Menstrual Cycle I Flashcards
1
Q
How is the menstrual cycle controlled and what is the key requirement to maintain this axis?
A
- Control of the menstrual cycle is via the HPG axis
- Pulsatile release is the key requirement to maintain this axis; cessation of the complete cycle if GnRH is administered/released continuously, because the release of LH and FSH is stopped (along with the downstream effects).
- Pulsatile release of both GnRH and gonadotrophins maintains the HPG axis and, hence, fertility.
2
Q
Describe the length of the normal menstrual cycle (MC = most cycles).
A
- The length of a menstrual cycle is the number of days between the first day of menstrual bleeding of one cycle to the onset of menses of the next cycle.
- Day 1 is always the first day of bleeding (menses)
- Median duration of MC is 28 days with most cycles between 25-30 days
- Luteal phase is fixed (14 days independent of the life cycle of the corpus luteum). It is the follicular phase that varies. Variable cycles are a result of the variation in the follicular phase. This can make it a bit tricky when timing ovulation for either contraceptive purposes or family planning purposes.
- Menstruation lasts 3-8 days, written clinically as 7/28 or 5-6/27-32
- MC<21 days=polymenorrheic; MC>35 days=oligomenorrheic
- Menstrual cycle typically most irregular around extremes of reproductive life i.e menarche and menopause
- PCOS can cause irregular cycles throughout their reproductive life (not just at the extremes).
3
Q
Draw the hormonal profile with LH (IU), FSH (IU), progesterone (nmol) and oestrogen (nmol) starting from the late luteal phase.
A
- Important to link hormonal profile to follicle growth (follicular stage). They are linked and involved feedback.
- There is an intercycle rise in FSH
- Two distinct peaks of oestrogen
- As FSH comes down, oestradiol is rising
- As oestradiol rises, there is a decrease in FSH. LH surge has a distinct peak. Progesterone (from the corpus luteum) is predominantly in the luteal phase.
4
Q
Which steroids are made where during the menstrual cycle (Two-cell-two-gonadotrophin theory)?
A
- Two-cell-two-gonadotrophin theory; the cells produce different steroids due to the different enzymes present in different compartments, e.g. aromatase is only found in the granulosa cell compartments. Since aromatase catalyses the conversion of the androgens, either androstenedione or testosterone, to oestrone or oestradiol respectively, that means formation of the oestrogens can only occur in the granulosa cells. The gonadotrophin that drives this is FSH. The enzymes catalyse reactions to make the steroids.
- Two cells in this context = theca and granulosa, two gonadotrophins = LH and FSH
- In small antral follicles steroidogenesis is organised. Theca cells produce androgens in response to LH, but granulosa cells do not (do not express CYP17A1). Androgens diffuse from theca cells to granulosa cells, where they are substrates for estrogen formation in response to FSH because granulosa cells differentially express CYP19A1, which encodes aromatase. Granulosa cells primarily express FSH receptors and respond to FSH by producing oestrogen.
- Formation of the oestrogens only occurs in the granulosa cells.
- Similarly, in the theca, there are only certain enzymes present (which are not present in the granulosa cells), so most of the androgens are made there. There is one exception.
- Majority of the androgens are made in the theca
5
Q
What are the phases of the menstrual cycle?
A
1) Late luteal, early follicular phase = progesterone declines. This results in a selective increase in FSH (intercycle rise in FSH). The death of the corpus luteum (due to lack of pregnancy) causes this drop in progesterone production. FSH rises as progesterone is no longer negatively feeding back to the hypothalamus and pituitary. FSH stimulates the follicles.
2) Mid follicular = Antral follicles grow and are producing oestrogen. As oestrogen rises, this reexerts negative feedback onto FSH and so FSH levels decrease. This rise and fall in FSH allows selection of the dominant follicle.
3) Mid cycle = Produces dramatically increased levels of oestrogen. Sustained oestrogen exceeding the threshold (300pmol) is required for around 48h. Feedback switches from negative to positive. This results in the LH surge which brings about ovulation of the dominant follicle
4) Mid luteal = The remainder of the follicle becomes the corpus luteum. Produces a lot of progesterone which exerts negative feedback, keeps FSH and LH low. The peak of oestrogen comes from the corpus luteum, but the progesterone dominates so negative feedback is always maintained in the mid luteal phase.
6
Q
What are the differences between the oestrus cycle and the menstrual cycle?
A
- All female mammals have cyclical ovarian function & the same reproductive system (in terms of HPG axis) to produce a mature egg(s) and the necessary sex steroids
- Menstrual cycles occur only in humans, primates (apes and monkeys) & some bats; named for the regular appearance of menses i.e. shedding of the endometrial lining
- Oestrus cycle in animals named because of:
1) The cyclic appearance of behavioural sexual activity (heat or oestrus)
2) They do not menstruate – the endometrium is reabsorbed if fertilization does not occur
3) Day 0 of the oestrous cycle is the day of beginning sexual receptivity
4) Ovulation usually occurs early in cycle as high oestrogen levels stimulate sexual behaviour as well as exerting positive feedback
5) Different species have different lengths of cycles; Some are poly-oestrous i.e. go into heat several times/year (cats, cows, pigs); others are di-oestrous (twice/year) and some have only one breeding season/year i.e. mono-estrous (eg. Bears, foxes, wolves) and usually in spring
6) Rabbits have no oestrous cycles and are induced to ovulate by mating and can conceive at any arbitrary moment. - Most other mammalian species do not shed their endometrial lining after it has been built up to receive implanting embryos; it is absorbed instead
- The oestrus cycle is a period of sexual receptivity and sexual activity (behaviour). They don’t menstruate; the endometrium is just reabsorbed if fertilisation does not occur.
- In research and when looking at models of reproduction, it is important to consider what animals are being used. They may not always fit in with the menstrual cycle. Monkeys, specifically apes, would be make an ideal animal model but would there are a lot of ethical issues.
7
Q
Outline the HPO axis in detail, including activins and inhibins.
A
- This is controlled by feedback and the feedback varies.
- It is not just about oestrogen and progesterone, there are also activins and inhibins
- The hypothalamus secretes GnRH which acts on the anterior pituitary.
- Stimulates release of LH and FSH which acts on the ovary.
- Ovaries produce oestrogen and progesterone which feed back on the pituitary and hypothalamus. This can be positive or negative feedback. Can also positively feed back on the ovary.
- Negative feedback = luteal phase, Negative & positive feedback = follicular phase
- Ovary produces activins and inhibins that can positively/negatively feed back on the pituitary.
8
Q
Outline the HPO axis in detail, including activins, follistatins and inhibins.
A
- This is controlled by feedback and the feedback varies.
- It is not just about oestrogen and progesterone, there are also activins and inhibins
- The hypothalamus secretes GnRH which acts on the anterior pituitary.
- Stimulates release of LH and FSH which acts on the ovary.
- Ovaries produce oestrogen and progesterone which feed back on the pituitary and hypothalamus. This can be positive or negative feedback. Can also positively feed back on the ovary.
- Negative feedback = luteal phase, Negative & positive feedback = follicular phase
- Ovary produces activins and inhibins that can positively/negatively feed back on the pituitary.
- Follistatin binds to activin with high affinity, so prevents activin from feeding back and acting on the pituitary to inhibit FSH (neutralizes FSH-stimulating ability of activin).
- Lots of input from other systems; insulin, insulin resistance, body fat, nutritional status etc. All of this is about a woman being seen to be fit to reproduce. There are multiple signals which can feed in; stress hormones can affect the axis and feed into this. These can all disrupt the menstrual cycle.
Many factors affect the menstrual cycle and a women’s ability to reproduce.
9
Q
When were inhibins, activins and follistatin discovered?
A
- Postulated for long time that gonadal factor involved in feedback regulation of FSH secretion, but only found recently.
- 1985 purified Inhibin → This was found to be produced in both males and females. Produced by testis (Sertoli cells) and ovary (Granulosa cells); Disulphide-linked protein dimers (two chains)= Common α-subunit with different β-subunits giving two forms of Inhibin. Both forms specifically suppress (INHIBIT) FSH secretion by pituitary without affecting LH secretion.
- 1986 – isolated Activins from follicular fluid which stimulate FSH secretion
- 1987 – isolated another FSH-suppressing protein from follicular fluid called Follistatin. It binds to activin with high affinity, so prevents activin from feeding back and acting on the pituitary to inhibit FSH (neutralizes FSH-stimulating ability of activin).
10
Q
What are the sub-types of inhibins and activins?
A
- Biosynthesis of inhibins and activins occurs from 3 genes, makes precursor protein:
1) α- protein, specific for Inhibin
2) βA- protein, can form either Activin/Inhibin
3) βB- protein, can form either Activin/Inhibin - These alpha and beta subunits are all members of TGF-beta superfamily of proteins. The genes encode for larger precursor proteins which are then processed proteolytically. The products are the sub-units which will combine at the time of release from the cell.
- Made as bigger hormones that have been cleaved down. Made from three genes which make these precursor proteins.
- The alpha protein is specific for inhibin, while the beta proteins are of different subunits (there is beta-A and beta-B). Inhibins are made up of a common alpha unit with different beta units. Beta A subunit gives its name to inhibin A and beta B to inhibin B. Inhibins take 2 forms depending on β-chain composition = Inhibin A and Inhibin B
- Activins have no alpha subunit, just combinations of the beta subunits. With the activins, if they had the same beta subunit, they are known as homodimers, otherwise they are heterodimers. Activins take 3 forms depending on β-chain composition = Activin A (βA-homodimer), Activin B (βB-homodimer) & Activin AB (βAβB-heterodimer).
11
Q
What are the different forms of inhibin and activin with their dimer subunits?
A
- All of these are members of the TGF beta super family of proteins.
- With the activins, if they had the same beta subunit, they are known as homodimers, otherwise they are heterodimers.
INHIBIN - inhibits FSH secretion
1) Inhibin A = alpha + beta A
2) Inhibin B = alpha + beta B
ACTIVIN - stimulates FSH secretion
1) Activin A = beta A + beta a
2) Activin A = beta A and beta B
3) Activin B = beta B and beta B
12
Q
How does the production of activin and inhibin correlate with follicle stage?
A
- Activins correlate with FSH rise in early follicular phase of MC and Inhibin with FSH fall in late follicular of MC – also levels of activin high at start of MC (EFP) but fall by luteal phase of MC, whereas inhibin start to rise in LFP of MC and peak in luteal phase.
- They are both produced from the granulosa cells. Activins activate FSH while inhibins inhibit FSH. However, it is not just simply about how much is produced, it is about the ratio of both.
- In the earlier antral stages, as a follicle is growing, activins are produced in higher levels compared to inhibins. As folliculogenesis progresses, the inhibins increase in concentration, so the ratio will vary. Link it back to the menstrual cycle graph.
Include what activins and inhibins are doing in exams (not just oestrogen and progesterone!!! - As follicle grows, FSH stimulates the alpha-subunit and get increase in Inhibin B (activin A, then activin AB and then activin B). Preantral follicles = activin B.
- Dominant follicle =Increase in Inhibin A (because of betaA subunit) and decrease in Inhibin B (inhibin B then inhibin A).
- CL = Inhibin A but no Inhibin B
- Activin A = InhbA+InhbA; Activin AB = InhbA+InhbB; Activin B = InhbB+InhbB
InhibinA = InhA+InhbA; InhibinB = InhA+InhbB
13
Q
What experimental evidence shows how activins and inhibins work at different stages of development?
A
- How do know? Inject rats in the late antral phase with Inhibin anti-serum – what will anti-serum to Inhibin do? Bind it and prevent it from working – hence see an increase in FSH. However, if inject with normal serum, there is no peak of FSH as normally expected at this stage of MC.
- Experimental evidence of how this works at different stages
- There is an inter cycle rise in FSH and then it drops. It is predominantly due to oestrogen feedback as oestrogen increasing from the antral follicles will feedback and exert negative feedback.
- Dealing with the late follicular phase which corresponds with larger antral follicles. In these experiments, they took female rats and injected them either with normal rabbit serum or with an antiserum against inhibin. They did this in the late follicular phase where there are the big follicular follicles. In this late follicular phase, when measuring FSH, a drop in FSH would be seen in this stage of the cycle. In the rats which were treated with the antiserum, there was a rise in FSH instead. This is because the antiserum to inhibin will bind inhibin, preventing it from working (inhibiting FSH).
- The dip in FSH normally occurs due to negative feedback of both oestrogen and inhibin.
14
Q
What is Anti-Müllerian Hormone (AMH)?
A
- AMH is a glycoprotein and also a member of the TGFβ superfamily
- In males, it is expressed from week 8 of development
causes regression of the Müllerian ducts by a wave of apoptosis. - In 1980s, found to be expressed in rodent ovaries
- Over the last decade a new and interesting role for AMH has emerged in the ovary
- It is expressed by ovarian granulosa cells of follicles with levels peaking in selectable follicles (large preantral and small antral follicles), then decreasing. The levels of AMH peak in selectable follicles (early antral follicles). The granulosa cells will produce AMH and it reaches its peak at that stage of folliculogenesis. It can be measured in serum (not just in follicular fluid). It actually can be released and measured in serum, then it will decrease.
- AMH production in preantral follicles is variable, but has been detected from the primary stage onwards. Studies have been conflicting and it is thought to be different species (species variation).
15
Q
How is AMH distributed in follicles at different stages?
A
- Ovaries cut into small 5 micron sections and then every other section is stained to find the follicles (don’t know where they will be). The adjacent section can be used to detect any protein of interest.
- The subsequent section was stained with AMH antibody. The egg is really sticky; binds to any protein during immunohistochemistry. The granulosa cells of these small, early antral follicles shows intense brown staining (AMH protein). The theca shows no staining (AMH is not being produced by the theca). Can see the staining really decreases in the large, dominant preovulatory follicles. Again, no staining in the theca and very low staining (practically none) in the large follicle. This shows the change in AMH at different stages of follicle growth.
