Menstrual cycle and its hormonal control

 

There are two closely linked reproductive cycles in human  females. Both are controlled by cyclic patterns of endocrine  signaling. Cyclic events in the ovaries define the Ovarian Cycle, Once per cycle a follicle matures and an oocyte is released. Changes in the uterus define the Uterine Cycle, which in humans and some other primates is a Menstrual Cycle. In each menstrual cycle, the endometrium (lining of the uterus) thickens and develops a rich blood supply before being shed through the cervix and vagina if pregnancy does not occur. 


By linking the ovarian and uterine cycles, 

hormone activity synchronizes ovulation with the establishment of a uterine lining that can support embryo implantation and development.

If an oocyte is not fertilized and pregnancy does not occur, the uterine lining is sloughed off, and another pair of ovarian and uterine cycles begins. The cyclic shedding of the bloodrich endometrium from the uterus, a process that occurs in a flow through the cervix and vagina, is called menstruation. The first menstruation begins at puberty and is called menarche.


The ovarian cycle in human females, as in males, the hypothalamus has a central role in regulating reproduction. The ovarian cycle begins when the hypothalamus releases GnRH, which stimulates the anterior pituitary to secrete small amounts of FSH and LH. Follicle-stimulating hormone, stimulates follicle 

growth, aided by LH, and the follicles start to make estradiol. Estradiol concentration slowly rises during most of the follicular phase, when follicles grow and oocytes mature (Several follicles begin to grow with each cycle, but usually only one matures; the others disintegrate.) Low levels of estradiol inhibit secretion of pituitary hormones, keeping levels of FSH and LH relatively low. In this portion of the cycle, regulation of hormones closely parallels the regulation in males. When estradiol secretion by the follicle begins to rise steeply, the FSH and LH levels increase markedly. Why? [Comment below]. Whereas a low level of estradiol inhibits secretion of pituitary gonadotropins, a high concentration has the opposite effect, It stimulates gonadotropin secretion by causing the hypothalamus to increase output of GnRH. A high estradiol concentration also increases the GnRH sensitivity of LH-releasing cells in the pituitary, further increasing LH levels. The maturing follicle, containing a fluid-filled cavity, enlarges to form a bulge at the surface of the ovary. The follicular phase ends at ovulation, about a day after the LH surge. In response to FSH and the peak in LH level, the follicle and adjacent wall of the ovary rupture, releasing the secondary oocyte. At or near the time of ovulation, women may feel a pain in the lower abdomen. On the same side as the ovary that released the oocyte, the luteal phase follows ovulation. Luteinizing hormone stimulates the remaining follicular tissue to form the corpus luteum, a glandular structure. Stimulated by LH, the corpus luteum secretes progesterone and estradiol, which in combination exert negative feedback on the hypothalamus and pituitary. This feedback greatly reduces LH and FSH secretion, preventing maturation of another egg when a pregnancy may be under way. If pregnancy does not occur, low gonadotropin levels at the end of the luteal phase cause the corpus luteum to disintegrate, triggering a sharp decline in estradiol and progesterone concentration. This decline liberates the hypothalamus and pituitary from negative feedback. The pituitary can then secrete enough FSH to stimulate the growth of new follicles, initiating the next ovarian cycle.



The Uterine (Menstrual) Cycle prior to ovulation, ovarian steroid hormones stimulate the uterus to prepare for support of an embryo. Estradiol secreted in increasing amounts by growing follicles signals the endometrium to thicken. In this way, the follicular phase of the ovarian cycle is coordinated with the proliferative phase of the uterine cycle. After ovulation,the estradiol and progesterone secreted by the corpus luteum stimulate maintenance and further development of the uterine lining, including enlargement of arteries and growth of endometrial glands. These glands secrete a nutrient fluid that can sustain an early embryo even before it implants in the uterine lining. Thus, the luteal phase of the ovarian cycle is coordinated with the secretory phase of the uterine cycle. If an embryo has not implanted in the endometrium by the end of the secretory phase, the corpus luteum disintegrates. The resulting drop in ovarian hormone levels causes arteries in the endometrium to constrict. Deprived of its circulation, the uterine lining largely disintegrates, releasing blood that is shed along with endometrial tissue and fluid. The result is menstruation, the menstrual flow phase of the uterine cycle. During this phase, which usually lasts a few days, a new set of ovarian follicles begin to grow. By convention, the first day of flow is designated day 1 of the new uterine (and ovarian) cycle.


About 7% of women of reproductive age suffer from a  disorder called endometriosis, in which some cells of the uterine lining migrate to an abdominal location that is abnormal, or ectopic (from the Greek ektopos, away from a place). Having migrated to a location such as an oviduct, ovary, or large intestine, the ectopic tissue responds to hormones in the bloodstream. Like the uterine endometrium, the ectopic tissue swells and breaks down during each ovarian cycle, resulting in pelvic pain and bleeding into the abdomen. Researchers have not yet determined why endometriosis occurs, but hormonal therapy or surgery can be used to lessen discomfort. 


After about 500 cycles, a woman undergoes menopause, the cessation of ovulation and menstruation. Menopause usually occurs between the ages of 46 and 54. During this interval, the ovaries lose their responsiveness to FSH and LH, resulting in a decline in estradiol production. Menopause is an unusual phenomenon. In most other species, females and males can reproduce throughout life. Is there an evolutionary explanation for menopause? One intriguing hypothesis proposes that during early human evolution, undergoing menopause after bearing several children allowed a mother to provide better care for her children and grandchildren, thereby increasing the chances for survival of individuals who share much of her genetic makeup.