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Oogenesis (/ ˌ oʊ. ə ˈ dʒ ɛ n ɪ s ɪ s /) or ovogenesis is the differentiation of the ovum (egg cell) into a cell competent to further develop when fertilized. [1] It is developed from the primary oocyte by maturation. Oogenesis is initiated before birth during embryonic development.
During oogenesis, the oogonia become primary oocytes. Oocytes (immature ova) residing in the primordial follicle of the ovary are in a non-growing prophase arrested state, but have the capacity to undergo highly efficient homologous recombinational repair of DNA damages including double-strand breaks. [ 1 ]
During oogenesis, meiosis arrests twice. The main arrest occurs during the diplotene stage of prophase 1, this arrest lasts until puberty. The second meiotic arrest then occurs after ovulation during metaphase 2 and lasts for a much shorter time than the first arrest.
An immature ovum is a cell that goes through the process of oogenesis to become an ovum. It can be an oogonium , an oocyte , or an ootid . An oocyte, in turn, can be either primary or secondary, depending on how far it has come in its process of meiosis .
In the case of pre-meiotic doubling, recombination, if it happens, occurs between identical sister chromatids. [ 28 ] If terminal fusion (restitutional meiosis of anaphase II or the fusion of its products) occurs, a little over half the mother's genetic material is present in the offspring and the offspring are mostly homozygous. [ 33 ]
During fertilization, the sperm provides three essential parts to the oocyte: (1) a signalling or activating factor, which causes the metabolically dormant oocyte to activate; (2) the haploid paternal genome; (3) the centrosome, which is responsible for maintaining the microtubule system.
During sexual intercourse, sperm cells are ejaculated into the vagina through the penis, resulting in fertilization of an ovum to form a zygote. [ 1 ] While normal cells contain 46 chromosomes (23 pairs), gamete cells contain only half that number, and it is when these two cells merge into one combined zygote cell that genetic recombination occurs.
Maternal effect genes [11] are expressed during oogenesis by the mother (expressed prior to fertilization) and develop the anterior-posterior and dorsal ventral polarity of the egg. The anterior end of the egg becomes the head; posterior end becomes the tail. the dorsal side is on the top; the ventral side is in underneath.