Oogonia enter meiosis during embryonic development, becoming oocytes. Meiosis begins with DNA replication and meiotic crossing over. It then stops in early prophase. Maintenance of meiotic arrest[ edit ] Mammalian oocytes are maintained in meiotic prophase arrest for a very long time -- months in mice, years in humans.
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Oogonia enter meiosis during embryonic development, becoming oocytes. Meiosis begins with DNA replication and meiotic crossing over.
It then stops in early prophase. Maintenance of meiotic arrest[ edit ] Mammalian oocytes are maintained in meiotic prophase arrest for a very long time -- months in mice, years in humans.
Initially the arrest is due to lack of sufficient cell cycle proteins to allow meiotic progression. However, as the oocyte grows, these proteins are synthesized, and meiotic arrest becomes dependent on cyclic AMP . The cyclic AMP is generated by the oocyte by adenylyl cyclase in the oocyte membrane.
The adenylyl cyclase is kept active by a constitutively active G-protein-coupled receptor known as GPR3 and a G-protein, Gs, also present in the oocyte membrane . Maintenance of meiotic arrest also depends on the presence of a multilayered complex of cells, known as a follicle, that surrounds the oocyte.
Removal of the oocyte from the follicle causes meiosis to progress in the oocyte. The granulosa cells produce a small molecule, cyclic GMP, that diffuses into the oocyte through the gap junctions.
Luteinizing hormone acts on receptors in the outer layers of granulosa cells of the follicle, causing a decrease in cyclic GMP in the granulosa cells . Because the granulosa cells and oocyte are connected by gap junctions, cyclic GMP also decreases in the oocyte, causing meiosis to resume . Meiosis then proceeds to second metaphase, where it pauses again until fertilization.
Luteinizing hormone also stimulates gene expression leading to ovulation . Oogenesis in Eukaryotic Cells. A oogonium where the mitotic division occurs B differentiation and meiosis I begins C primary oocyte D meiosis I is completed and meiosis II begins E secondary oocyte F first polar body G ovulation must occur and the presence of the sperm penetration fertilization induces meiosis II to completion H ovum I second polar body Oogenesis[ edit ] Oogenesis starts with the process of developing primary oocytes, which occurs via the transformation of oogonia into primary oocytes , a process called oocytogenesis.
Number of primary oocytes[ edit ] It is commonly believed that, when oocytogenesis is complete, no additional primary oocytes are created, in contrast to the male process of spermatogenesis, where gametocytes are continuously created.
Two publications have challenged the belief that a finite number of oocytes are set around the time of birth. Ootidogenesis[ edit ] The succeeding phase of ootidogenesis occurs when the primary oocyte develops into an ootid. This is achieved by the process of meiosis. In fact, a primary oocyte is, by its biological definition, a cell whose primary function is to divide by the process of meiosis.
In late fetal life, all oocytes, still primary oocytes, have halted at this stage of development, called the dictyate. After menarche , these cells then continue to develop, although only a few do so every menstrual cycle. Meiosis I[ edit ] Meiosis I of ootidogenesis begins during embryonic development, but halts in the diplotene stage of prophase I until puberty.
The mouse oocyte in the dictyate prolonged diplotene stage actively repairs DNA damage, whereas DNA repair is not detectable in the pre-dictyate leptotene , zygotene and pachytene stages of meiosis. As a result of meiosis I, the primary oocyte has now developed into the secondary oocyte and the first polar body. However, this process is also halted at the metaphase II stage until fertilization , if such should ever occur. When meiosis II has completed, an ootid and another polar body have now been created.
Main article: Folliculogenesis Synchronously with ootidogenesis, the ovarian follicle surrounding the ootid has developed from a primordial follicle to a preovulatory one.
Maturation into ovum[ edit ] Both polar bodies disintegrate at the end of Meiosis II, leaving only the ootid, which then eventually undergoes maturation into a mature ovum.
The function of forming polar bodies is to discard the extra haploid sets of chromosomes that have resulted as a consequence of meiosis. In vitro maturation[ edit ] Main article: In vitro maturation In vitro maturation IVM is the technique of letting ovarian follicles mature in vitro. It can potentially be performed before an IVF. Rather, oocytes can mature outside the body prior to IVF. Hence, no or at least a lower dose of gonadotropins have to be injected in the body. 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. In ascaris , the oocyte does not even begin meiosis until the sperm touches it, in contrast to mammals, where meiosis is completed in the estrus cycle. In female Drosophila flies, genetic recombination occurs during meiosis. This recombination is associated with formation of DNA double-strand breaks and the repair of these breaks.
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