IP1 UNIT 3 - lect 5

Female Reproduction and Folliculogenesis

  • Female reproductive system is responsible for ovulation, folliculogenesis, and the development of the corpus luteum.

  • Folliculogenesis refers to the maturation of ovarian follicles in the ovaries, which is crucial for the fertility of females.

Oocyte and Follicular Structure

Key Components of the Follicle

  • Oocyte: The female gamete involved in reproduction.

  • Membrana granulosa: A layer of cells surrounding the oocyte that provides nourishment.

  • Antrum: A fluid-filled cavity within the follicle that aids in the maturation process.

  • Basement membrane: Separates the membrana granulosa from the thecal layer.

  • Theca interna: The layer of the follicle that produces hormones, particularly androgens.

  • Graafian follicle: A mature ovarian follicle ready for ovulation.

Graafian Follicle – Hormonal Control

  • The Graafian follicle is characterized by high levels of oestrogen.

Hormonal Interactions

  • Oestrogen Impact:

    • Stimulates the production of Follicle-Stimulating Hormone (FSH) and Luteinizing Hormone (LH) by the pituitary gland.

    • Promotes expression of LH receptors on theca cells.

  • FSH Role:

    • Triggers proliferation of granulosa cells.

  • LH Action:

    • Androgen production is stimulated in theca cells, which is then converted to oestrogen in granulosa cells.

    • This creates a positive feedback loop enhancing FSH and LH levels significantly, demonstrating a rare physiological positive feedback mechanism.

Graafian Follicle - Ovulation

Process of Ovulation

  • Extremely high oestrogen levels induce a surge in LH.

  • LH surge leads to:

    • Secretion of proteolytic enzymes (like lysosomal enzymes and collagenase) that weaken the follicle wall.

    • Increased blood flow to the ovary aiding in follicle wall pressure, leading to fluid accumulation in the antrum.

  • The follicle ruptures at the stigma point releasing the oocyte into the oviduct.

Ovulation Details

  • The fimbriae of the oviduct seek the ruptured follicle on the ovary.

  • Once the follicle ruptures, the fimbriae collect the oocyte, completing the ovulation process.

Oocyte Structure

  • Zona pellucida: A protective glycoprotein layer surrounding the oocyte.

  • Cumulus cells: Supportive cells that surround the oocyte.

  • Polar body: A small cell that is a byproduct of oocyte division.

  • Perivitelline space: The area between the zona pellucida and the oocyte, crucial for fertilization.

Corpus Luteum (CL)

  • Following ovulation, the ovary enters the luteal phase where the corpus hemorrhagicum forms due to rapid blood vessel invasion.

  • LH Role: It induces luteinisation of remaining granulosa and theca cells to form the corpus luteum.

  • Progesterone Production: The corpus luteum produces progesterone, essential for maintaining pregnancy if it occurs.

Luteal Regression (Sheep)

  • Corpus luteum lifespan is contingent upon pregnancy; it remains until the end of the luteal phase or when pregnancy does not occur.

  • Prostaglandin F2α: A hormone that causes luteolysis (breakdown of the corpus luteum) necessary for the commencement of a new follicular phase, secreted by the uterus about 10-15 days post-formation of the corpus luteum.

Revision Summary

  • Follicles contain both granulosa and theca cells responsible for secreting oestrogen and progesterone.

  • FSH specifically targets granulosa cells, while LH targets theca cells, resulting in an LH surge that instigates ovulation.

  • The corpus luteum's function is to secrete progesterone; in sheep, prostaglandin F2α leads to luteolysis.