Lecture Notes: The Ovarian Cycle

Overview

Ovaries: Two structures in the female reproductive system that produce eggs.
Ovarian Cycle: Maturation process of eggs each month, leading to the creation of a secondary oocyte, which can be fertilized by sperm.
Menstrual Cycle: A physical manifestation of the ovarian cycle.

Oocyte Development:
- Eggs develop in structures called follicles (purple circles).
- Begins as primordial follicles with a primary oocyte surrounded by granulosa cells.
- Granulosa cells increase in number and secrete hormones like estrogen, progesterone, inhibin.

Timeline:
- 28-day cycle.
- Day 0: Primordial follicle stage.
- Day 13: Secondary oocyte ejection.
- Day 14: Ovulation.
- Days 15-28: Return to primordial follicle stage.
Follicle Development:
- Primary follicles: Granulosa cells develop, separated by the zona pellucida.
- Pre-antral follicle: Increase in granulosa cells and formation of theca cells.
- Theca Cells: Have receptors for luteinizing hormone (LH) and produce androstenedione, which granulosa cells convert to estrogen.
- Antrum Formation: Fluid-filled space within the follicle, helps follicle expansion.

Selection: Multiple follicles form, but only one becomes the dominant follicle and is ovulated.
Atresia: Process where other developing follicles degenerate.
Mature Follicle: Enlarges, forming a structure called cumulus oophorus, which helps separate the egg from follicle wall.
Ovulation (Day 14): Enzymes break down the wall between the mature follicle and ovary, releasing the secondary oocyte.

Corpus Luteum Formation: Old follicle transforms into corpus luteum, producing estrogen, progesterone, inhibin.
- Inhibin: Lowers FSH from the pituitary.
- Apoptosis: If not fertilized, corpus luteum degenerates around Day 25.
Fertilization: If the egg is fertilized, corpus luteum persists to support pregnancy.

Age 50-51: Menstrual cycles become irregular and stop entirely.
Ovarian Failure: Ovaries lose response to LH and FSH due to follicle degeneration.