Trophoblast stem cell derivation, cross-species comparison and use of nuclear transfer: new tools to study trophoblast growth and differentiation.

The trophoblast is a supportive tissue in mammals that plays key roles in embryonic patterning, foetal growth and nutrition. It shows an extensive growth up to the formation of the placenta. This growth is believed to be fed by trophoblast stem cells able to self-renew and to give rise to the differentiated derivatives present in the placenta. In this review, we summarize recent data on the molecular regulation of the trophoblast in vivo and in vitro. Most data have been obtained in the mouse, however, whenever relevant, we compare this model to other mammals. In ungulates, the growth of the trophoblast displays some striking features that make these species interesting alternative models for the study of trophoblast development. After the transfer of somatic nuclei into oocytes, studies in the mouse and the cow have both underlined that the trophoblast may be a direct target of reprogramming defects and that its growth seems specifically affected. We propose that the study of TS cells derived from nuclear transfer embryos may help to unravel some of the epigenetic abnormalities which occur therein.