Concise Notes on the Follicular Phase in Reproductive Cyclicity

Follicular Phase Overview

  • Composed of two stages: Proestrus and Estrus
  • Events from regression of Corpus Luteum (CL) to ovulation.

Major Events of the Follicular Phase

  1. Gonadotropin release from the anterior pituitary
  2. Follicular dynamics and preparation for ovulation
  3. Sexual receptivity
  4. Ovulation

Hormonal Regulation

  • Estradiol (E2): Dominant hormone from developing follicles, causing biochemical changes.
  • Gonadotropin: Regulated by the hypothalamus-anterior pituitary-ovary interaction.
  • High E2 and low progesterone favor follicular development.

Gonadotropin Release Mechanism

  • Hypothalamus: Produces Gonadotropin-Releasing Hormone (GnRH).
  • Tonic Center: Maintains basal secretion of GnRH (small pulses).
  • Surge Center: Triggers preovulatory surge of GnRH leading to LH surge and ovulation.

Folliculogenesis Stages

  1. Preantral Phase: Gonadotropin-independent, oocyte growth.
  2. Antral Phase: Gonadotropin-dependent, includes recruitment, selection, dominance, and atresia.
Antral Phase Processes
  • Recruitment: Primordial follicles begin growing due to low inhibin and high FSH.
  • Selection: Only some follicles continue developing; influenced by moderate E2 and inhibin.
  • Dominance: A selected follicle outgrows competitors; produces high E2 and inhibin.
  • Atresia: Degeneration of follicles via apoptosis during all phases before ovulation.

Ovulation Process

  • Induced by a surge in GnRH due to high E2: Triggers LH release.
  • Leads to:
    • Resumption of meiosis
    • Cumulus expansion
    • Stigma formation
    • Follicular rupture

Reproductive Behavior

  • Elevated E2 and low progesterone induce sexual receptivity.
  • Behavior changes allow copulation prior to ovulation.