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

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