Female reproductive physiology

Question 1

List the phases and duration of the menstrual cycle:

Answer 1

Menstrual cycle duration 28-35 days.

Follicular phase:

• Starts on first day of menses
• Finishes day before LH surge.
• Variable duration 14-21 days.

Luteal phase:

• Starts on day of LH surge
• Finishes on first day of menses.
• Fixed duration 14 days.

Question 2

Describe the process of oogenesis:

Answer 2

Before life:

• Primordial germ cells migrate from extraembryonic tissue near yolk sac to gonadal ridge.
• Mitosis: oogonia undergo mitotic division increasing in number and peak in late 2nd trimester.
• Oogonia begin meiosis but stop prior to metaphase I; now oocytes and stay at this point until ovulation.
• Oocytes develop into primordial follicles and have a thin layer of cells around them; oocytes that do not develop into primordial follicles are lost so that number of oocytes decreases dramatically before birth.

Question 3

Regarding the menstrual cycle:

Describe the changes that occur during the follicular phase:

Answer 3

Start of period/early follicular:

• Loss of negative feedback from progesterone, oestradiol and Inhibin A –> increase in GnRH pulses –> increase in FSH secretion.
• High Inhibin B: negative feedback on FSH.

Mid-follicular phase:

• Rising FSH –> folliculogenesis and oestradiol production.
• Granulosa cells produce increasing levels of oestradiol and Inhibin A.

Late follicular phase:
- Oestradiol and inhibin A –> negative feedback on FSH and LH –> decreased FSH and LH.

Question 4

Regarding the menstrual cycle:

Describe the changes that occur during the luteal phase:

Answer 4

Luteal phase:

• Oestradiol concentrations peak on day before ovulation –> sudden switch from negative to positive feedback on LH –> 10 x LH surge
• Dominant follicle oocyte completes first meiotic division and oocyte ovulates 36 hours after LH surge.
• Corpus luteum begins secreting progesterone.

Mid- to late luteal phase:

• Increasing progesterone secretion from corpus luteal –> negative feedback on LH secretion.
• Inhibin A also secreted by CL and peaks.
• Note: there is no Inhibin B secretion during the luteal phase.
• No implantation occurs: CL involutes and oestradiol and progesterone levels fall –> withdrawal of negative feedback control of HPA –> FSH begins rising again in next cycle.

Question 5

Describe the process of folliculogenesis and ovluation:

Answer 5

Primary follicle development:

• Oocytes begin to increase in size and pregranulosa cells enlarge.
• Theca cells develop.
• Antral cavity appear with follicular fluid.

Secondary/antral follicle:

• 15-20 follicles develop over 2 weeks due to FSH stimulation.
• Follicles in turn secrete increasing amounts of oestradiol (theca cells produce androgen that are aromatised to oestradiol by granulosa cell aromatase).
• Oestradiol negatively feedsback to pituitary to reduce FSH which causes smaller follicles to undergo atresia.

Tertiary/Graafian follicle:

• A dominant follicle around 20 mm develops.
• Granulosa cells become receptive to LH as well to compensate for reduction in FSH secretion.
• Midcycle LH surge causes oocyte to enter metaphase I and will complete meiosis by the time of fertilisation.
• PGs responsible for follicle wall thinning and ovulation (release of oocyte from follicle).
• Luteinisation of granulosa cells: being producing progesterone.

Question 6

Describe the process of luteolysis:

Answer 6

Luteal progesterone is dependent on LH.
As corpus luteum matures, it becomes less sensitive to LH and gradually produces less progesterone and eventually involutes if implantation does not occur.

Question 7

List the secretory products of the corpus luteum:

Answer 7

• Progesterone
• Inhibin A
• Oestradiol