Final Study Guide: Reproductive System

Endometrial Cycle (Menstrual Cycle) and Ovulation

1. Menstruation (Menses): During menstruation, the functional layer of the endometrium disintegrates and is discharged through the vagina. This marks the beginning of the menstrual cycle.
2. Follicular/Proliferative Phase:
   • Hormonal Regulation: The release of GnRH and the balance between activin and inhibin from granulosa cells contribute to the rise in FSH levels, which stimulates the development of multiple follicles. Between days 5 to 7 of the cycle, the pulsatile secretion of FSH from the anterior pituitary gland rescues a dominant ovarian follicle from undergoing apoptosis.
   • Follicular Development: Estrogen and FSH increase the number of FSH receptors on granulosa cells in the primary follicle, making them more sensitive to FSH. FSH and estrogen work together to induce the production of LH receptors on granulosa cells, which further stimulates the follicle and promotes rapid secretion of follicular estrogen.
   • Endometrial Growth: As estrogen levels rise, FSH levels drop due to increased secretion of inhibin-B from granulosa cells in the dominant follicle. The decline in FSH levels reduces the growth of less-developed follicles. Estrogen also causes the cells of the endometrium to proliferate and stimulates the production of LH.
3. Luteal/Secretory Phase:
   • Ovulation: Ovulation marks the beginning of the luteal or secretory phase. The ovarian follicle transforms into a corpus luteum.
   • Progesterone Production: Pulsatile secretion of LH from the anterior pituitary stimulates the corpus luteum to secrete progesterone, which initiates the secretory phase of endometrial development. The endometrial glands and blood vessels branch and curl, and the glands begin to secrete a thin, glycogen-containing fluid, preparing the endometrium for possible implantation.
   • Conception: If conception occurs, the nutrient-rich endometrium is ready for implantation. Human chorionic gonadotropin (HCG) is secreted by blastocytes 3 days after fertilization, which maintains the corpus luteum once implantation occurs around day 6 or 7. HCG can be detected in maternal blood and urine 8 to 10 days after ovulation.
4. Ischemic/Menstrual Phase:
   • If Conception Does Not Occur: The corpus luteum degenerates and stops producing progesterone and estrogen if conception and implantation do not occur. Without these hormones, the endometrium becomes ischemic and disintegrates, leading to menstruation and marking the beginning of another cycle.
5. Ovulatory Cycle Duration:
   • Cycle Length: Ovulatory cycles typically range from 24 to 26.5 days in length. The primary ovarian follicle requires 10 to 12.5 days to develop, and the luteal phase is relatively fixed at 14 days (±3 days).
   • Menstrual Blood Flow: Menstrual blood flow usually lasts 3 to 7 days, though it may vary, lasting as long as 8 days or as short as 1 to 2 days and still be considered within normal limits.