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The normal human menstrual cycle is 28 days, but no woman is always precisely regular, and cycles as short as 21 days or as long as 35 days are not abnormal. It is customary to call the first day of the menstrual period the first day of the cycle, although menstruation is the end rather than the beginning of a process.
On this basis the cycle is described as starting with about five days of menstruation, followed by a proliferative phase that lasts to about the 14th day, and then a secretory phase that lasts until the next menstruation.The external manifestation of menstruation depends upon cyclical change in the lining of the body of the uterus. The lining, called endometrium consists of tubular glands that open into the uterine cavity. The glands lie in a vascular framework, or stroma, and are separated by it.
At the end of menstruation, just at the beginning of the proliferative phase, the endometrium is thin, with short, straight glands, and the ovary is quiescent. Under the influence of the gonadotropic hormones from the pituitary gland an ovarian follicle (occasionally more than one) ripens in one of the ovaries.
Thisovarian follicle contains the ovum, which is a cell about 0.14 millimetre (0.006 inch) in diameter, surrounded by a group of smaller cells, calledgranulosa cells. The granulosa cells multiply, with the ovum situated in the wall of the rounded structure that they form, andsecrete an estrogenic hormone, estradiol. This hormone causes proliferative changes in the endometrium, so that the glands become taller and the whole endometrium becomes thicker and more vascular.
At about mid-cycle ovulation occurs: The ovum is discharged out of the follicle and from the surface of the ovary, to be received into the fallopian tube, down which it is carried to the uterus. After ovulation the granulosa cells lining the follicle from which the ovum has been extruded accumulate yellow lipid and are therefore calledlutein cells, from the Latin wordluteus, "saffron-yellow."
The altered follicle is calledcorpus luteum.The corpus luteum continues to secrete estrogens but now also secretes progesterone; this additional hormone induces the secretory phase in the endometrium.
The endometrial glands are distended with secretion and become very tortuous, while the stromal cells are swollen. The appearance of the endometrium at the end of the menstrual cycle is indistinguishable from that of early pregnancy, and this endometrial change is a preparation for the reception of the ovum.
If it is fertilized, the ovum liberated at mid-cycle reaches the uterine cavity at a time when the endometrium is in the secretory phase, and the ovum embeds itself in the endometrium and starts its growth. If the ovum is not fertilized the endometrium breaks down and menstruation occurs.Menstruation has therefore been described as the outward evidence of the abortive close of one cycle and the hopeful commencement of the next.
When the ovum dies, the corpus luteum degenerates and ceases to produce hormones. On the withdrawal of estrogens and progesterone there is sudden spasm of the endometrial blood vessels, and all but the basal layer of the endometrium dies. The disintegrating endometrium is shed, together with some blood. The endometrium containsplasmin, an enzyme that dissolves blood clots, so that the menstrual discharge is normally fluid. The total blood loss does not ordinarily exceed 50 millilitres (1.69 U.S. fluid ounces).
After menstruation the endometrium regenerates from the residual basal layer during the proliferative phase of the next cycle.
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