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It is well established that the presence of the diploid chromosome set in cell nuclei of the majority of tissues is a characteristic of vertebrates. This peculiarity was supplemented by the rule of the relative constancy of DNA in nuclei with diploid chromosome set (Boivin et al., 1948). However, some cells contain doubled or multiplied chromosome set as compared to other cells of the organism or other individuals of the species. As distinct from the generative polyploidy in which polyploidy covers all cell types including gametes, somatic polyploidy extents to cells of several tissues (Brodsky, Uryvaeva, 1985). At present there are numerous data that the cells of some tissues of animal and plants appear to be polyploid. The incidence of polyploidy in the tissues of animals and plants as well as levels of ploidy is described in detail in quite a few books and reviews (Geitler, 1953; Tschermak-Woess, 1971; Nagl, 1978, 1981, 1995; Brodsky, Uryvaeva, 1985; Raikov, 1982; Zybina, 1986; D’Amato, 1989; Zybina, Zybina, 1996; Edgar, Orr-Weaver, 2001; Ravid et al., 2002). A large body of data on incidence of polyploidy in animal and plant tissues enabled W. Nagl to make a conclusion that polyploidy is a widespread phenomenon characteristic of the overwhelming majority of eukaryotes (Nagl, 1978; 1981; 1995). According to Nagl, Vertebrata represent a specific taxon that differs from other eukaryotes in the extent of polyploidy among the differentiated cells. Thus, in insects and higher plants, nearly all normal, differentiated cells are polyploid. The majority of Mammalia and, most probably, other Vertebrates, are characterized by the diploid chromosome set in most cells including both proliferative and the tissue-specific cells. Polyploidy in Vertebrata is present in many cell types, i.e., hepatocytes, cardyomyocytes, megakaryocytes and some others (Brodsky, Uryvaeva, 1985; Hancock et al, 1993; Kudryavtsev et al, 1993; Nagl, 1995; Anatskaya et al, 2001). It suggests that propagation of vertebrates is, probably, due to an advantage of the mitotic cycle that involves a machinery of accurate reproduction of diploid cells over other cell reproduction cycles that result in polyploidization. It cannot be ruled out that the biological advantage of mitosis has made it possible the wide distribution of Vertebrata. Nevertheless, genome multiplication characteristic of wide range of highly differentiated cells in different taxons suggests an undoubtedly important biological significance of polyploidy in the lifespan of some specific cell types.

Cell Cycle Modification in Trophoblast Cell Populations in the Course of Placenta Formation