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7422869 
Journal Article 
Embryonic Development of the Ovary, Sexual Reproduction and Meiosis 
Vázquez-Nin, GH; Escobar, ML; Echeverría, OM; , 
2011 
Springer Netherlands 
Dordrecht 
Cell Death in Mammalian Ovary 
25-48 
WOS:000292219800002 
Sex first evolved in the common ancestor of all eukaryotes some two billons years ago. Fertilization is the union of two haploid gametes of different sex; the resulting diploid cell is the zygote. In mammalian species gametes are derived from precursors termed primordial germ cells (PGCs). Specification of the germline occurs through: (1) repression of somatic differentiation; (2) reacquisition of potential pluripotency; (3) genome wide epigenetic reprogramming. In several mammals the maintenance of germ cell linage is not due to a preformed "germ plasma", but is induced by environmental signals. In mammals PGCs originate in extraembryonic region and eventually move to gonadal ridges, located in the medial part of the urogenital ridge. Observations and experimental evidence demonstrate that PGCs move actively from the hindgut to the gonadal ridges. However, some observations suggest that most displacements of these cells may be explained by global growth movements. In most mammals sex determination is genetically controlled by the presence or absence of Y chromosome and the expression of gene Sry. However, the ovary differentiation requires in addition the action of a precise gene cascade. The main process that marks PCGs/oogonia sex differentiation within fetal ovary is the beginning of meiosis. During the progression of the first meiotic prophase many germ cells are eliminated, around 70% in mice, rats and humans. Prenatal development of the follicles is regulated by a complex interplay of hormones produced by the hypophysis (FSH, LH), and paracrine regulatory factors produced by the oocytes and the surrounding somatic cells. Epigenetic signals are covalent modifications of histones or DNA that convey a specific "meaning" to the stretch of chromatin on which they are found. Frequently they induce changes in chromatin conformation and gene expression. Pluripotent embryonic germ cells are capable of giving rise to all tissues, thus they must be able to reorganize their mechanisms of gene regulation to allow multiple lines of cell differentiation. Besides epigenetic modifications regulate X inactivation/reactivation in the female embryo and mark the paternal and maternal genome of germ cells, making the expression of a group of genes monoallelic, crucial for embryo development termed for modification such as "imprinted" genes. 
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