Reproduction: The Menstrual Cycle Flashcards
What are some of the aims of the menstrual cycle?
- Selection of a single oocyte
- Regular spontaneous ovulation
- Preparation of the uterus for potential developing embryo
- Support of the fertilised dividing egg
Give a brief overview of the different stages of the menstrual cycle
- Day 1 of menstrual cycle is first day of bleeding
- Days 1-14 are known as follicular phase - growth and selection of dominant follicle
- Ovulation then occurs at end of follicular phase
- Days 14-28 are known as luteal phase - dominated by corpus luteum
Once the ovum is ovulated out of the dominant follicle what does the empty follicle become known as?
Empty follicle becomes corpus luteum
If the mature ovum doesn’t get fertilised what happens at the end of the luteal phase?
Menstration will occur at the end of the luteal phase and the cycle will then start again
During the luteal phase of the menstrual cycle what type feedback is placed on the HPO-axis and what gonadotrophin hormone/s are responsible for this?
- During luteal phase of the menstrual cycle there is negative feedback on the pituitary and hypothalamus
- This negative feedback is provided by progesterone
During the follicular phase of the menstrual cycle what type feedback is placed on the HPO-axis and what gonadotrophin hormone/s are responsible for this?
- Initially during the follicular phase negative feedback placed on hypothalamus and pituitary by progesterone is removed
- This increases pulsatile release of GnRH from hypothalamus and LH and FSH from pituitary
- This results in increase in oestrogen secretion from granulosa cells of follicle.
- Oestrogen release then causes negative feedback to be re-instated
- Towards end of follicular phase feedback switches from negative to positive
Explain how feedback within the HPO-axis changes at the end of the luteal phase/start of the follicular phase of the menstrual cycle
- No pregnancy means that the corpus luteum isn’t needed and so begins to die
- Because corpus luteum secretes progesterone the fact that it dies means that progesterone levels begin to decrease
- Progesterone was placing negative feedback on hypothalamus and pituitary so decreasing levels of progesterone causes the release of the negative feedback on hypothalamus/pituitary
- This causes increased secretion of FSH from the pituitary
What is the increase in FSH secretion from the pituitary as a result of the removal of negative feedback placed on it and the hypothalamus called?
Inter-cycle rise in FSH
Explain how feedback within the HPO-axis changes at the mid follicular phase of the menstrual cycle
- Increase in FSH secretion will cause the antral follicles that have gone through follicle initiation to continue to grow and be recruited into the menstrual cycle
- FSH also causes the granulosa cells within the antral follicles to produce and secrete oestrogens
- This increased secretion of oestrogens will begin to exert negative feedback on the hypothalamus and anterior pituitary
- This results in a decrease in FSH secretion from the anterior pituitary
What effect does the drop in FSH caused by increased secretion of oestrogen from the antral follicles recruited into the menstrual cycle have on those antral follicles?
- Drop in FSH causes all of the antral follicles apart from the dominant follicle to die
Explain how feedback within the HPO-axis changes at the middle phase of the menstrual cycle (ovulation)
- As the dominant follicle grows it produces lots of oestrogens (estradiol)
- When oestrogen levels reach >300 pmol for 2 days oestrogen begins to exert positive feedback on the anterior pituitary/hypothalamus
- This causes a massive surge in LH secretion from anterior pituitary
What effect does the LH surge have on the dominant follicle?
- LH surge causes mature ovum to be ovulated from the dominant follicle
- Ovulated follicle becomes corpus luteum
Explain how feedback within the HPO-axis changes at the mid-luteal phase of the menstrual cycle
- Corpus luteum begins to secrete high levels of progesterone
- Progesterone exerts negative feedback on the anterior pituitary/hypothalamus which causes a decrease in FSH/LH secretion
- This causes a decrease in oestrogen secretion from the granulosa cells of the dominant follicle
Why is the inter-cycle rise and then fall of FSH important within the menstrual cycle?
- Inter-cycle rise causes growth of antral follicles that have gone through follicle initiation and causes them to be recruited into mentrual cycle
- Inter-cycle fall particularly important because it allows for the selection of the dominant follicle which will go on to be ovulated
- Without fall all antral follicles recruited into menstrual cycle to grow
Explain the concept of the “window of opportunity” with respect to the menstrual cycle
- Raised FSH levels as a result of inter-cycle rise in FSH presents a “window” of opportunity for antral follicles that are at right stage to be recruited into menstrual cycle and continue growth
- Dominant follicle then produces Oestrogens which initially exert negative feedback on hypothalamus/pituitary which causes decrease in FSH
- This causes “window of opportunity” to close because fall in FSH prevents any other follicles from being selected
Describe the changes in the level of secretion of hormones secreted by the ovaries during pregnancy
- Oestrogen levels increase throughout pregnancy
- Progesterone levels increase throughout pregnancy
- hCG levels increase rapidly after 4 weeks but at around 10th week hCG levels decrease rapidly throughout preganancy
