Oogenesis in Mammals
In mammals, the first part of oogenesis starts in the germinal epithelium, which gives rise to the development of ovarian follicles, the functional unit of the ovary.
Note that this process, important to all animal life cycles yet unlike all other instances of cell division, occurs completely without the aid of oo spindle-coordinating centrosomes.
Oogenesis consists of several sub-processes: oocytogenesis, ootidogenesis, and finally maturation to form an ovum (oogenesis proper). Folliculogenesis is a separate sub-process that accompanies and supports all three oogenetic sub-processes.
| Cell type | ploidy | Process | Process completion |
| Oogonium | diploid | Oocytogenesis (mitosis) | third trimester (forming oocytes) |
| primary Oocyte | diploid | Ootidogenesis (meiosis 1) (Folliculogenesis) | Dictyate in prophase I for up to 50 years |
| secondary Oocyte | haploid | Ootidogenesis (meiosis 2) | Halted in metaphase II until fertilization |
| Ovum | haploid |
Oogonium --(Oocytogenesis)--> Primary Oocyte --(Meiosis I)-->First Polar Body (Discarded afterward) + Secondary oocyte --(Meiosis II)--> Secondary Polar Body (Discarded afterward) + Ovum
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