Lecture on Oogenesis and Reproductive Development

Introduction

• Focus on the process of oogenesis
• Comparison with popular media (reference to the movie with Matt Damon)

What is Oogenesis?

• Definition: The process where mature female gametes or ova are produced from germ cells.
• Occurs in the outermost layer of the ovaries.
• Begins before birth.

Process of Oogenesis

Early Development

• Begins in fetal development:
  • Primordial germ cells originate from the Primitive streak.
  • Migrate through the hind gut to the gonadal ridge, which becomes the gonads.
  • In absence of a Y chromosome, gonads become fetal ovaries.
• Mitosis:
  • Primordial germ cells undergo mitosis, producing oogonia (immature diploid cells).

Oogonia Multiplication

• Occurs during weeks 9-22 of gestation.
• By week 24, up to 7 million oogonia in female ovaries.
• Reduction: Most oogonia die, remaining become primary oocytes.
• At Birth: Approximately 2 million primary oocytes present.

Arrest in Prophase 1

• Primary oocytes enter meiosis but get arrested in prophase 1.
• Remain in this arrested state throughout childhood.

Adolescence and Beyond

Ovulation and Meiosis

• Puberty onwards:
  • Primary oocytes complete meiosis 1, producing a haploid secondary oocyte and the first polar body.
  • Haploid Nature: Single set of chromosomes.

Arrest in Metaphase 2

• Secondary oocyte enters meiosis 2 but is arrested in metaphase 2.

Fertilization

• Completion of Meiosis 2: Triggered by fertilization.
• Secondary oocyte completes meiosis 2 to form:
  • Mature ovum
  • Second polar body
• Formation of Zygote:
  • Haploid sperm pronucleus fuses with haploid female pronucleus.
  • Forms diploid zygote with two full sets of chromosomes.

Summary

• Key Steps:
  1. Differentiation of primordial germ cells into oogonia, proliferation via mitosis.
  2. Oogonia become primary oocytes, arrested in prophase 1 until puberty.
  3. Ovulation triggers completion of meiosis 1, secondary oocyte arrested in metaphase 2 until fertilization.
  4. Fertilization results in completion of meiosis, forming a mature ovum and creating a diploid zygote.