Menstrual Cycle

Question 1

What is the main aim of the menstrual cycle?

What are the 4 phases involved in the menstrual cycle?

Give a brief outline of the Follicular phase

Give a brief outline of the Luteal phase

Answer 1

• Prepares the uterus and provides support for the fertilised egg
• Bleeding → Follicular phase (Variable) → Ovulation → Luteal phase (Fixed = 14 days)
• • Growth of follicles till Ovulation
  • OESTRADIOL production from Dominant follicle
• • Formation of Corpus Luteum (CL)
  • Progesterone production from CL

Question 2

What hormonal change occurs in the Late Luteal/Early Follicular phase?

What hormonal change occurs in the Mid Follicular phase?

What hormonal change occurs in the Mid Cycle (Ovulation) phase?

What hormonal change occurs in the Mid Luteal phase?

Answer 2

• Progesterone declines to selectively RAISE FSH = INTER-CYCLE RISE
• Oestradiol increases negative feedback, leading to FALL in FSH
• High Oestradiol over a couple of days causes a switch to Positive feedback = LH SURGE
• Progesterone increases to cause negative feedback = Low LH/FSH

Question 3

Follicular Selection:
What does the Inter-cycle rise in FSH allow for?

What is the FSH threshold hypothesis?

How is the Dominant follicle able to survive the fall in FSH?

What other 2 properties does the Dominant follicle have?

Answer 3

• Allows for Selection of a single follicle and a Window of opportunity to recruit antral/secondary follicles that are at the right stage to continue growth
• In a pool of antral follicles, One of them is at just the right time to survive the declining FSH, and becomes the dominant follicle for ovulation
• Dominant follicle acquires LH receptors on its Granulosa cells, so is able to receive stimulation during LH surge to survive the fall in FSH, while other follicles aren’t stimulated (=Atresia)
• • Increased sensitivity to FSH = More FSH receptors
  • Increased number of Granulosa cells = more Oestrogen production

Question 4

LH Surge:
Why does this happen?

What does this trigger?

Answer 4

• At the end of the Follicular phase, Oestradiol levels were raised high and long enough for the feedback to switch to Positive = Huge release of LH from pituitary
• The Ovulation Cascade - Release of egg, Luteinisation (Formation of CL), then Oestradiol (E2) decreases and Progesterone increases

Question 5

Ovulation:
What occurs to the follicle during this time?

What causes the breakdown of the ovary wall?

Once the ovary wall breaks down, what is released?

Where can the follicular fluid pour out into?

How is the oocyte drawn in and moved along the uterine tube?

Answer 5

• Increased blood flow to follicle and vascular permeability leads to increased Intrafollicular pressure = Appearance of Apex on ovary wall
• Local releases of Proteases and Inflammatory mediators
• Oocyte with Cumulus cells still attached
• Pouch of Douglas (Rectouterine)
• Drawn in by Fimbriae and moved by peristalsis and cilia

Question 6

Preparation of the Oocyte:
Once ovulated, what occurs to the oocyte?

How does it then divide?

What occurs after this division?

What is the function of the Secondary oocyte?

Answer 6

• LH surge allows the oocyte to complete the 1ST MEIOTIC division, but it doesn’t divide
• ½ of the chromosomes are stored in the 1st Polar body and the rest of the egg with most of the cytoplasm is now called the Secondary oocyte
• Oocyte begins 2ND MEIOTIC division, but arrests and stays in this stage again
• Supports early cell divisions of the dividing embryo before implantation

Question 7

Corpus Luteum (CL):
What does it produce? What do these do?

What structural property does it have?

In pregnancy, what is produced and what does it do?

What’s its lifespan?

Why is the removal of the CL necessary to initiate a new cycle?

Answer 7

• • Progesterone - SUPPORTS OOCYTE, MAINTAINS CL, PREPARES ENDOMETRIUM, Controls cells of Uterine tubes, Alters Cervical secretions
  • Oestradiol - SUPPORTS ENDOMETRIUM
• Has a large number of LH receptors, so is supported by LH and hCG (if pregnant)
• hCG produced to bind to the LH receptors to prevent the CL from dying off, therefore allowing the CL to support the implanting embryo till a placenta is formed
• 14 days if there isn’t fertilisation (no hCG support)
• Once gone, it allows for a drop in progesterone and negative feedback