Describe the events and associated hormonal control of the ovarian cycle. In a logical progression, discuss the major developmental events from fertilization to early embryonic development (up to gastrulation).
...e maturation of an egg is called the ovarian cycle, which has two major phases –the follicular phase and the luteal phase. The follicular phase is the maturation of a primordial follicle to the mature state and involves several events. The first stage of the follicular phase is when the primordial follicles are activated. On day one of the cycle, rising levels of GnRH from the hypothalamus stimulate increased production and release of FSH and LH by the anterior pituitary. The follicle is now called a primary follicle as FSH and LH stimulate follicle growth and maturation and estrogen secretion. FSH exerts its main effects on the follicle cells and LH targets the thecal cells at this stage. Next, follicular cells proliferate, forming a stratified epithelium around the oocyte. The rising estrogen levels exert negative feedback on the anterior pituitary, inhibiting its release of FSH and LH. Within the ovary, estrogen increases the estrogen output by intensifying the effect of FSH on follicle maturation. After this process, the cells take on the name granulosa cells. A signal passes from the granulosa cells to the oocyte telling the oocyte to grow. In the next stage estrogen reaches a critical blood concentration and exerts positive feedback on the brain and anterior pituitary. In this stage the thecal and granulosa cells cooperate to produce estrogens. The granulosa cells simultaneously secrete a glycoprotein-rich substance that forms the zona pellucida around the oocyte. Moving on to the next phase, high estrogen levels result in a release of accumulated LH. Clear liquid accumulates between the granulosa cells and coalesces to form the antrum, which distinguishes the new secondary follicle from the primary follicle. The antrum expands with fluid until it isolates the oocyte on a stalk on one side of the follicle. When the follicle reaches its full size it becomes a vesicular follicle. The LH also triggers ovulation around day fourteen of the ovarian cycle. Ovulation occurs when the ovary wall ruptures and expels a secondary oocyte, which results from the surge of LH that stimulates the primary oocyte to complete its first meiotic division and the second oocyte continues on to metaphase II. The surge of LH also transformed the ruptured follicle into a corpus luteum and stimulates the newly formed endocrine gland to produce progesterone and estrogen almost immediately after it’s formed. Due to the intense stretching of the ovarian wall during ovulation many women experience mittelshmerz pain in their lower abdomen. In the ovaries of adult females there are always several follicles at different stages of maturation but only one of these follicles can become the dominant follicle and is at peak stage of maturation when LH stimulus is given. 3 After ovulation the Luteal Phase begins. When the ruptured follicle collapses, the antrum fills with blood. The remaining granulosa cells increase in size and form a new endocrine gland called the corpus luteum, with help of internal thecal cells. Corpus luteum enhances the inhibitory effect of rising progesterone and estrogen blood levels on anterior pituitary release of LH and FSH. If pregnancy does not occur LH blood levels fall, the stimulus for luteal activity ends and the corpus luteum degenerates and in about ten days hormonal output ends, and a scar remains called the corpus albicans. If pregnancy does occur, the corpus luteum does not degenerate but instead persists until the placenta is ready to take over its hormone-producing duties in approximately three months. 3 An oocyte is only fertilizable for up to 24 hours and most sperm are viable within the female reproductive tract for one to three days. Sperm must be able to survive the hostile environment of the vagina and become capacitated to fertilize an egg. Also, hundreds of sperm must release acrosomal enzymes to break down the egg’s corona radiata and zona pellucidia before fertilization can occur. When one sperm binds to receptors ...