Physiology of the Menstrual Cycle
The menstrual cycle is a cyclic process in females that, on average, occurs every 28 days. The purpose of the menstrual cycle is to help prepare the body for potential fertilization, implantation, and, consequently, pregnancy. The happenings of the menstrual cycle are consequences of the simultaneously occurring ovarian cycle and uterine cycle. The ovarian cycle occurs in the ovaries and carries out the follicular phase, which ps the first 14 days of a new menstrual cycle. The result is ovulation.
The uterine cycle works concurrently with the ovarian cycle and carries out the menstrual, proliferative, and secretory phases. Hormonally, gonadotropin-releasing hormone (GnRH) is released in the hypothalamus and travels to the anterior pituitary of the brain, thus increasing levels of follicule-stimulating hormone (FSH) and leutinizing hormone (LH). The brain acts directly on the follicle cells (in the follicular phase) and the corpus luteum (in the luteal phase) to stimulate a response.
The result of hormonal secretions of the hypothalamus and anterior pituitary along with the ovarian and uterine cycle is menstruation. The overall cycle is regulated by the hormones that result from a release of GnRH from the brain. Negative feedback occurs in the early follicular phase and luteal phase of th ovarian cycle as GnRH is released, which causes an increase in FSH and LH levels in the anterior pituitary. In the early follicular phase, these hormones send signals through neurotransmitters that reach receptors on follicle cells and follicular development is enhanced.
Simultaneously, plasma levels of estrogen increase and return to the brain to hinder continued LH and FSH secretion. At the end of the follicular phase, positive feedback occurs as high estrogen levels are passed back to the brain and estrogen continues to amount to high levels. In the luteal phase, the brain sends hormonal signals to the corpus luteum. As a result, levels of estrogen and progesterone are heightened and progesterone is fed back to the brain to maintain stable levels of LH and FSH.
This demonstrates how brain function and hormonal regulation carry out essential, complex processes in the body, such as those of the menstrual cycle. The ovaries originally contain primordial follicles, which are made up of an oocyte (an immature egg) and a layer of follicle cells that envelops it. In the beginning stages of the follicular phase of the menstrual cycle, some of these follicles start to develop, becoming granulosa cells. A fluid-filled cavity called an antrum appears in the follicle and begins to increase in size only to eventually engulf the oocyte.
The follicle is now called the Graafian follicle, which soon bursts, and the oocyte leaves the ovary and travels into the fallopian tube, spurring ovulation. The leftover Graafian follicle becomes what is called the corpus luteum. The corpus luteum (“yellow body”) is a gland that secretes estrogen and progesterone when the menstrual cycle is not in full swing. If fertilization of the oocyte ensues, the corpus luteum remains intact for a fair amount of time in order to regulate the release of estrogen and progesterone.
If the oocyte fails to become fertilized, the corpus luteum will disintegrate relatively quickly. At the point of the follicle rupturing, there is a major spike in LH and FSH plasma levels. The uterine cycle is carried out in three phases. The menstrual phase begins on the first day of the menstrual cycle and corresponds with the follicular phase. It lasts about three to five days and sheds the uterine lining. During this phase is when actual menstruation occurs due to a decline in estrogen and progesterone plasma levels that occurred because the corpus luteum had fully deteriorated.
The inner lining of the uterus, the endometrium, has blood vessels that now narrow and the endometrial tissue begins to die and shed from the uterus. As it sheds, blood vessels burst and blood and tissue are dispelled from the vagina. The proliferative phase is next and allows the uterus to refurbish as blood vessels become engulfed again, allowing the uterine lining to thicken. It occurs around day five and lasts until day fourteen, when ovulation has happened. During the proliferative phase, estrogen levels peak.
The next phase is the secretory phase, which creates a setting that readily allows for embryo development as glands become engorged due to increased blood flow and secrete fluids that contain glycogen that may act as a food source of glucose for a potential embryo. The secretory phase lasts about the last fourteen days of the menstrual cycle-at the same time as the luteal phase. By the end of this cycle, the corpus luteum has completely diminished and, because it can no longer act as a source of stimulation for estrogen and progesterone release, these two hormone levels drop and menstruation commences as the menstrual phase begins again.
Works Cited
- Bowen, R. “Luteinizing and Follicle Stimulating Hormones. ” Arbl. cvmbs. colostate. edu. 13 May 2004. Web. 09 Feb. 2011. <http://www. vivo. colostate. edu/hbooks/pathphys/endocrine/hypopit/lhfsh. html>.
- Cohen, Hilary. “Menstrual Cycle Home. ” Endocrinology of the Menstrual Cycle. 2000. Web. 09 Feb. 2011. <http://sprojects. mmi. mcgill. ca/menstrualcycle/home. html>.
- Stanfield, Cindy L. , William J. Germann, Mary Jane. Niles, and Joseph G. Cannon. Principles of Human Physiology. San Francisco: Pearson/Benjamin Cummings, 2008. Print.
