Lecture on Primordial Germ Cells and Follicle Development

Primordial Germ Cells (PGCs) in Female Fetus

• PGCs are precursors to gametes (oocytes).
• Migration: PGCs migrate from the yolk sac to the fetal ovary.
• Proliferation:
  • Undergo mitosis in the fetal ovary.
  • Cytokinesis is often incomplete, forming interconnected cells (germ cell cysts/nests).
• Apoptosis:
  • Many PGCs undergo apoptosis, reducing their numbers.
• Meiosis:
  • Some PGCs start meiosis, forming primary oocytes.
  • Arrested at prophase I of meiosis I until ovulation.

Development of Primordial Follicles

• Formation:
  • Oocytes are enclosed by granulosa cells, forming a primordial follicle.
  • Female is born with ~1 million primordial follicles.
• Regulation:
  • Believed to be regulated by factors within the oocyte.
  • Environmental contaminants can affect formation, sometimes leading to follicles with multiple oocytes, which are selected for degradation.
  • Fewer follicles can result in a smaller oocyte reserve, affecting reproductive lifespan.

Primary and Secondary Follicle Development

• Primary Follicle:
  • The oocyte remains in prophase I.
  • Granulosa cells proliferate and change shape (cuboidal).
  • Appearance of zona pellucida (glycoprotein layer essential for fertilization).
• Secondary Follicle:
  • Introduction of thecal cells (internal and external) for structural support.
  • Development from primordial to secondary is gonadotropin-independent.

Role of Follicle and Oocyte-Secreted Factors

• OSFs (Oocyte Secreted Factors): Members of the TGF-beta superfamily, e.g., GDF9 and BMP15.
• Granulosa Cell Factors:
  • Kit Ligand: Promotes follicle development.
  • AMH (Anti-Müllerian Hormone): May restrain development, linked to ovarian reserve and reproductive lifespan.

Tertiary Follicle Development (FSH Dependent)

• Tertiary Follicle:
  • Development is dependent on FSH from the hypothalamus and anterior pituitary.
  • Differentiation of granulosa cells to cumulus granulosa and mural granulosa cells.
  • Formation of the antral cavity by secretion of fluid by granulosa cells.
  • Tertiary follicles start producing sex steroids.

Key Takeaways

• Follicle development requires a delicate balance of intrinsic factors and hormonal signals.
• Environmental factors can significantly impact follicle formation and reserve.
• The transition from secondary to tertiary follicle marks the start of sex steroid production.