![]() ![]() Thus, a pool of primordial follicles each containing an oocyte arrested at the end of prophase one is established and represents the population of germ cells available for the reproductive lifespan in female mammals ( Pepling, 2012). More recent work has provided evidence that some oocytes play a supporting role similar to nurse cells in Drosophila ( Lei and Spradling, 2016). In addition, other potential functions of germ cell loss have been proposed including selection for the highest quality oocytes or as support for a subset of the cyst cells ( Pepling et al., 1999). As oocytes separate and follicles form, a large number of oocytes are lost by programmed cell death that in part aid in individualization of surviving cells ( Greenfeld et al., 2007). Concurrently, the oocytes contained in germ cell nests separate and each individual oocyte is surrounded by somatic pregranulosa cells forming structures called primordial follicles ( Pepling and Spradling, 2001). Oocytes gradually arrest near the end of prophase one with some oocytes reaching arrest as early as 17.5 dpc and most by postnatal day (PND) 5 ( Cohen et al., 2006). Oocytes remain associated during fetal development though cysts may fragment and reassociate as germ cell nests ( Lei and Spradling, 2013). Oogonia enter meiosis in a wave from anterior to posterior and become oocytes beginning at 13.5 dpc in the mouse ( Menke et al., 2003 Bullejos and Koopman, 2004). However, cytokinesis is not complete and the oogonia remain connected by intercellular bridges in structures called germ cell cysts ( Pepling and Spradling, 1998). The germs cells divide by mitosis until 13.5 dpc and are referred to as oogonia during this time. Oocytes differentiate from primordial germ cells (PGCs) that migrate to the genital ridge starting at 10.5 days post coitum (dpc) in the mouse (see Figure 1 Molyneaux et al., 2001). In the mammalian female embryo, meiotic division of the oocyte is preceded by several rounds of mitosis. In meiosis, cells replicate their DNA once, followed by two rounds of division: meiosis one (MI)- a reductional division, and then meiosis two (MII)-an equational division analogous to mitotic division. Meiosis is a special type of cell division that generates haploid gametes important for sexual reproduction. Introduction: Mammalian Oocyte Development and Meiosis Therefore, elucidating the mechanisms regulating meiotic progression is important to provide a foundation for developing improved treatments of female infertility. These events are prerequisites for proper chromosome segregation in meiotic divisions and if they go awry, chromosomes mis-segregate resulting in aneuploidy. ![]() This review focuses on events and mechanisms controlling the progression through meiotic prophase one, which include recombination, synapsis and control by signaling pathways. At postnatal day 5, most of the oocytes have reached the late diplotene (or dictyate) substage of prophase one where they remain arrested until ovulation. As meiotic prophase one proceeds, chromosomes find their homologous partner, synapse, exchange genetic material between homologs and then begin to separate, remaining connected at recombination sites. ![]() Specifically in the mouse, oocytes enter meiosis at 13.5 days post coitum. If fertilization occurs, the oocyte completes meiosis two followed by fusion with the sperm nucleus and preparation for zygotic divisions otherwise, it is passed into the uterus and degenerates. ![]() After puberty, luteinizing hormone induces ovulation and meiotic resumption in a cohort of oocytes, driving the progression from meiotic prophase one to metaphase two. Oocytes transition through the prophase one substages consisting of leptotene, zygotene, and pachytene, and are finally arrested at the diplotene substage, for months in mice and years in humans. In female mammals, meiotic prophase one begins during fetal development. Department of Biology, Syracuse University, Syracuse, NY, United States. ![]()
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