4. The Menstrual Cycle

Question 1

Why is GnRH released in a pulsation manner?

Answer 1

Persistent presence of GnRH would lead to desensitisation of its receptors on gonadotrophs - FSH and LH production cease, gonadal steroid production ceases

Question 2

What are the 2 phases of the ovarian cycle?

Answer 2

Follicular phase - early and later

Luteal phase

Question 3

What happens in the early follicular phase?

Answer 3

Days 0-5

• No ovarian hormone production
• Granulosa cells secrete activin - increase FSH and FSH receptors on granulosa cells, reduced production of androgens in theca cells (less oestrogen)
• Stimulates follicle growth
• theca interna disappears
• follicle now capable of enhanced oestrogen production
• dominant follicle granulosa cells switch to inhibin

Question 4

When is the late follicular phase?

Answer 4

Days 5-14

Question 5

What happens in the late follicular phase?

Answer 5

• Dominant follicle granulosa cells produce oestrogen and inhibin
• Oestrogen and inhibin continue to rise
• Negative feedback to reduce FSH production
• Increased LH receptors on granulosa cells
• Inhibin reduces FSH production, helps LH to increase theca cell androgen production

Question 6

When is the ovulatory phase?

Answer 6

~day 14

Question 7

What happens in the ovulatory phase?

Answer 7

• Oestrogen continues to rise independent of FSH
• Oestrogen exert positive feedback, surge in LH production
• Ovulation occurs
• Granulosa cells that’s secreting progesterone (LH action)

Question 8

What happens in the luteal phase?

Answer 8

• Follicle is luteinised (corpus luteum)
• Secreted oestrogen and progesterone in large quantities
• Inhibin also secreted
• Oestrogen and progesterone exert negative feedback on LH
• FSH remains low

Question 9

How long does the luteal phase last?

Answer 9

14 +/- 2 days

Question 10

What are the 3 phases of the uterine cycle?

Answer 10

Menstrual, proliferation and secretory

Question 11

What impact does oestrogen have on the endometrium?

Answer 11

Proliferation epithelium - causes it to proliferate

Question 12

What impact do progesterone and oestrogen together have on the endometrium?

Answer 12

Secretory epithelium

Question 13

What are the layers of the uterus?

Answer 13

Endometrium - functional layer and basal layer
Myometrium
Perimetrium

Question 14

Which layer of the uterus responds to hormones?

Answer 14

Functional layer of endometrium - gets shed in a period

Question 15

What happens at the end of the menstrual cycle?

Answer 15

Corpus luteum -> corpu albicans
Dramatic fall in oestrogen, progesterone and inhibin
Loss of negative feedback, slow rising FSH
Menstruation occurs

Question 16

What happens if fertilisation occurs?

Answer 16

Syncytiotrophoblast produces hCG

Extends lifespan of corpus uterus before placenta forms

Question 17

What is the function of oestrogen in menstrual cycle?

Answer 17

During follicular phase

• thickening of endometrium, Fallopian tube function, grow and motility of myometrium
• thin alkaline cervical mucus, vaginal changes, changes in skin, hair and metabolism

Question 18

What is the function of progesterone in the menstrual cycle?

Answer 18

During luteal phase

• further thickening of endometrium (secretory), thickening of myometrium and reduction of motility
• thick acidic cervical mucus, development of breast tissue, increased body temp, metabolic changes, electrolyte changes

Question 19

What is normal duration of cycle length?

Answer 19

21 to 35 days

Variation is due to length of follicular phase

Question 20

What can cause disruptions to cycle?

Answer 20

Physiological factors - pregnancy, lactation
Emotional stress
Body weight
Infertility