Ovarian cycle and its regulation.

 

The ovarian cycle has follicular (FSH, estrogen), ovulation (LH flow), and luteal (progesterone) phases, regulated by the hypothalamic–pituitary–ovarian axis through a delicate balance of feedback mechanisms.

1. Ovarian Cycle – Phases

The ovarian cycle lasts about 28 days (average) and has three main phases:

A. Follicular Phase (Day 1–14, variable)

  • Begins with menstruation.

  • FSH stimulates growth of several ovarian follicles.

  • One follicle becomes dominant (Graafian follicle) → secretes estrogen.

  • Rising estrogen → proliferation of endometrium (preparing for implantation).

B. Ovulation (Around Day 14)

  • High estrogen levels (positive feedback) → LH surge.

  • LH surge causes:

    • Final maturation of oocyte.

    • Rupture of Graafian follicle → release of ovum.

C. Luteal Phase (Day 15–28, fixed ~14 days)

  • Remaining follicle forms corpus luteum.

  • Corpus luteum secretes progesterone (main) + estrogen.

  • Progesterone prepares endometrium (secretory phase) for implantation.

  • If fertilization does not occur → corpus luteum degenerates → progesterone falls → menstruation starts.

  • If fertilization occurs → hCG from trophoblast maintains corpus luteum → continued progesterone secretion.

2. Regulation of Ovarian Cycle

Hormonal Control (Hypothalamic–Pituitary–Ovarian Axis)

  • GnRH (hypothalamus) → stimulates anterior pituitary.

  • FSH → follicular growth, estrogen secretion.

  • LH → ovulation, corpus luteum formation, progesterone secretion.

Feedback Mechanisms

  • Early Follicular Phase: Low estrogen → negative feedback → maintains baseline LH & FSH.

  • Late Follicular Phase: High estrogen (sustained for ~48h) → positive feedback → LH surge → ovulation.

  • Luteal Phase: High progesterone (with estrogen) → negative feedback → suppresses GnRH, LH, FSH.

  • If pregnancy occurs: hCG maintains corpus luteum until placenta produces enough progesterone.


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