![]() 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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