S4: The Menstrual Cycle

Question 1

What are the aims of the menstrual cycle?

Answer 1

• Selects a single oocyte that is mature enough to be fertilised, this involves selection of the dominant follicle.
• Regular sponataneous ovulation to maintain fertility.
• Need to ensure that there are the correct number of chromosomes in the eggs.
• There are cyclical changes in the vagina, cervix and uterine tubes.
• There is preparation of the uterus, ready to receive a potential embryo and support it.

Question 2

What is the main control of the menstrual cycle?

Answer 2

The hypothalamic/pituitiary/gonadal axis (HPG) or more specifically, the hypothalamic/pituitary/ovarian axis. The products of the ovary (oestrogen and progesterone) feedback and control the production of GnRH and gonadotrophins (FSH/LH).

Question 3

List brief summary of steps in menstrual cycle.

Answer 3

The menstrual cycle consists of two phases separated by ovulation. Most women’s menstrual cycles are about 28 days long, remember that it is a cycle so this cycle occurs over and over again, with a new one starting roughly every 28 days.

1. The menstrual cycle begins on day 1 and this is the first day of bleeding (menses).
2. The next 14 (usually) days are the follicular phase where there is growth of the preantral follicle.
3. Ovulation occur at the end of the follicular phase.
4. The empty follicle collapses and transforms into the corpus luteum.
5. The next 14 days are the luteal phase which is dominated by the corpus luteum that secretes progesterone.
6. Menstruation occurs at the end of the luteal phase.

Question 4

What are the two phases the menstrual cycle can be divided into (based on germ cells)?

Answer 4

1. First phase is the follicular phase where there is growth of the recruited early antral follicles. One will be selected to keep growing and this is the dominant follicle. Remember that these follicles would have started off growing almost 3 months prior starting off as the primordial follicles, the local factors would have caused the early growth of the pre-antral follicles. This process is slow. Once there is recruitment of certain antral follicles that grow large enough at the right time, there is more rapid growth. The follicular phase lasts until ovulation.
2. The second phase of the menstrual cycle is called the luteal phase. The luteal phase is named after the structure formed from the remainder of the dominant follicle after the cumulus oocyte complex has been ovulated, the corpus luteum. The corpus luteum is the dominant structure of the luteal phase and the phase is dominated by progesterone secretion from the corpus luteum.

Question 5

How does the hypothalmic/pituitary/ovarian axis control the luteal phase?

Answer 5

• Negative feedback by progesterone.
• In the luteal phase, there is lots of progesterone released by the corpus luteum which negatively feeds-back to the hypothalamus and pituitary. This turns down the secretion of GnRH and gonadotrophins. In males there is also negative feedback of testosterone.
• Therefore in the luteal phase, there is high progesterone and low GnRH, LH and FSH.

Question 6

How does the hypothalmic/pituitary/ovarian axis control the follicular phase?

Answer 6

• Variable feedback.
• At the end of the luteal phase, the corpus luteum dies and progesterone levels fall. This releases the negative feedback that was occurring (i.e. stops exerting negative effect on hypothalamus/pituitary) causing a rise in FSH (intercycle rise in FSH).
• Then as the antral follicles grow, oestrogen starts to be produced which then reinstates the negative feedback so LH/FSH will fall.
• However, as the dominant follicle matures, we get closer to the middle of the cycle and oestrogen levels shoot up even higher and there is a switch to positive feedback. So oestrogen feeds-back positively increasing levels of the gonadotrophins.

Question 7

Difference in hypothalmic/pituitary/ovarian axis control of follicular and luteal phase

Answer 7

In the luteal phase there is only have negative feedback, so gonadotrophins kept low. But in the follicular phase there is switching from a lack of negative feedback, to negative feedback, to positive feedback.

Question 8

List main stages of the menstrual cycle

Answer 8

• Late luteal/ early follicular
• Mid follicular
• Mid cycle
• Mid luteal

Question 9

Describe the late luteal/ early follicular stage

Answer 9

• At the end of the luteal phase/beginning of follicular phase, the corpus luteum dies and progesterone levels decline.
• This releases the negative feedback/control on the hypothalamus/pituitary like letting go off the brake.
• In response to this, there is a selective increase in FSH levels called the inter-cycle rise in FSH.
• This intercycle rise in FSH, is what will signal/allow recruitment of the early antral follicles that are large enough, to be allowed to continue growing.

Question 10

Describe the mid follicular stage

Answer 10

• The antral follicles that have been recruited will start to produce lots of E2 (from granulosa cells).
• This will negatively feedback on the hypothalamus/pituitary and cause a big decline in FSH levels.
• The decline in FSH results in the death of all the follicles except one and this will become the dominant follicle which will survive and grow. So it is the rise and fall in FSH that ultimately selects the dominant follicle.

Question 11

Describe the mid cycle stage

Answer 11

• The dominant follicle will undergo very rapid growth and the granulosa cells will proliferate causing E2 levels to rise further.
• After two days of sustained high E2 (above 3000pmol), there will be a switch to a positive feedback relationship.
• So the E2 will not positively increase levels of gonadotrophins, preferentially LH causing LH surge.
• It is this LH surge that causes the final maturation of the oocyte within the dominant follcile to complete meiosis I and ovulation.
• The high LH then causes the empty follicle to become the corpus luteum which will then start to produce progesterone.

Question 12

Describe the mid luteal stage

Answer 12

• High levels of progesterone secreted by the corpus luteum result in negative feedback causing levels of LH/FSH to decline.
• This fall happens because during this time, we hope that the oocyte that has been ovulated has been fertilised and implantation will occur. If this happens, we want to try and sustain this fertilised embryo and not start growing new follicles. So by turning off FSH/LH it would enable a single embryo to start growing.
• If at the end of 14 days there has been no pregnancy, the corpus luteum will die and progesterone levels will fall again, the -ve feedback released and the cycle will restart.

Question 13

What is the intercyle rise and fall of FSH?

Answer 13

The intercycle rise and fall in FSH is very important because it allows selection of the dominant follicle. The intercycle rise occurs at the beginning of the menstrual cycle (late luteal/early follicular), when the progesterone is decreasing which releases the negative feedback. This causes selective increase in FSH.
The increase in FSH enables a select group (cohort) of antral follicles to continue to grow, as they grow they start to produce oestrogen and this exerts a negative feedback causing FSH to fall. As FSH falls, the dominant follicle in the cohort of recruited follicles will be selected and grow rapidly.

Question 14

Describe follicle selecting with the window of opportunity and FSH threshold hypothesis

Answer 14

The point in the cycle where the dominant follicle is selected is called the window of opportunity. A rise and fall in FSH presents the window that is needed to select a dominant follicle called the FSH threshold hypothesis.

• There is a cohort of preantral follicles that started growing several months previously.
• When these follicles are at the right stage reach the right size, they require FSH to progress.
• So several follicles reach the antral stage before there is the rise in FSH, they can’t sustain their growth so die off.
• The lucky follicles that reach the right size and right stage of growth when there is the rise in FSH, will be recruited into the menstrual cycle.
• These recruited antral follicles will begin to grow more and start to release E2, this causes FSH to fall, therefore any small follicles here that reach antral will not be recruited and die off. This presents the window.
• One of the recruited follicles will be selected as the dominant follicle due it it reaching the right stage at the right time in the ovary. There will be expansion of granulosa cells etc. causing the E2 levels to increase further and reinstate the -ve feedback and suppress other recruited follicles (due to falling FSH) so only the dominant one will grow. The other follicles die off by atresia.
• The dominant follicle will then go on to ovulate.
• This whole process is known as follicle selection.

Question 15

How does the dominant follicle survive the fall in FSH (which prevents other recruited antral follicles from growing)?

Answer 15

• As the FSH falls, LH increases.
• The dominant follicle switches on genes and acquires LH receptors on the granulosa cells. The other recruited antral follicles do not and so lose their stimulant to grow and then die off.
• It also has increased sensitivity to FSH due to increased FSH receptors and these are better coupled with their downstream signalling. It is therefore more sensitive to FSH levels, despite them being lower it is enough to maintain growth.
• Increased number of granulosa cells. Dominant follicle is not always the biggest recruited antral follicle, but it has to most granulosa cells to multiply This increases E2 production because of increased aromatase levels.

Question 16

Describe the growth of the dominant follicle

Answer 16

• Once the dominant follicle is selected around mid-follicular phase, it grows rapidly doubling in diameter in just 7 days. Going from 7mm – 14mm.
• The dominant follicle requires masses of growth factors, nutrients and steroids, there is rapid neoangiogenesis to enable this.
• The granulosa cells also release high amounts of E2 into the circulation.

Question 17

What receptors are found on theca and granulosa cells on the follicles? How do they affect the menstrual cycle through steroidgenesis?

Answer 17

• The theca cells always have LHr and never FSHr. LH drives androgen and progesterone production from the theca. The androgens are made in the theca only (testosterone/androstenedione), these can then be aromatised in the granulosa cells to oestrogens.
• Granulosa have FSHr, they then aquire LHr from the midfollicular phase onwards in the dominant follicle. The FSH and LH drive oestrogen production in the follicular phase. Only granulosa cells contain aromatase.
• Steroids originate from cholesterol and originate from cleavage of C atoms from the cholesterol by a variety of enzymes. These enzymes are located in different parts of the cell, some in mitochondria some in cytosol.

Question 18

What causes LH surge?

Answer 18

• Throughout the follicular phase, E2 feedback was negative, however nearing the end of the follicular phase E2 levels have been raised for long enough that the feedback switches from negative to positive.
• This results in a massive release of LH from the pituitary, as a consequence there is an exponential rise in serum LH.

Question 19

What does the LH surge cause?

Answer 19

LH surge triggers the ovulation cascade which involves:

• The cumulous oocyte complex gets released from the dominant follicle. This is the oocyte surrounded by cumulous cells.
• The empty follicle undergoes lutenisation which forms the corpus luteum.
• There is lutenisation of granulosa and theca cells.
• E2 production falls and progesterone secretion is stimulated and increases.
• Oestrogen is still present but progesterone is the dominant steroid at this point in the cycle.
• Later, the nucleus of the oocyte in the dominant follicle completes the first meiotic division.

Question 20

Describe mechanism of ovulation

Answer 20

• Blood flow to the pre-ovulatory follicle increases dramatically. There is an increase in vascular permeability also which increases intra-follicular pressure.
• Follicle has moved back to the cortex of the ovary, this produces an appearance of an apex on ovary wall.
• There is local release proteases in order to enzymatically breakdown the protein of the ovary wall.
• 18 hours after peak LH surge, a hole appears in follicle wall and ovulation occurs.
• So the oocyte with the cumulus cells is extruded from the ovary under pressure (cumulus oocyte complex). During this process fluid may pour into the Pouch of Douglas.
• Cumulus cells expand due to LH surge and they become a sticky mass. So the cumulus-oocyte complex allows it to be picked up by fimbriae. The cumulus cells also provides a big barrier for the sperm.
• The egg is collected by the fimbria of the uterine tube, the egg then progresses down the uterine tube by peristalsis and action of the cilia. Patent tubes is needed (tubes open and close)

Question 21

What is the state of the oocyte in the follicle before ovulation?

Answer 21

• The oocyte as discussed is packaged up into the follicles etc. but crucially we need the oocyte to ultimately be haploid at fertilisation.
• From the oocytes initial formation in the foetal ovary up until it is ovulated it remains arrested in the meiotic I division at anaphase. This allows the oocyte to retain all its DNA and remain as large as possible during its long wait.

Question 22

Describe meiosis, formation of secondary oocyte and extrusion of the polar body (oocyte in response to LH surge after ovulation)

Answer 22

• Half of the chromosomes are put into a small “package” in the egg called the 1st polar body.
• The egg is now a secondary oocyte. It is an unequal division (cytokinesis), with the secondary oocyte containing almost all the cytoplasm, the organelles, proteins etc. This is because the egg contains all the nutrients to sustain the early pregnancy, whereas the sperms function is to simply transport half the chromosomes to the egg.
• The 1st polar body plays no further part in the process and doesn’t divide again and it gets extruded.
• The secondary oocyte begins meiosis II but arrests again, it will stay in meiosis II until it is fertilised.
• If the egg doesn’t get fertilised it is lost.

Question 23

Describe fate of secondary oocyte - fertilisation

Answer 23

• Unlike sperm, we only want a single oocyte, the oocyte is also the largest cell in the body while sperm are the smallest. This is because the oocyte has to support all of the early cell divisions of the dividing embryo until it establishes attachment to the placenta.
• The oocyte will spend a few days in the uterine tube, where it may meet the sperm and be fertilised. It will then finish meiosis II to produce a second polar body containing the identical sister chromatids and oocyte becomes truly haploid and the two nuclei will meet and you will have a diploid cell.
• Half the chromosomes from the mother, with the 23 homologous chromosomes coming from the fathers sperm.

Question 24

Describe corpus luteum formation and role

Answer 24

• After ovulation the empty follicle collapses, it gets lutenised into the corpus luteum (“yellow body”). The LH surge is responsible.
• The corpus luteum secretes progesterone which increases greatly, also there is some E2 secretion.
• The corpus luteum contains a large number of LH receptors so is maintained by low levels LH and hCG if a pregnancy occurs, if a pregnancy doesn’t occur it will die after 14 days.
• Corpus luteum is maintained for so long because it keep progesterone going to prevent another cycle from starting and maintain uterus lining.
• If pregnancy does occur, the trophoblast of the embryo will produce hCG which maintains the corpus luteum. The corpus luteum then persists to support the embryo until the placenta is formed.

Question 25

What are the secretions of the corpus luteum and their function?

Answer 25

1. Progesterone.
   - Supports the oocyte along its journey.
   - Prepares the endometrium.
   - Controls cells in the uterine tubes.
   - Alters secretions of the cervix (more thick and gloopy cervical mucus).
2. Oestrodiol.
   - Required for successful endometrial functioning and implantation.

Question 26

Describe the demise of the corpus luteum

Answer 26

If fertilisation doesn’t occur, the corpus luteum dies after its lifespan of 14 days.

• Removal of the corpus luteum is essential to initiate the next cycle.
• Cell death occurs, the vasculature breaks down and CL shrinks. However the process is still not well understood.

Question 27

What hormone changes in pregnancy?

Answer 27

Levels of hCG are very high during first few months of pregnancy to maintain the corpus luteum until placenta is well established and can take over.

Question 28

Describe some clinical applications for the menstrual cycle that arise from the concept of feedback of HPG axis

Answer 28

Our understanding of GnRH action and feedback have permitted us to develop pharmacological agents to manipulate the MC.

• So analogues of GnRH which are not broken down reduce gonadotrophin release or you can give pulatile to stimulate it.
• Injections of LH/FSH for infertility.
• OCP which prevents LH and FSH release as it is constant negative feedback on pituitary.
• Clomid given for infertility treatment as it disrupts negative feedback.
• Hormone replacement therapy.

Question 29

What are activins and inhibins?

Answer 29

They are other factors produced from the ovary called activins and inhibins which control feedback in HPG axis.

Question 30

List some other factors not in 3 tier system that feed into HPG axis

Answer 30

• Nutritional systems.
• Adipocytes via leptin.
• Pancreas feeds into the cycle via insulin.
• Liver feeds in via IGF-1.
• Adrenals feed in e.g. corisol.
• VEGF, TGF-beta also feed in.
  We can see the menstrual cycle is very much effected by the environment of the body and this is because we have to be fit to reproduce and the environment must be right. The body needs to know the woman can cope with the burden of carrying a baby.

Question 31

How is the menstrual cycle clinically noted?

Answer 31

Day 1 of the menstrual cycle is the first day of bleeding and the bleeding usually lasts 3-8 days. Clinically we would write this in the notes as 7/28 (7 days of bleeding out of a 28 day cycle). For someone with whose periods vary slightly we would write this as 5-6/27-32.

• 3-8 days is usual for mensuration.
• Clinically you should ask a women how long there cycle is from the 1st day of bleeding to the 1st day of bleeding in the next cycle – NOT do you have regular cycles!

Question 32

What causes the variation of mensuration in the menstrual cycle?

Answer 32

This variation in cycles occurs in the follicular phase, it is the follicular phase that is variable, the luteal phase is constant.
The reason for this is the corpus luteum has a set lifespan of 14 days, after which it dies (constant). So if an individual has a 32 day cycle one month, it means that the follicular phase has been extended, if the next month she has 27 day cycle, it means the follicular phase was shorter. (27-14 = 13 day follicular phase).

Question 33

What is a regular cycle?

Answer 33

A regular cycle should have no more than 4 days variation from month to month. So going from 28 in one month to 32 in the other is still regarded as having regular cycle.

Question 34

How does a patient work out when they are going to ovulate (ovulation prediction)? This is for pregnancy

Answer 34

• If a woman’s cycles are regular, then it is easy, simply 14 days from the first day of bleeding you will ovulate. But if cycles are irregular due to whatever reason (e.g. anorexic) you could try monitor your temperature which increases when you’re going to ovulate. So you would have to keep a chart of your temperature. This method is not hugely reliable however.
• Another option is to have intercourse 3 times a week, as sperm can survive for 5 days after it means that by chance you’ll have sex around the time you ovulate. However for many couples that are busy, this isn’t feasible.
• The best option would be ultrasound monitoring, so you can see the dominant follicle. They appear black on ultrasound as they are fluid filled. This again isn’t practical because the majority of people do not have ultrasound machines.
• There are changes in cervical mucus, closer to ovulation time is becomes thin and more runny so easier for sperm to travel.
• Ovulation prediction is commonly done using “ovulation sticks”. Can measure metabolite of oestrogen in urine and LH surge –> allow women to identify days of high fertility leading up to ovulation.
  These detect LH because roughly 18hrs after the LH surge there is ovulation. Problem with this is they have to be used very often and are expensive.

Question 35

Signs of ovulation

Answer 35

• A slight rise in basal body temperature, typically 0.5 to 1 degree fahreneheit, measured by a thermometer. Need to keep a chart of basal body temp from day 1 of LMP (period) to see fluctuation.
• Tender breasts.
• Abdominal bloating.
• Light spotting.
• Changes in cervical mucus.
• Slight pain or ache on one side of the abdomen.

Question 36

The menstrual cycle as a whole summary

Answer 36

1. (End of first mentural cycle). At the end of the late luteal phase the corpus luteum is dying, this causes a decrease in progesterone levels, as a result the negative feedback is released and there is the intercycle rise in FSH
2. The intercycle rise in FSH presents the window of opportunity, where a cohort of late pre-antral/early antral follicles will be recruited into the menstrual cycle to continue their growth.
   3 As the cohort grows, their granulosa cells will secrete oestrogen, this will cause serum oestrogen to rise which reinstates the negative feedback. Thus there is a fall in FSH levels.
3. As FSH levels fall, a single dominant follicle will be recruited to continue its growth. This will secrete even more oestrogen which will result in further –ve feedback and further decrease in FSH.
4. The other follicles not selected will die off due to the low FSH
   However the dominant follicle continues to grow as the granulosa cells begin to express LH receptors.
5. The dominant follicle doesn’t die like the other follicles due to the presence of the LH receptors it develops that allow continued stimulation of the follicle. As the FSH falls, LH increases.
6. After around 2 days of this sustained high oestrogen, nearing the end of follicular phase it results in a switch to positive feedback. LH increases massively, this is the LH surge.
7. 1. 18hrs after the LH surge meiosis I gets completed and the cumulous oocyte complex is ovulated leaving behind the empty follicle which becomes the corpus luteum. Now in the luteal phase.
8. The corpus luteum secretes progesterone, which causes negative feedback, so FSH and LH fall. There is a rise in oestrogen but it is not as big as progesterone which dominates the luteal phase.
9. Other follicles that have matured enough will die off because the FSH is very low.