Menstrual Cycle

The menstrual cycle is a monthly cycle (roughly 28 days) in which the endometrium thickens, becomes secretory, and then degenerates and sloughs off. This is a continuous cycle which periodically prepares the uterus for implantation of a fertilised egg by proliferating and secreting nutritious fluid during the period of ovulation. The cycle is split into four phases:-

Menstrual Phase (Days 1 - 5)

The first day of the cycle is the first day of menstruation - the process in which the necrotic functional layer of the endometrium is sloughed off and discharged through the vagina as menses. This usually lasts for five days and is termed the menstrual phase. After this phase the endometrial lining is thin and ready to grow again.

Proliferative Phase (Days 5 - 14)

Oestrogen, produced by the developing ovarian follicles, stimulates the endometrium to rapidly proliferate and thicken. The thickness of the endometrium nearly triples during these nine days and the recently shed epithelium on the surface regenerates. The spiral arteries in the tissue expand and the endometrial glands increase in size and number. This phase ends when the ovulation occurs.

Luteal Phase (Days 14 - 27)

After ovulation progesterone, produced by the corpus luteum of the recently ovulated oocyte, stimulates the endometrial epithelium to secrete a mucous fluid rich in glycogen from its glands. This provides nutrition for the implanting blastocyst if fertilisation has occurred. The glands have become cavernous and twisted under the influence of oestrogen, and the job of progesterone is to maintain this environment suitable for growth during the second half of the cycle. If fertilisation has occurred then the corpus luteum is kept functioning by human chorionic gonadotrophin (hCG), which means production of oestrogen and progesterone is continued.

Ischemic Phase (Day 28)

If fertilisation does not occur then progesterone levels decrease as the corpus luteum degenerates, and the spiral arteries begin to constrict and cut off blood supply to the endometrial functional layer. The glandular secretions stop and the endometrium shrinks. The tissue becomes ischemic and begins to necrotise, and vessels within the wall rupture and allow blood to seep into the tissue, forming coagulating pools of blood which break off. These are the beginnings of menstruation.

Menstrual Cycle

An overview of the menstrual cycle

This cycle continues along with the ovarian cycle until menopause – the point at which a female simply runs out of follicles which can develop into oocytes. At this point both cycles have permanently stopped, and the female doesn’t produce the cyclic hormones and doesn’t menstruate. This usually happens around the age of 50, and can lead to clinical problems such as osteroporosis due to the loss of oestrogen production, for which oestrogen is administered in the form of Hormone Replacement Therapy (HRT).