<P> Recently, however, two publications have challenged the belief that a finite number of oocytes are set around the time of birth . The renewal of ovarian follicles from germline stem cells (originating from bone marrow and peripheral blood) has been reported in the postnatal mouse ovary . In contrast, DNA clock measurements do not indicate ongoing oogenesis during human females' lifetimes . Thus, further experiments are required to determine the true dynamics of small follicle formation . </P> <P> 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 . </P> <P> However, although this process begins at prenatal age, it stops at prophase I. 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 . </P> <P> 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 . For those primary oocytes that continue to develop in each menstrual cycle, however, synapsis occurs and tetrads form, enabling chromosomal crossover to occur . As a result of meiosis I, the primary oocyte has now developed into the secondary oocyte and the first polar body . </P>

The stage of prophase i in which mammalian oogenesis can arrest for years is