**2. Poly- and aneuploidy, their origin and significance**

Recently, a notion dominates that the multifold genome multiplication is achieved by modified cell cycles. Among them the shortest one leading to the highest levels of ploidy is characterized by alternating DNA synthesis (S) and Gap (G) phases in the absence of intervening mitoses, karyokinesis, and cytokinesis; a series of these shortened cycles allows cells to achieve high level of ploidy that may exceed 1000c [10–13].

The trophoblast cells that form a barrier between semiallogenic fetal (trophoblastic) and maternal (decidua) tissues probably require mechanism(s) to sustain maternal-fetal tolerance achieved by different mechanisms. For example, the trophoblast cells secrete a range of cytokines and chemokines thereby contributing to the process of immune regulation at the placental–maternal interface [14, 15]. On the other hand, as we stated previously, the TGC multifold genome multiplication also may protect their genome from mutagenic effect of the DNA of the phagocytosed maternal cells [16, 17]. Besides, some of the TGC functions of a barrier may be performed due to their giantism. TGC produce enormous keratin-positive sprouts that allow them to phagocytose accumulations of decidual cells and simultaneously to sustain the continuous TGC layer at the border with decidua [18]. Destruction of the cytokeratin 8 and 19 results in disruption of integrity of the murine giant trophoblast cell layer [19], which result in embryo death.

In distinct from the primary and secondary TGC, the low-ploid trophoblast cells in rat and mouse placenta show high proliferative activity and, being protected by a TGC barrier, accumulate a great bulk of cells that differentiate into a range of trophoblast cell subtypes, some of them form placental barrier supplying embryo by nutrition and oxygen; other subtypes are involved in glycogen storage, hormone production and deep intrauterine invasion [20–26].

The lifespan of the endoreduplicated TGC ends in depolyploidization via non-mitotic division of the giant nucleus or nuclear whole-genome fragmentation. In this case, division is achieved without complete chromosome condensation and their arrangement in metaphase plate, spindle formation and poleward chromosome movement. DNA content as well as nucleoli, heterochromatin and gonosomal chromatin bodies distributed into "subnuclei" according to their ploidy levels [3, 27–30]. By now, it is possible to consider it as variant of so called "polyploidy cycle" [31–33] that implies alternation of diploid and polyploid state in a cell lineage. It should be noted that such a phenomenon is fairly rare encountered in the cell lifespan and may be found in the "ancient" organisms like Protists [31–33] and some Invertebrates [34]. In the multicellular Invertebrates and Plants a wast majority of the differentiated cell types are endopolyploid [35, 36]. In Vertebrates, most cells are diploid, and the mammalian trophoblast cells, probably, represent an example of a recapitulation to some ancient forms of cell cycle and cell lifespan similar to protists and Invertebrates.

As to depolyploidization in TGC of rodent placenta, it should be emphasized that they do not belong to the complete polyploid cycle because they do not give rise to the cells capable of self-reproduction because they cease DNA replication shortly before the birth that probably prevents a massive proliferation of semiallogenic embryonic cells inside the maternal tissues.

As we stated in our previous paper, depolyploidization probably may result in aneuploidy in the trophoblast cells [5] because such a way of cell dividion, most probably does not ensure precise distribution of all chromosomes into dauther cells. Surprisingly, aneuploidy combined with polyploidy were recently reported as a factor of adaptation to the stressful conditions [37, 38]. Hepatocytes represent a cell

**63**

*Genome Modifications Involved in Developmental Programs of the Placental Trophoblast*

type capable of high mitotic activity [39]. In the polyploid cells multipolar mitoses are encountered that may result in cells of lower ploidy, some of them were aneuploid [40]. Thus, in mice with knockdown of the genes *E2f7* and *E2f8* that regulate polyploidy in the liver, the amount of polyploid hepatocytes reduced fourfold; interestingly, nearly all hepatocytes became euploid [36]. Therefore, aneuploid cell resulted from polyploid ones. These mice were bred to tyrosinemia mice. As a result, although tyrosinemic mice were more susceptible to morbidities and death, they developed regenerating nodules similar to the control mice. Notably, the nodules in tyrosinemic livers were generated by aneuploidy; moreover, the mutation of *E2f7* and *E2f8* were inactivated [40]. The authors state that polyploid hepatocytes are necessary for the formation of aneuploid cells that can facilitate adaptation to

In the placental trophoblast wide variability in different mammalian species show aneuploidy. In rodents, TGC of rat and mice undergone genome segregation via nuclear fragmentation, showed a great number of DNA values intermediate between ploidy classes [27]. It may be accounted for the deviation from the regular chromosome distribution rather than S-phase because at this developmental stage TGC do not proceed cell cycle and DNA replication that would be a reason of intermedial DNA content values. In contrast, in another rodent, field vole *Microtus rossiaemeridionalis*, TGC demonstrate the clear-cut classes of ploidy from 1c to 16 c [28]. Much more DNA content variability was found in the silver fox placenta [41, 42]. By contrast, silver fox placenta, especially its invasive trophoblast, shows a notable fluctuation of ploidy and a variability of patterns of polyploidization within the same cell lineage including aneuploidy and genome multiplication pathways

It may reflect the necessity of different strategies that may be useful for maintaining the lengthy pregnancy [5, 42]. In the silver fox placenta, upon polyploidiza-

and there were a great variety of intermediate values suggesting a significant incidence of aneuploidy [41]. We suggested that it may serve a source of genome variability, in particular, hetero- and homozygosity that may be useful to select a more specific response to stress factors. Unlike small rodents such as mouse, rat and field vole, whose pregnancy do not exceed 30 days, in the fox placenta aneuploid trophoblast cells may have a protective effect during 6 months of intrauterine

**3. Underreplication and amplification of some genes and clasters** 

The cells undergone endoreduplication and formation of the classic polytene chromosomes are known to underreplicate a significant amounts of DNA [43, 44]. Recently it has been found underreplication (UR) of some chromosome regions and genes in the murine giant trophoblast cells. TGCs of the mouse placenta contain 47 regions, totaling 138 Megabases, where genomic copies are underrepresented [45]. UR domains originate from a subset of late-replicating heterochromatic regions containing gene deserts and genes involved in cell adhesion and neurogenesis. Interestingly, both size and degree of depletion of UR domains gradually progresses during early gestation. Thus, all UR domains at 9.5th day are also present at 8.0th day, and UR domains at 9.5 gestation day are also more numerous, larger and more depleted. However, unlike between 8.0th and 9.5th day, where the degree of depletion expanded, there were no significant change from 9.5th and 16.5th day.

**regulate the giant trophoblast cells differentiation**

)c was found, with a tendency to 2<sup>n</sup>

× 3c,

(endoreduplication, classic endomitosis, depolyploidization).

tion, a considerable deviation from (2<sup>n</sup>

development [5].

*DOI: http://dx.doi.org/10.5772/intechopen.97247*

chronic liver diseases.

#### *Genome Modifications Involved in Developmental Programs of the Placental Trophoblast DOI: http://dx.doi.org/10.5772/intechopen.97247*

type capable of high mitotic activity [39]. In the polyploid cells multipolar mitoses are encountered that may result in cells of lower ploidy, some of them were aneuploid [40]. Thus, in mice with knockdown of the genes *E2f7* and *E2f8* that regulate polyploidy in the liver, the amount of polyploid hepatocytes reduced fourfold; interestingly, nearly all hepatocytes became euploid [36]. Therefore, aneuploid cell resulted from polyploid ones. These mice were bred to tyrosinemia mice. As a result, although tyrosinemic mice were more susceptible to morbidities and death, they developed regenerating nodules similar to the control mice. Notably, the nodules in tyrosinemic livers were generated by aneuploidy; moreover, the mutation of *E2f7* and *E2f8* were inactivated [40]. The authors state that polyploid hepatocytes are necessary for the formation of aneuploid cells that can facilitate adaptation to chronic liver diseases.

In the placental trophoblast wide variability in different mammalian species show aneuploidy. In rodents, TGC of rat and mice undergone genome segregation via nuclear fragmentation, showed a great number of DNA values intermediate between ploidy classes [27]. It may be accounted for the deviation from the regular chromosome distribution rather than S-phase because at this developmental stage TGC do not proceed cell cycle and DNA replication that would be a reason of intermedial DNA content values. In contrast, in another rodent, field vole *Microtus rossiaemeridionalis*, TGC demonstrate the clear-cut classes of ploidy from 1c to 16 c [28]. Much more DNA content variability was found in the silver fox placenta [41, 42]. By contrast, silver fox placenta, especially its invasive trophoblast, shows a notable fluctuation of ploidy and a variability of patterns of polyploidization within the same cell lineage including aneuploidy and genome multiplication pathways (endoreduplication, classic endomitosis, depolyploidization).

It may reflect the necessity of different strategies that may be useful for maintaining the lengthy pregnancy [5, 42]. In the silver fox placenta, upon polyploidization, a considerable deviation from (2<sup>n</sup> )c was found, with a tendency to 2<sup>n</sup> × 3c, and there were a great variety of intermediate values suggesting a significant incidence of aneuploidy [41]. We suggested that it may serve a source of genome variability, in particular, hetero- and homozygosity that may be useful to select a more specific response to stress factors. Unlike small rodents such as mouse, rat and field vole, whose pregnancy do not exceed 30 days, in the fox placenta aneuploid trophoblast cells may have a protective effect during 6 months of intrauterine development [5].
