The menstrual cycle consists of cyclical changes occurring in the female reproductive system regarded as periodic preparation for fertilization and pregnancy. During the menstrual cycle or the fertility cycle, changes occur in each of the female reproductive organs, the ovaries, uterus, cervix and the breasts.
The most conspicuous feature of the menstrual cycle is the periodic vaginal bleeding which occurs along with shedding of the uterine mucosa. The average length of a cycle is around 28 days.
While it is called the menstrual cycle in humans and some primates, it goes by the name of estrous cycle in other animals. The main difference is that the cyclical vaginal bleeding which occurs in the menstrual cycle does not accompany the estrous cycle.
Ovulation Cycle – Changes in the Ovaries
From birth, each ovary contains thousands of cells called the primordial follicles, which actually contain the immature ovum or the egg. After the onset of menstruation, at the start of each menstrual cycle, several of these follicles enlarge with the formation of a cavity around them. In humans, only one of these follicles in one ovary rapidly enlarges to become the dominant follicle and the others start to regress.
Ovulation occurs when this dominant follicle ruptures releasing the egg into the abdominal cavity on about the 14th day. The open end of the fallopian tube, one of two tubes attached to the uterus, picks it up and transports it to the uterus, ready to be fertilized by sperm. If it does not get fertilized in that cycle, it is extruded through the vagina.
The ruptured follicle in the ovary, meanwhile, starts secreting hormones like estrogen and progesterone and is now called the corpus luteum. If fertilization occurs and pregnancy follows, the corpus luteum persists, continuing to produce hormones. Otherwise, it degenerates before the start of the next cycle.
Uterine cycle – Changes in the Uterus During Menstrual Cycle
At the start of the cycle, the endometrium, the innermost layer of the uterine cavity starts to regrow (after being shed in the previous cycle) and rapidly increases in thickness until the 14th day. This occurs under the influence of the hormone estrogen and is called the proliferative phase.
With ovulation, the secretory or luteal phase begins under the influence of the hormone progesterone produced by the corpus luteum. The endometrium has by now become thick, highly engorged with blood vessels, and starts to secrete a clear fluid. The uterus is actually getting ready to receive the embryo for implantation in the event that fertilization occurs. When it does not, the endometrium gradually becomes thinner and is eventually shed through the vagina as menstrual bleeding.
Normal Menstrual Cycle – Changes in the Cervix and Breast
The cervix does not undergo any cyclical changes unlike the ovary and the uterus. But before ovulation, under the influence of estrogen, the cervix produces a thin mucus-like secretion which assists in transporting the sperm.
Estrogen causes proliferation of mammary ducts in the breast while progesterone causes growth of the glands. The surge of these hormones during the regular menstrual cycle causes some women to experience pain, tenderness and swelling of the breasts in the later half of the cycle.
Normal menstrual cycles occur in preparation for pregnancy under the influence of the hormones estrogen and progesterone and are accompanied by changes in the ovaries, uterus and the breasts.
Sources:
Ganong, William F. “Review of Medical Physiology” (McGraw Hill; 22nd edition, 2005)
Lakshmi Ananth - Lakshmi Ananth is a practising ENT, head and neck surgeon and has faculty position at a medical college. She is specially interested in ...
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