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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 ]
Fertilization was not understood in antiquity. Hippocrates believed that the embryo was the product of male semen and a female factor. Aristotle held that only male semen gave rise to an embryo, while the female only provided a place for the embryo to develop, [5] a concept he acquired from the preformationist Pythagoras.
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.
During these stages, there is also little to no transcription of mRNA from the zygotic genome, [5] but zygotic transcription is not required for MBT to occur. [3] Cellular functions during early cleavage are carried out primarily by maternal products – proteins and mRNAs contributed to the egg during oogenesis .
During this stage, the zygote divides in a process called cleavage. A blastocyst is then formed and implants in the uterus . Embryogenesis continues with the next stage of gastrulation , when the three germ layers of the embryo form in a process called histogenesis , and the processes of neurulation and organogenesis follow.
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 the primary oocyte stage of oogenesis, the nucleus is called a germinal vesicle. [ 2 ] The only normal human type of secondary oocyte has the 23rd (sex) chromosome as 23,X (female-determining), whereas sperm can have 23,X (female-determining) or 23,Y (male-determining).