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 & 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 & 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)
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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 & is known as the Graafian follicle
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4. LH binds to the ovaries causing;
- Production of oestrogen which is required for ovulation & 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
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5. Oestrogen, Progesterone & Inhibin all cause -ve feedback on the pituitary & hypothalamus
6. This results in reduction of GnRH, FSH & LH production
7. In pregnancy GnRH, FSH & LH all remain inhibited, causing cessation of menstruation
Phases of the menstrual cycle
Follicular Phase
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
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7. Oestrogen causes;
- Endometrial thickening
- Thinning of cervical mucous to allow easier passage of sperm
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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 & then into a mature ovum
13. The ovum is then released into the peritoneal space & is taken into the Fallopian tube via fimbriae (finger like projections)
Luteal Phase
14. Once ovulation has occurred the hormones LH & FSH cause the remaining graffian follicle to develop into the corpus luteum
15. The corpus luteum then begins to produce the hormone progesterone
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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 & FSH (both needed to maintain the corpus luteum)
- Increase in the woman’s basal body temperature
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17. As the levels of FSH & LH fall, the corpus luteum degenerates
18. This results in progesterone no longer been produced
18. The falling level of progesterone triggers menstruation & the entire cycle starts again
19. However if an ovum is fertilised it produces hCG which is similar in function to LH
20. This prevents degeneration of the corpus luteum (continued production of progesterone)
21. Continued production of progesterone prevents menstruation
22. The placenta eventually takes over the role of the corpus luteum (from 8 weeks)
The Uterine Cycle
The uterus has it’s 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.
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It is composed of 2 layers;
- Functional layer – this grows thicker in response to oestrogen & 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 & menstrual phases
Proliferative phase
During the proliferative phase the endometrium is exposed to an increase in oestrogen levels caused by FSH & LH stimulating the ovaries. This oestrogen causes repair & growth of the functional endometrial layer allowing recovery from the recent menstruation & further proliferation of the endometrium.
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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
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Secretory phase
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
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Menstrual phase
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 & necrotic
As a result the functional endometrium is shed & 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
Women may also monitor symptoms that suggest they are about to ovulate such as;
- Basal body temperature measuring – it spikes during the LH surge 24-48 hours before ovulation
- Thinning of cervical mucous
Symptoms experienced in the menstrual cycle
Abdominal pain & cramps
Heavy vaginal bleeding
Vaginal pain
Nausea
Diarrhoea
Sweating
Fatigue
Irritability
Dysphoria (unhappiness)
References
1. Uterine cycle image - http://legacy.owensboro.kctcs.edu/gcaplan/anat2/notes/Image719.gif