Half of the chromosomes are put into a small ‘package’ in the egg called the ‘first polar body’. The egg (part with most of the cytoplasm) 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.
How long does the oocyte spend in the fallopian tube?
About 2-3 days, where if it meets with a sperm, fertilisation will occur.
Describe corpus luteum formation.
After ovulation, the follicle collapses. The corpus luteum is then formed, often called ‘the yellow body’.
Progesterone production greatly increases, along with oestrogen.
The corpus luteum contains large numbers of LH receptors. The corpus luteum is supported by LH and hCG (from the implanting embryo, if a pregnancy occurs).
