The Ovarian Cycle

Overview

• The ovarian cycle is the maturation process of eggs in the ovaries.
• Results in a secondary oocyte that can be fertilized, leading to pregnancy.
• Also responsible for the menstrual cycle.

Egg Development

• Location: Eggs develop in the ovaries within structures called follicles.
• Initial Stage: Primordial follicles, consisting of a primary oocyte surrounded by granulosa cells.

Ovarian Cycle Timeline

• Duration: 28 days.
• Day 0: Primordial follicle.
• Day 13: Development of follicles.
• Day 14: Ovulation, ejection of secondary oocyte.
• Days 15–28: Return to primordial follicle stage.

Hormone Production

• Granulosa cells secrete hormones: estrogen, progesterone, and inhibin.
• Hormone levels increase as follicles develop.

Follicle Development Stages

• Primary Follicles: Formation of zona pellucida between granulosa cells and oocyte.
• Pre-antral Follicles: Increase in granulosa cell number; formation of the theca layer.
  • Theca cells produce androstenedione, converted to estrogen by granulosa cells.
• Early Antral Follicles: Formation of an antrum filled with fluid, expanding the follicle.
• Mature Follicle: Dominant follicle enlarges and prepares for ovulation.

Ovulation and Post-Ovulation

• Ovulation: Occurs on day 14, releasing the egg.
• Corpus Luteum: Formed from the follicle post-ovulation, produces hormones.
  • If the egg isn't fertilized, corpus luteum degenerates.
  • If fertilization occurs, corpus luteum persists to support pregnancy.

Follicle and Egg Loss

• Multiple follicles form but only one ovulates; others undergo atresia (degeneration).
• Women lose 15–25 eggs per cycle to atresia.

Menopause

• Onset: Around age 50–51.
• Process: Menstrual cycles become irregular and eventually stop.
• Cause: Ovarian failure due to loss of response to LH and FSH, depletion of follicles and eggs through atresia.