What is the menstrual cycle?
The menstrual cycle is a complex series of physiological changes occurring in women on a monthly basis. It results in production of an ovum and thickening of the endometrium to allow for implantation if fertilisation should occur. The menstrual cycle is orchestrated by the endocrine system through the complex interaction of the hypothalamus, pituitary and gonads. The entire cycle lasts around 28 days, with the cycle beginning on the first day of menstruation and ovulation occurring around day 14.
How is it controlled?
1. The hypothalamus produces Gonadotrophin Releasing Hormone (GnRH).
2. This binds to the pituitary stimulating release of:
- Luteinizing hormone (LH)
- Follicle Stimulating Hormone (FSH)
3. FSH binds to the ovaries stimulating:
- Development of ovarian follicles
- Secretion of oestrogen
- Secretion of inhibin
The follicle most sensitive to FSH becomes dominant and is known as the Graafian follicle.
4. LH binds to the ovaries causing:
- Production of oestrogen which is required for ovulation and thickening of the endometrium
- Conversion of the Graafian follicle into the progesterone producing corpus luteum
- Progesterone causes the endometrium to become receptive to implantation of a fertilised ovum
5. Oestrogen, Progesterone and Inhibin all cause negative feedback on the pituitary and hypothalamus.
6. This results in reduction of GnRH, FSH and LH production.
7. In pregnancy GnRH, FSH and LH all remain inhibited, causing cessation of menstruation.
Phases of the menstrual cycle
1. At the start of the cycle levels of FSH rise causing stimulation of a few ovarian follicles.
2. As follicles mature they compete with each other for dominance.
3. The 1st follicle to become fully mature will produce large amounts of oestrogen.
4. This inhibits the growth of the other competing follicles.
5. The 1 follicle reaching full maturity is called the Graafian follicle (oocyte develops within this).
6. The Graafian follicle continues to secrete increasing amounts of oestrogen.
7. Oestrogen causes:
- Endometrial thickening
- Thinning of cervical mucus to allow easier passage of sperm
8. Oestrogen also initially inhibits LH production from the pituitary gland.
9. However, when the ovum is mature, oestrogen reaches a threshold level which conversely causes a sudden spike in LH around day 12.
10. The high amounts of LH cause the membrane of the Graafian follicle to become thinner.
11. Within 24-48 hours of the LH surge, the follicle ruptures releasing a secondary oocyte.
12. The secondary oocyte quickly matures into an ootid and then into a mature ovum.
13. The ovum is then released into the peritoneal space and is taken into the fallopian tube via fimbriae (finger-like projections).
14. Once ovulation has occurred the hormones LH and FSH cause the remaining Graafian follicle to develop into the corpus luteum.
15. The corpus luteum then begins to produce the hormone progesterone.
16. Increased levels of progesterone result in:
- Endometrium becoming receptive to implantation of the blastocyst
- Increased production of oestrogen by the adrenal glands
- Negative feedback causing decreased LH and FSH (both needed to maintain the corpus luteum)
- Increase in the woman’s basal body temperature
17. As the levels of FSH and LH fall, the corpus luteum degenerates.
18. This results in progesterone no longer being produced.
19. The falling level of progesterone triggers menstruation and the entire cycle starts again.
20. However, if an ovum is fertilised it produces hCG which is similar in function to LH.
21. This prevents degeneration of the corpus luteum (continued production of progesterone).
22. Continued production of progesterone prevents menstruation.
23. The placenta eventually takes over the role of the corpus luteum (from 8 weeks).
The Uterine Cycle
The uterus has its own cycle which is driven by the cyclical release of hormones by the ovaries which we’ve previously talked about. The inside lining of the uterus is known as the endometrium. The endometrium is the part of the uterus most affected by these changes in hormone levels.
It is composed of 2 layers:
- Functional layer – this grows thicker in response to oestrogen and is shed during menstruation
- Basal layer – this forms the foundation from which the functional layer develops – it is not shed
Phases of the uterine cycle
The uterine cycle has 3 phases known as the proliferative, secretory and menstrual phases.
During the proliferative phase, the endometrium is exposed to an increase in oestrogen levels caused by FSH and LH stimulating the ovaries. This oestrogen causes repair and growth of the functional endometrial layer allowing recovery from the recent menstruation and further proliferation of the endometrium.
Continued exposure to increasing levels of oestrogen causes:
- Increased endometrial thickness
- Increased vascularity –spiral arteries grow into the functional endometrial layer
- Development of increased numbers of secretory glands
The secretory phase begins once ovulation has occurred.
This phase is driven by progesterone produced by the corpus luteum.
It results in the endometrial glands beginning to secrete various substances.
These secretions make the uterus a more welcoming environment for an embryo to implant.
At the end of the luteal phase, the corpus luteum degenerates (if no implantation occurs).
The loss of the corpus luteum results in decreased progesterone production.
The decreasing levels of progesterone cause the spiral arteries in the functional endometrium to contract.
The loss of blood supply causes the functional endometrium to become ischaemic and necrotic.
As a result, the functional endometrium is shed and exits out through the vagina.
This is seen as the 3-5 day period of menstruation a woman experiences each month.
Window of fertility
A woman’s most fertile period is between 5 days before ovulation until 1 to 2 days after.
Women can, therefore, use knowledge of their cycle to improve chances of conception.
Symptoms experienced during the menstrual cycle
- Abdominal pain and cramps
- Vaginal bleeding