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