Oocytes are maintained in this state until the oocyte resumes meiosis (just before ovulation) or degenerates. 67 Once the diplotene stage of the first meiotic prophase is complete, oocyte progression through meiosis becomes arrested at the dictyate stage. During the first meiotic prophase, maternal and paternal genetic material is exchanged through the process of recombination between homologous chromosomes, a unique characteristic of meiotic cell division (for review see Cohen and colleagues). Pre-meiotic DNA replication is followed by the transitory stages of the first meiotic prophase: preleptotene-leptotene-zygotene, pachytene, and diplotene ( Fig. There is an increasing body of evidence that demonstrates the existence of stem cells with germ cell–like characteristics in the adult ovary in some species, 58 but whether such ovarian stem cells contribute new oocytes to the follicle pool under normal conditions in vivo remains a matter of intense debate. 55, 56 Isolation of these stem cells in mice can generate new oocytes and even support the development of live offspring when reintroduced into a donor ovary, 55, 57 challenging the concept that postnatal neoformation of oocytes does not occur in most mammals. 51, 52 In the postnatal and adult mouse oogonial stem cells have been identified 53, 54 and subsequently in human ovaries as well. 47–50 However, the ovaries of adult prosimians (lower primates) are an exception to this rule, wherein DNA synthesis can be detected in oogonia-like germ cells. The absence of detectable DNA synthesis in germ cells in the postnatal ovary has been taken as evidence that neoformation of oocytes does not occur later in life that is, the entire pool of oogonia initiate meiosis and differentiate into oocytes before or around birth. This DNA must last until the oocyte has been successfully ovulated and fertilized, which in humans may occur up to 40 years later. In the mammalian ovary, meiosis commences with a round of pre-meiotic DNA replication, doubling the DNA content of the germ cell from 2c (i.e., copies of) DNA to 4c. Extensive degeneration, however, reduces the number of germ cells drastically during the rest of fetal life, leaving only 300,000 to 2.5 million oocytes in the ovaries of the newborn girl. 36 The term oogenesis denotes the transformation of an oogonium into an oocyte, and maturation into a fertilizable oocyte. The first meiotic stages can be identified in week 9 pc. 43, 44 When the oogonia stop mitotic divisions and enter meiosis, they are termed oocytes. The number of mitotically dividing oogonia increases exponentially from approximately 25,000 in the 6-week-old human embryo to approximately 250,000 in the 9th week, 36, 42, 43 reaching a maximum of around 7 million in the 20th week ( Fig. Polin MD, in Fetal and Neonatal Physiology, 2022 Meiotic Prophase of Oocytes
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