The ovarian cycle is the cycle of follicle and oocyte development in the ovaries. It is divided into follicular and luteal phases. In the follicular phase, rising FSH and LH stimulate follicular growth and development. Primary oocytes develop within their primary follicles at the beginning of each menstrual cycle. Within these follicles, thecal cells are stimulated by LH to convert cholesterol to androgens which diffuse to granulosa cells. At the same time, the granulosa cells are stimulated by FSH to convert androgens to estrogens.
Ovulation is initiated by a mid-cycle surge of LH due to elevated estrogen levels. Mature (Graafian) follicles containing secondary oocytes fill with fluid and rupture at ovulation. Ova are released from the ovaries into the interstitial space where they are captured by fimbriae. Ovarian (Fallopian) tubes (oviducts) transport ova to meet sperm and become fertilized. Muscular contractions and cilia lining the oviducts assist transport of the ovum. Once fertilized meiosis II is completed prior to fusion of the nucleus of the sperm with the now haploid nucleus of the ovum.
After ovulation, the luteal phase begins. The corpus luteum develops from the burst Graafian follicle. In the luteal phase of the ovarian cycle, the corpus luteum produces estrogen and progesterone. These hormones prepare the uterus for implantation of the developing embryo. LH stimulates development and maintenance of the corpus luteum. The zygote begins to develop as it continues toward the uterus where it implants in the uterine lining. Implantation results in the production of hCG (human chorionic gonadotropin) that continues to maintain the corpus luteum. If fertilization and implantation do not occur, the corpus luteum degenerates after 12 days, forming a corpus albicans.
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