**Abstract**

The placental trophoblast cells give an example of profound genome modifications that lead to whole-genome multiplication, aneuploidy, under-replication of some genes or their clusters as well as, by contrast, gene amplification. These events are included into program of differentiation of functionally different cell lineages. In some cases the trophoblast cell differentiation involves depolyploidization achieved by nonmitotic division. Aneuploidy may be also accounted for by the unusual mitoses characteristic of Invertebrates and plants; in mammalian it may result from hypomethylation of centromere chromosome regions. The giant (endopolyploid) trophoblast cells organization includes "loose nucleosomes" accounted for by the non-canonical histone variants, i.e. H2AX, H2AZ, and H3. 3 . In the human extravillous trophoblast cells that, like murine TGC, invade endometrium, there occured significant changes of methylation as compared to non-invasive trophoblast cell populations . Meantime, some genes show hypermethylation connected with start of trophoblast lineages specification. Thus, despite the limited possibilities of chromosome visualization trophoblast cells represent an interesting model to investigate the role of modification of gene copy number and their expression that is important for the normal or abnormal cell differentiation.

**Keywords:** trophoblast, genome, chromatin, chromosomes, histones, polyploidy, aneuploidy, gene amplification
