Exam 22: The Reproductive System

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The ovarian cycle is divided into two phases: the follicular phase and the luteal phase. In the beginning of the follicular phase, about 10- 25 follicles develop. Within seven days, one of those follicles (the dominant follicle)will develop to full maturity. The remaining follicles will undergo atresia and be lost. As the follicles begin to develop, the oocyte grows in size. At the same time, the granulosa cells increase in size and number. This growth is stimulated by
follicle- stimulating hormone (FSH)and the estrogens secreted by the follicles. There is also a thick layer of
non- cellular material forming around the oocyte called the zona pellucida. At the outer layer of the follicle, theca cells begin to develop. A fluid- filled cavity (antrum)begins to form within the granulosa cell layer. This is the point at which many of the follicles stop their development. Follicles that have not developed adequately to maintain estrogen secretion in the face of decreasing FSH undergo atresia. As follicles develop further, the antrum continues to grow, displacing some of the cellular tissue around the oocyte, leading to a protrusion from the follicular wall (corona radiata). Eventually, the first meiotic division occurs and the secondary oocyte detaches from the follicle wall and floats freely in the antral fluid. The antrum continues to grow until it forms the Graafian follicle, just prior to ovulation. The luteinizing hormone (LH)surge that induces ovulation begins the luteal phase, during which the follicle bursts and the oocyte is released. The granulosa and theca cells of the ruptured follicle are transformed into the corpus luteum that secretes the estrogen and progesterone that prepare the uterus for implantation. In the absence of fertilization, the corpus luteum begins to degrade after ten days, reducing plasma estrogens and progesterone concentration, thereby initiating menstruation.