Oogenesis
From Devonshire
Oogenesis or rarely oögenesis is the creation of an ovum (egg cell).
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Oogenesis in animals
In mammals, oogenesis occurs in the ovarian follicle of the ovary. Oogonial transformation into oocytes (oocytogenesis) is completed either before or shortly after birth. Further development comes to a rest during prophase I of meiosis, known as the dictyate stage, until puberty triggers oogenesis. In ascaris, the oocyte does not even begin meiosis until the sperm touches it.
Oogenesis is the step in which the primary oocyte turns to an ovum undergoing meiosis. At first there is a diploid oocyte called the primary oocyte. Meiosis I occurs in which synapsis occurs and tetrads form and Crossing over as well and this becomes the secondary oocyte and the first polar body (2n). The haploid secondary oocyte undergoes meiosis II forming an ootid and another polar body (n). Both polar bodies at the end of Meiosis II disintegrate leaving only the ootid which undergoes maturation and eventually matures into an ovum.
It is interesting to note that such an important process in animal life cycles is done completely without the aid of spindle-coordinating centrosomes.
Oogenesis in other organisms
Many protists produce egg cells in structures termed archegonia. Some algae and the oomycetes produce eggs in oogonia. In the brown alga Fucus, all four egg cells survive oogenesis, which is an exception to the rule that generally only one product of female meiosis survives to maturity.
In plants, oogenesis occurs inside the female gametophyte via mitosis. In many plants such as bryophytes, ferns, and gymnosperms, egg cells are formed in archegonia. In flowering plants, the female gametophyte has been reduced to an eight-celled embryo sac within the ovule inside the ovary of the flower. Oogenesis occurs within the embryo sac and leads to the formation of a single egg cell per ovule.
