5- The Menstural Cycle

Question 1

What are the aims of the menstrual cycle?

Answer 1

• 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

Question 2

Why does GnRH secretion need to be pulsatile?

Answer 2

to maintain LH secretion

Question 3

How many phases are in the menstrual cycle and what are these phases separated by?

Answer 3

2 phases - separated by ovulation
follicular phase
luteal phase

Question 4

What is the follicular phase of the menstrual cycle?

Answer 4

The first half of the menstrual cycle before Ovulation
The growth of follicles up to ovulation → dominated by oestradiol production from follicles

Question 5

What is the luteal phase of the menstrual cycle?

Answer 5

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

Question 6

When does the menstrual cycle begin?

Answer 6

It begins on the first day of bleeding - and the cycle can last up to 28 days

Question 7

When does ovulation occur in the menstrual cycle?

Answer 7

Ovulation occurs at the end of the follicular phase, the middle of the cycle

Question 8

Describe how the feedback in the follicular phase of the menstrual cycle is variable

Answer 8

• Release of negative feedback
• Negative feedback then reinstated, then
• Switch from negative to positive feedback

Question 9

What is the level of feedback in the luteal phase of the menstrual cycle?

Answer 9

Negative feedback - dominated by progesterone

Question 10

Describe the hormonal changes that occur during the menstrual cycle

Answer 10

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

Question 11

Why is the inter-cycle rise and fall in FSH important?

Answer 11

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

Question 12

Describe the process of follicle selection

Answer 12

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.

Question 13

How does the dominant follicle survive the fall in FSH?

Answer 13

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.

Question 14

What are some rules of receptors on follicles?

Answer 14

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

Question 15

Describe the LH surge during the menstrual cycle.

Answer 15

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.

Question 16

What is the ovulation cascade and what triggers it?

Answer 16

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

Question 17

Explain the cascade of events that occur during ovulation?

Answer 17

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

Question 18

Describe the divisions that occur in the dominant follicle in response to the LH surge

Answer 18

• 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.

Question 19

Describe the difference in appurtenance between the 1st polar body and the secondary oocyte

Answer 19

The first polar body maintains less cytoplasm than the secondary oocyte
The secondary oocyte is composed of majority of the cytoplasm after division

Question 20

Describe the properties of a secondary oocyte

Answer 20

• 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

Question 21

How long does the oocyte spend in the fallopian tube?

Answer 21

About 2-3 days, where if it meets with a sperm, fertilisation will occur.

Question 22

Describe the formation of the corpus luteum

Answer 22

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).

Question 23

What are the secretions of the corpus luteum and why are they important?

Answer 23

progesterone:
- supports oocyte in its journey
- Maintains the CL
- prepares the endometrium
- controls cells in Fallopian tubes
- alters secretions of cervix

oestradiol:
- for endometrium

Question 24

Describe the demise of the corpus luteum

Answer 24

• 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

Question 25

What are the signs of ovulation?

Answer 25

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

Question 26

Which hormone is detected in ovulation test kits?

Answer 26

LH