Ovarian Cycle Overview

Ovarian Cycle

• The ovarian cycle is a monthly series of events in the ovaries that prepares an egg for fertilization. It's comprised of two phases: follicular and luteal.
• Follicular Phase: This phase begins with the recruitment of several follicles (each containing an oocyte) from the ovarian reserve. The follicles grow and mature, driven by follicle-stimulating hormone (FSH). One follicle, the dominant follicle, develops more rapidly and eventually releases the mature egg – ovulation.
• Ovulation: Triggered by a surge in luteinizing hormone (LH), the mature follicle ruptures, releasing the ovum into the fallopian tube. This process typically occurs around day 14 of a 28-day cycle.
• Luteal Phase: Following ovulation, the ruptured follicle transforms into the corpus luteum. The corpus luteum secretes progesterone, a hormone that prepares the uterine lining for pregnancy. If fertilization doesn't occur, the corpus luteum degenerates into the corpus albicans, eventually disappearing.

Uterine Cycle

• The uterine cycle is a monthly series of changes in the endometrium (lining of the uterus) in preparation for possible implantation of a fertilized egg. The cycle is closely coordinated with the ovarian cycle. It also comprises distinct phases: menstrual, proliferative, and secretory.
• Menstrual Phase: If fertilization doesn't occur, the corpus luteum degenerates, leading to a decrease in progesterone levels. This decline causes the breakdown and shedding of the thickened endometrial lining, resulting in menstrual bleeding. This phase typically lasts about 3-7 days.
• Proliferative Phase: During this phase (roughly correlating with the follicular phase of the ovarian cycle), elevated estrogen levels cause the endometrium to thicken and regenerate. The endometrial glands and blood vessels increase in number.
• Secretory Phase: The rising progesterone levels (from the corpus luteum) during the luteal phase stimulate the development of the endometrial glands. The glands secrete nutrients that nourish a potential embryo. The endometrium becomes increasingly vascular and edematous (swelling with fluid). If fertilization occurs, implantation will take place during the secretory phase.
• Correlation between ovarian and uterine cycles: The timing of the uterine phases coincides with the phases of the ovarian cycle. The proliferative phase overlaps with the follicular phase, while the secretory phase occurs during the luteal phase. This intricate coordination ensures the uterus is appropriately prepared for the potential arrival and implantation of a fertilized egg.