Oogenesis

Overview

• Oogenesis 
• development of mature oocytes, or haploid gametes capable of fertilization by sperm

Oogenesis

• Pre-natal oogenesis
  • oogenesis, or maturation of oocytes, begins in fetal life
    • primordial germ cells migrate from fetal hind gut to gonadal ridge
      • primordial germ cells differentiate to oogonia
    • in fetal ovary, oogonia proliferate by mitotic divisions
      • by week 20, proliferation ends and oogonia numbers peak at 7 million
    • oogonia (2N, diploid) differentiate to primary oocytes (2N, diploid)
      • primary oocytes begins meiosis I and arrest in prophase I  
        • primary oocytes arrest in “dictyotene stage” until just before ovulation
        • primary oocyte is surrounded by pre-granulosa cells in primordial follicle
• Post-natal oogenesis
  • oogenesis, or maturation of oocytes, continues in post-puberty adult life
    • in follicular (proliferative) phase of menstrual cycle, development of primordial follicles yields one dominant, pre-ovulatory Graafian follicle
    • primary oocyte (2N, diploid) differentiates to secondary oocyte (2N, haploid)
      • primary oocyte completes meiosis I, yielding secondary oocyte and first polar body
      • secondary oocyte begins meiosis II and arrests in metaphase II
    • at ovulation, Graafian follicle ruptures and releases secondary oocyte
      • in response to fertilization, secondary oocyte completes meiosis II, yielding mature ovum (N, haploid) and second polar body
        • ovum nucleus condenses and forms female pronucleus that fuses with male pronucleus to form zygote