Lecture 8- The Menstrual Cycle

Question 1

The Hypothalamic-Pituitary-Gonadal (HPG) Axis

Answer 1

The hypothalamus, anterior pituitary gland and gonads (ovaries) work together to regulate the menstrual cycle.

Question 2

Gonadotropin releasing hormone (GnRH) from the hypothalamus stimulates

Answer 2

luteinising hormone (LH) and follicular stimulating hormone (FSH) release from the anterior pituitary gland.​

Question 3

LH and FSH are gonadotropins that act primarily on the ovaries in the female reproductive tract: FSH binds to

Answer 3

FSH binds to granulosa cells to stimulate follicle growth, permit the conversion of androgens (from theca cells) to oestrogens and stimulate inhibin secretion

Question 4

LH and FSH are gonadotropins that act primarily on the ovaries in the female reproductive tract: LH binds to

Answer 4

acts on theca cells to stimulate production and secretion of androgens

Question 5

Moderate oestrogen levels exert a ………….feedback on the HPG axis

Answer 5

negative

Question 6

High oestrogen levels (in the absence of progesterone (due to increased inhibin) ………..feedback on the HPG axis

Answer 6

positively

Question 7

Oestrogen in the presence of progesterone exerts …………….feedback on the HPG axis

Answer 7

negative

Question 8

Inhibin selectively inhibits …………at the anterior pituitar

Answer 8

FSH

Question 9

summary of the feedback systems acting on the HPG axis

Answer 9

Question 10

the menstrual cycle is

Answer 10

Menstrual cycle

• 2 cycles happening in parallel- ovarian and uterine
  
  • Ovarian cycle- 2 phases
    
    • Pre ovulation- follicular phase
    • Post ovulation- luteal phase
  • Uterine cycle- 2 phases
    
    • Pre-ovulation
      
      • Period
      • Proliferative
    • Post-ovulation
      
      • secretory

Question 11

average length of menstrual cycle

Answer 11

• Average length =28 days

Question 13

GnRH release in females

*

Answer 13

• Female HPG axis is pulsatile
• Persistent presence of GnRH would lead to desensitisation of its receptors on gonadotrophs
  
  • FSH and LH production cease
  • Gonadal steroid production ceases

Clinical importance

• GnRH analogous e.g. Decapeptyl aka Triptorelin pamoate- used in treatment of endometriosis and infertility

Question 14

the ovarian cycle involves the

Answer 14

follicular phase- pre-ovulatory

Luteal phase- post ovulatory

Question 15

The follicular phase marks the

Answer 15

beginning of a new cycle as follicles (oocytes surrounded by stromal cells) begin to mature and prepare to release an oocyte.

Question 16

At the start of a new cycle (menses) there is little

Answer 16

ovarian hormone production and the follicle begins to develop independently of gonadotropins or ovarian steroids.

Question 17

Due to the low steroid and inhibin levels at the begining of the follicular phase….

Answer 17

there is little negative feedback at the HPG axis, resulting in an increase in FSH and LH levels. These stimulate follicle growth and oestrogen production.

Question 18

how many follicles can contunue to maturity and compelte each menstrual cycle

Answer 18

Only one dominant follicle can continue to maturity and complete each menstrual cycle.

Question 19

As oestrogen levels rise,

Answer 19

negative feedback reduces FSH levels (inhibin), and only one follicle can survive, with the other follicles forming polar bodies.

Question 20

when is positive feedback at the HOG axis initiated

Answer 20

when oestrogen eventually becomes high enough

This…..increaseslevels of GnRH and gonadotropins.

Question 21

why do we only see a surge in LH when oestrogen levels initiate a positive feedback on the hypothalamus and pituitary

Answer 21

due to the increased follicular inhibin, selectively inhibiting FSH production at the anterior pituitary.

Granulosa cells become luteinised and express receptors for LH.

(LH SURGE= ovulation)

Question 22

Ovulation is in response to

Answer 22

to the LH surge , the follicle ruptures and the mature oocyte is assisted to the fallopian tube by fimbria. Here it remains viable for fertilisation for around 24 hours.

Question 24

what happens to the empty follicle after ovulation

Answer 24

the follicle remains luteinised, secreting oestrogen and now also progesterone, reverting back to negative feedback on the HPG axis. This, together with inhibin (inhibits FSH) stalls the cycle in anticipation of fertilisation.

Question 25

luteal phase represents the

Answer 25

corpus luteum development and maintainence

Question 26

what is the corpus luteum and what does it produce

Answer 26

is the tissue in the ovary that forms at the site of a ruptured follicle following ovulation. It produces: * oestrogens * progesterone

Question 27

why is the corpus luteum important

Answer 27

maintains conditions for fertilisation and implantation.

Question 28

At the end of the cycle, in the absence of fertilisation

Answer 28

the corpus luteum spontaneously **regresses** after 14 days. There is a significant fall in hormones, relieving negative feedback, resetting the HPG axis ready to begin the cycle again.

Question 29

If fertilisation occurs, what stimulates (lutenises) the CL

Answer 29

the syncytiotrophoblast of the embryo produces human chorionic gonadotropin (HcG), exerting a luteinising effect, maintaining the corpus luteum. It is supported by placental HcG and it produces hormones to support the pregnancy. At around 4 months of gestation, the placenta is capable of production of sufficient steroid hormone to control the HPG axis.

Question 30

summary of the ovarian cycle

Answer 30

1. initially low oestrogen, inhibin and progesterone will send negative feedback to the H and AP --\> inhibitng release of LH 2. as the follicle grows it will produce more and more oestrogen and inhibin (inhibits FSH) which will at a set point change the negative feedback to positive feedback 3. causing a surge in LH 4. LH surge casuses rupture of the ovum from the follicle 5. the follicle will turn into a corpus luteum, which is stimulated by HcG and produces oestrogen, progesterone to help maintain conditions for fertilisation and implantation

Question 31

The uterine cycle

Answer 31

* Pre-ovulation * Proliferative * Post-ovulation * secretory

Question 32

proliferative phase

Answer 32

Following menses, the proliferative phase runs alongside the follicular phase, preparing the reproductive tract for fertilisation and implantation.

Question 33

oestrogens role in the proliferative phase

Answer 33

initiates fallopian tube formation, thickening of the endometrium, increased growth and motility of the myometrium and production of a thin alkaline cervical mucus (to facilitate sperm transport).

Question 34

Secretory Phase

Answer 34

The secretory phase runs alongside the luteal phase.

Question 35

progesterone in the secretory phase

Answer 35

stimulates further thickening of the endometrium into a glandular secretory form, thickening of the myometrium, reduction of motility of the myometrium, thick acidic cervical mucus production (a hostile environment to prevent polyspermy), changes in mammary tissue and other metabolic changes. * endometrial thickness at maximum, very coiled glands, coiled arterioles

Question 36

Menses

Answer 36

Menses marks the beginning of a new menstrual cycle. It occurs in the absence of fertilisation once the corpus luteum has broken down and the internal lining of the uterus is shed.

Question 37

Menstrual bleeding usually lasts between

Question 38

**Oestrogen** * **Highest in**

Answer 38

* **Follicular phase** * Thickening of endometrium * Increases fallopian tube function- cilia which waft oocyte along fallopian tube * Growth and motility of myometrium * Produces thin alkaline cervical mucus- for sperm * Vaginal changes * Changes skin, hair, metabolism

Question 39

* **Progesterone is Highest in**

Answer 39

* **Luteal phase** * Further thickening of endometrium (secretory) * Thickening of myometrium and reduction of motility- to stop implanted embryo from being expelled * Thick acidic cervical mucus * Development of breast tissue * Increased body temp * Metabolic changes * Electrolyte changes

Question 41

**Cycle length- what is normal** * *

Answer 41

* Normal duration 21-35 days * **Variation is due to length of follicular phase** * Luteal phase

Question 42

Disruption to cycle

Answer 42

* Physiological factors * Pregnancy * Lactation * Emotional stress * Body weight * Infertility