Menstruation, or the period, is the result of an endless dialogue between the command centers within the brain (the hypothalamus and pituitary gland); the ovary and the uterus. In particular the uterine lining, the endometrium, is responsive to the growth and maturation hormones provided by the ovary. The two major hormones responsible for preparing the endometrium for pregnancy are estrogen and progesterone. They are present at different levels during the menstrual cycle and together, they drive endometrial preparation and promote early development of the fetus and placenta alike. If the embryo is the seed, then the endometrium is the soil.
The menstrual cycle may be spoken of in terms of the two major players: the ovary and the uterus. Each player then can be subdivided according to function using ovulation (release of the egg from the ovary) as the gatekeeper for transitioning from one phase to the next. If pregnancy does not occur, the cycle begins anew.
There are two ovarian stages – the follicular phase and the luteal phase. The follicular phase corresponds to the preparation of the follicle, the protective bubble in which the egg resides. The luteal phase corresponds to the action of the residual follicle after the egg is released.
During the follicular phase, a single dominant follicle (and egg within) is selected from a collection of hundreds of eggs prepared for each cycle. The pituitary gland provides enough growth hormone for the development of one follicle unit. Out of the typical 200-500 follicles present each month, the hungriest follicle attracts the growth hormone (follicle stimulating hormone) and the other ones simply die off – it is the ultimate king of the hill game. The selected follicle unit is exposed to a flux of hormones which promote an orderly development of the egg within. As the egg and follicle develop, they produce the hormone estrogen.
Estrogen has two key roles. Its first job is to act as a surrogate marker for the pituitary gland. Its increasing levels signal to the brain that a follicle has been selected and is growing appropriately. These levels then can be used to gauge egg maturity. Secondly, estrogen is a growth factor that prepares a nutrition-laden and thickened endometrium. The growth phase of the endometrium is known as the proliferative phase and it is coincident of development of the egg within the follicle.
Eventually, the follicle receives the release signal from the pituitary gland in the form of a highly concentrated pulse of Leutenizing hormone (LH) which promotes a final maturation of the egg. There is a 36 hour window between the follicle receiving the signal and release of the egg. This time is needed for a final egg maturation and preparation for its release and eventual transit to the fallopian tube where it will await sperm for hopeful fertilization.
This surge of LH also signals the endometrium to prepare for an anticipated embryo. The remaining, empty follicle begins to produce the hormone progesterone (progestation-al hormone). Progesterone initiates the endometrium into becoming a more hospitable environment for the developing embryo.
If one were making a parallel to the building of a bridge over a body of water, estrogen would be the rafters to build the bridge both far and high. Progesterone is the rivet that secures the rafters to each other securing safe passage.
This hospitality, however is short-lived. There is a window for the embryo to implant which lasts only 24 hours. If the embryo does not implant; then this cycle was for naught. The command center within the brain then signals the body to dispense with the endometrium and start anew. This signal to begin a fresh cycle manifests in actual menstruation, or the period.
Ironically, often the period is thought of as the end of the menstrual cycle. In actuality, it is a time when the communications between the ovary and brain are in overdrive. As within the reproductive system, hope seems to spring eternal and the cycle continues.