Answer:
1. The follicular phase begins
2. FSH stimulates the maturation of follicles
3. Estradiol increases the dominant follicle's sensitivity to the gonadotropins
4. FSH secretion diminishes and non-dominant follicles degenerate.
5. A surge in LH stimulates the primary oocyte in the dominant follicle to complete meiosis
6. The secondary oocyte is formed and the dominant follicle ruptures (ovulation).
7. The follicle collapses and bleeds and becomes the corpus luteum.
8. The corpus luteum secretes progesterone helping to prepare the uterus for a possible fertilization
Explanation:
Ovocites production in all vertebrates is cyclic. It involves hormones interaction and changes in follicular cells and uterine walls. The hypothalamus is in charge of this cycle, and the hormones involved are the estrogens and progesterone, hypophysis hormones FSH and LH, and the hormone that releases gonadotrophins GnRH. In reduced concentrations, estrogens act by negative feedback inhibiting the production of FSH, GnRH, and LH. In high concentrations, by positive feedback, estrogens stimulate the GnRH secretion and enhancing the synthesis of FSH and LH hormones. Under the high concentration of progesterone, the secretion of all these hormones is inhibited.
At the beginning of the menstrual cycle, the hormonal levels are low. After a few days, oocytes and follicles start to maturate under the influence of the gonadotrophic hormones FSH and LH. As the follicle increases in size, it secretes estrogen that stimulates the endometrium´s regeneration to make the fertilized ovule implantation possible. The rapid increase in estrogens level in the middle of the cycle provokes an increase in LH release, which also stimulates the oocyte release (ovulation) that goes to the uterus. Under the stimulation of the LH, the follicle cells increase in size and produce the luteal body. Cells of this last one increase in size and secrete progesterone and estrogen. The high levels of these two hormones inhibit the secretion of GnRH and thus the secretion of FSH and LH.
If there is not fertilized and implanted egg, the luteal body is reabsorbed and the production of the ovarian hormones decreases. Under low hormones concentration, the endometrium loses part of its wall by eliminating as the menstrual fluid. When the ovarian hormones decrease, hypophysial gonadotrophic hormones increase again followed by the development of a new follicle and the increase in estrogen levels when the cycle starts all over again.
