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Menstrual Cycle

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Menstrual Cycle - Lesson Summary

There are three phases in the life of a living organism – juvenile, reproductive and senescence. During the reproductive phase, organisms give birth to offspring. A woman experiences her first menstrual cycle, called the menarche, at the onset of puberty, usually between the ages of 9 and 12. Occurring approximately every 28 days a single menstrual cycle consists of four phases – menstrual phase, follicular or proliferative phase, ovulatory phase and lastly the luteal or secretory phase. Each phase of the menstrual cycle is marked by simultaneous changes in the uterus, ovary and the secretion of hormones.

During the menstrual phase, the endometrial lining of the uterus and its blood vessels break down and mix with blood, mucus and other cell debris to form menses. The menses are discharged through the vagina as menstruation or menstrual flow which lasts for three to five days from the beginning of the menstrual cycle. Interestingly, the absence of menstruation indicates pregnancy or ovarian disorders such as polycystic ovarian disorder or even factors such as stress and ill health.

The menstrual phase is followed by the follicular phase, which occurs between days six and fourteen of the menstrual cycle. During this phase, the primary follicle in the ovary matures into a Graafian follicle. Simultaneously, the endometrium undergoes proliferation – a process of regeneration which results in the formation of a new, thick endometrium.

The follicular phase is followed by the ovulatory phase, which occurs during the middle of each menstrual cycle. During this phase, the Graafian follicle ruptures to release an ovum or egg cell into the pelvic cavity in a process called ovulation.

The ovulatory phase is succeeded by the secretory phase –the last phase of the menstrual cycle, which occurs between days 16 and 28 of the menstrual cycle. This phase sees the transformation of the remaining Graafian follicle into a yellow mass called the corpus luteum. The endometrium too thickens and its blood vessels become coiled and enlarged. However, the endometrium continues to thicken because of an increase in the level of progesterone. Incidentally, the corpus luteum will keep secreting progesterone if a fertilised egg gets embedded in the endometrium. However, in the absence of a fertilised egg, the corpus luteum stops releasing progesterone and degenerates. A drop in the level of progesterone, in turn, triggers menstruation as well the restart of the release of LH and FSH by the pituitary gland. Menstrual cycles normally stop once a woman reaches her fifties. This phase is called menopause. The menstrual cycle is an important process in a woman’s body, and it consists of four phases which are controlled by hormones released by the ovary and the pituitary gland.


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