7. Menstrual cycle and the HPG axis

Question 1

what are the difference in gametogenesis of makes and females ?

Answer 1

gametogenesis in men is continuous.

female = it takes time to prepare the uterus for
implantation of a fertilised oocyte. Therefore, release of this oocyte needs to be periodic, otherwise even if fertilisation occurs, the uterus will not be able to sustain it. A typical cycle lasts between 21-35 days.

Question 2

What are the 2 types of preparation that need to occur in a female and which cycles are responsible for them?

Answer 2

preparation of the ovary to provide an environment for oogenesis to occur and production of steroid hormones (‘the ovarian cycle’), and preparation of the uterus to receive a fertilised oocyte (‘the uterine cycle’).

Question 3

in relation to each other, when do the ovarian and uterine cycles occur?

Answer 3

2 cycles in parallel

Average length of 28 days

Question 4

what are the important hormones for the menstrual cycle

Answer 4

GnRH
 LH, FSH
 Oestrogen*, Progesterone, Inhibin, activin

Question 5

why is GnRH released in a pulsatile manner?

Answer 5

Persistent presence of GnRH would lead to desensitisation of it’s receptors on gonadotrophs
 FSH and LH production cease
 Gonadal steroid production ceases
results in infertility

Question 6

What are the stages of the ovarian cycle?

Answer 6

Follicular and luteal

Question 7

What are the stages of uterine cycle?

Answer 7

Proliferative and Secretory

Question 8

Define endometriosis.

Answer 8

Ectopic endometrial tissue, most commonly in ovaries, fallopian time or pelvic organs

Question 9

How can symptoms of endometriosis be relieved?

Answer 9

Giving continuous GnRH to such patients can ‘switch the axis off’ to alleviate their symptoms.

Question 10

What is the first hormone in he menstrual cycle that starts to rise and why?

Answer 10

FSH, low steroid and inhibit levels at start of the cycle, little inhibition of the hypothalamus or AP.

Question 11

What cells does FSH bind to and what effects does its initial rise have?

Answer 11

Stimulate further follicular development, Theca interna develops,

Question 12

What are the first hormones produced by the ovaries in the menstrual cycle and what is this in response to?

Answer 12

Oestrogen and inhibin, in response to FSH

Question 13

what is LH resposnsible for?

Answer 13

‘LH surge’ is the key factor that drives ovulation.

Question 14

what is the function of the ovarian cycle and what are the two stages?

Answer 14

Preparation of the gamete for release
2 phases:
 Follicular phase – Early and Late
 Luteal phase

Question 15

what happens in the early follicular phase?

Answer 15

• No ovarian hormone production = no inhibition on the hypothalamus or AP
• Granulosa cells secrete activin
- ↑ FSH production, ↑ FSH receptors on granulosa cells
- Reduced production of androgens by theca cells – less oestrogen (for now)
• FSH levels increase
- Stimulates follicle growth - number of granulosa cells increases, and causes the development of the
theca interna and externa cells
- The follicle will now produce oestrogen from both the granulosa cells and the theca cells
• Dominant Follicle granulosa cells switch to produce inhibin (from activin)

Question 16

what happens in the late follicular phase?

Answer 16

• Dominant Follicle granulosa cells produce oestrogen and inhibin.
• Oestrogen
- ↑ FSH receptors on follicle
- Initially exerts negative feedback to reduce FSH production
- ↑ LH receptors on granulosa cells – prep for ovulation

• Inhibin

• decreases FSH production
• Helps LH to ↑ Theca cell androgen production (converted to Oestrogen)

• Oestrogen + Inhibin continue to rise

Question 17

what happens in the late follicular phase after Oestrogen + Inhibin continue to rise ?

Answer 17

• Oestrogen continues to rise independent of FSH
• High levels of Oestrogen exert positive feedback on
hypothalamus + anterior pituitary → Surge in LH
production
• Ovulation occurs
• Rise is not seen in FSH levels due to presence of inhibin
• Granulosa cells starts secreting progesterone (LH action)

Question 18

why does inhibin inhibit FSH release?

Answer 18

to prevent FSH stimulating more than one dominant follicle.

Question 19

once ovulation occurs, is meiosis 1 and 2 complete?

Answer 19

MEIOSIS I completed
MEIOSIS II begun but not completed yet
Mature oocyte travels through the capsule of the ovary into the fallopian tube

Question 20

what happens in the luteal phase?

Answer 20

• Follicle is luteinised ( granulosa and theca cells become the Corpus Luteum)
• Secretes oestrogen and progesterone in large quantities
• Inhibin also secreted
• Oestrogen + Progesterone exert negative feedback on LH - (The high levels of oestrogen alone would cause positive feedback, however the presence of
progesterone promotes the negative feedback of oestrogen at the hypothalamus and AP)
• FSH levels remain low
• Corpus luteum lifespan 14 ±2
• WAITING PHASE

Question 21

what is the function of the luteal phase?

Answer 21

This phase happens after ovulation, and it is the stage of the cycle where the reproductive system is waiting to see if fertilization and implantation has occurred.

Question 22

what are the 3 phases of uterine cycle?

Answer 22

Menstrual, Proliferative and Secretory

Question 23

define myometrium and endometrium

Answer 23

The myometrium is the muscular wall of the uterus and the endometrium is the epithelial lining of the uterine cavity

Question 24

what is the endometrium divided into?

Answer 24

 Functional layer – responsive to hormone, shed in menstruation
 Basal layer – develops into new functional layer

Question 25

which hormones are responsible for the proliferative and secretory phases respectively?

Answer 25

Oestrogen → Proliferative epithelium

Progesterone + Oestrogen → Secretory epithelium

Question 26

what happens in the proliferative phase?

Answer 26

At the start of the cycle, the endometrium will proliferate and thicken in response to oestrogen produced by the ovary. There are simple, straight
glands within the endometrium but as this part of the cycle continues and the endometrium continues to develop, the glands become coiled and the functional layer doubles in size

Question 27

what happens in the secretory phase?

Answer 27

Once ovulation has occurred, the glands that have been produced in the proliferative stage then become secretory under the influence of progesterone. Towards the end of this stage, when the hormones from the corpus luteum begin to fall, the glands then lose their structure and the endometrium becomes ready to shed its functional layer if implantation has not occurred.

Question 28

what happens at the end of the menstrual cycle?

Answer 28

Corpus luteum → Corpus Albicans
Dramatic fall in oestrogen, progesterone and inhibin
Loss of negative feedback → slow rising FSH
New cycle: Menstruation occurs

Question 29

what is the action of oestrogen in the follicular phase?

Answer 29

The effect of oestrogen on the reproductive tract is intended to promote
fertilisation:
• Proliferation of endometrium
• Proliferation of myometrium
• Fallopian tube motility
• Thin, alkaline cervical mucus

Question 30

what is the action of progestrogen in the luteal phase?

Answer 30

The effect of progesterone on the reproductive tract is intended to sustain a viable pregnancy:
• Secretory function of endometrium
• Reduction of motility of myometrium (assumes fertilisation has occurred)
• Thick, acidic cervical mucus to prevent further entry of sperm

Question 31

what happens if fertilisation occurs?

Answer 31

Syncytiotrophoblast produces hCG (human chorionic
gonadotropin)(same function as LH)
Extends lifespan of corpus luteum → before placenta
forms

Question 32

what is a normal cycle length?

Answer 32

Normal duration: 21 to 35 days

Question 33

what may disrupt the cycle?

Answer 33

 Physiological Factors: Pregnancy, Lactation
 Emotional stress
 Body weight
 Infertility – various causes