4. Ovarian and Menstrual cycles

4.1. Ovarian Cycle (28-Day Cycle)

1.1      Overview

§  The monthly series of events associated with egg maturation is the ovarian cycle.

§  It consists of three phases.

1.     The follicular phase

2.     Ovulation phase.

3.     Luteal Phase

§  Regulated by the hypothalamic–pituitary–ovarian (HPO) axis.

§  Ovarian cycles may vary in length. Average length: 28 days (normal range: 21–35 days).

o   The follicular phase may range from seven to 26 days long. However, the luteal phase is constant 14 days.

1.2 Follicular Phase (Day 1–14)

  • Begins with menstruation.
  • Rising FSH stimulates a cohort of primordial follicles to develop into primary follicles.
  • Continued follicular growth results in fused vesicles to form a single antrum. This is a mature Graafian follicle
  • As the Graafian follicle grows, the primary oocyte finishes meiosis I to become a secondary oocyte (plus a polar body, which soon degenerates).
  • Dominant follicle selected by Day 7.
  • Granulosa cells produce estrogen.
    • Endometrial proliferation
    • Cervical mucus thinning
  • Estrogen peak → LH surge (positive feedback)→ ovulation.
  • The follicles that did not rupture degenerate and their eggs are lost.

1.3 Ovulation (Around Day 14)

  • The spike in LH causes ovulation: the most mature follicle bulges out of the ovary, ruptures and releases its egg (secondary oocyte arrested in metaphase II).
  • LH surge begins ~24 hours before ovulation. 
    • Stimulates Graafian follicle to rupture, releasing secondary oocyte
  • Fertilization must occur within 12–24 hours.
  • If not fertilized, the oocyte will degenerate after a few days.

1.4 Luteal Phase (Day 15–28)

  • Ruptured follicle → corpus luteum under the influence of LH
  • Secretes progesterone (dominant) + estrogen.
  • The progesterone facilitates the regrowth of the uterine lining and inhibits the release of further FSH and LH.
    • The inhibition of FSH and LH prevents any further eggs and follicles from developing, while the progesterone is elevated.
  • If no pregnancy: corpus luteum degenerates → corpus albicans →hormonal level falls → menstruation begins.
    • The decrease in progesterone also allows the hypothalamus to send GnRH to the anterior pituitary, releasing FSH and LH and starting the cycles again.
  • If the oocyte is fertilized and pregnancy occurs, the corpus luteum will persist for about three months until the placenta is formed and ready to take over its function