5- The Menstural Cycle Flashcards
What are the aims of the menstrual cycle?
- selection of a single oocyte
- correct number of chromosomes in eggs i.e. haploid
- regular spontaneous ovulation
- cyclical changes in the vagina, cervix and Fallopian tube
- preparation of the uterus
- support of the fertilised dividing egg
Why does GnRH secretion need to be pulsatile?
to maintain LH secretion
How many phases are in the menstrual cycle and what are these phases separated by?
2 phases - separated by ovulation
follicular phase
luteal phase
What is the follicular phase of the menstrual cycle?
The first half of the menstrual cycle before Ovulation
The growth of follicles up to ovulation → dominated by oestradiol production from follicles
What is the luteal phase of the menstrual cycle?
The second half of the menstrual cycle after ovulation.
The formation of corpus luteum from the empty follicle → dominated by progesterone production from corpus luteum
When does the menstrual cycle begin?
It begins on the first day of bleeding - and the cycle can last up to 28 days
When does ovulation occur in the menstrual cycle?
Ovulation occurs at the end of the follicular phase, the middle of the cycle
Describe how the feedback in the follicular phase of the menstrual cycle is variable
- Release of negative feedback
- Negative feedback then reinstated, then
- Switch from negative to positive feedback
What is the level of feedback in the luteal phase of the menstrual cycle?
Negative feedback - dominated by progesterone
Describe the hormonal changes that occur during the menstrual cycle
Late luteal/early follicular phase:
- progesterone declines
- selectively raises FSH = inter- cycle rise of FSH (recruits the antral follicles into the menstrual cycle)
Mid follicular:
- oestradiol increases
- negative feedback
- FSH falls
Mid cycle:
- after 2 days oestradiol levels > 300pmol
- positive feedback
- LH surge (release of egg)
Mid luteal:
- high progesterone
- negative feedback
- low LH/FSH
- progesterone overcomes oestradiol
Why is the inter-cycle rise and fall in FSH important?
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
- rise in FSH: allows for the recruitment of antrum follicles into the follicular phase
- fall in FSH: allows for the selection of a follicle that will ovulate
Describe the process of 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
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.
What is the ovulation cascade and what triggers it?
It is triggered by a surge in LH
- Egg is released
- Above result in changes in follicle cells = luteinisation i.e. formation of the corpus luteum
- Corpus luteum has both luteinised granulosa and theca cells
- E2 production falls, but still produced and P is stimulated & dominates
Explain the cascade of events that occur during ovulation?
Ovulation occurs via a cascade of events:
- blood flow to the follicle increases dramatically (the increase in vasculature permeability increase intra-follicular pressure)
- an appearance of an apex or stigma in the ovary wall
- the local release of proteases
- 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 fimbriae of the fallopian tube
- the egg progresses down the tube by peristalsis and the action of the cilia
Describe the divisions that occur in the dominant follicle 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.
- ½ the chromosomes are put into a small “package” in the egg called the 1st polar body
- The egg (with most of the cytoplasm) is now a secondary oocyte
- The 1st polar body plays no further part in the process and does not divide again
- Oocyte begins the 2nd meiotic division, but arrests again.
Describe the difference in appurtenance between the 1st polar body and the secondary oocyte
The first polar body maintains less cytoplasm than the secondary oocyte
The secondary oocyte is composed of majority of the cytoplasm after division
Describe the properties of a secondary oocyte
- Unlike sperm we only want a single oocyte
- The oocyte is the largest cell in the body (sperm are smallest…..but fastest!)
- The oocyte has to support all of the early cell divisions of the dividing embryo until it establishes attachment to the placenta
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 the formation of the corpus luteum
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).
What are the secretions of the corpus luteum and why are they important?
progesterone:
- supports oocyte in its journey
- Maintains the CL
- prepares the endometrium
- controls cells in Fallopian tubes
- alters secretions of cervix
oestradiol:
- for endometrium
Describe the demise of the corpus luteum
- If fertilisation does not occur, CL has finite lifespan of 14 days.
- The removal of CL essential to initiate new cycle
- Cell death occurs, vasculature breakdown, CL shrinks
- Process is not well understood
