Epithelial-mesenchymal transition in Rhesus monkey embryonic stem cell colonies: a model for processes involved in gastrulation?

A characteristic feature of embryonic stem (ES) cells is their ability to give rise to differentiated cell types that are derived from all three primary germ layers. In the embryo of higher vertebrates, formation of mesoderm and definitive endoderm (gastrulation) occurs at the primitive streak through a spatially highly ordered process of cell ingression, combined with epithelial-mesenchymal transition (EMT). With respect to ES cell differentiation in vitro, however, germ layer derivative formation has not been studied in much detail, and data on any degree of spatial order that may be attained here are lacking. In the investigations to be reviewed here, rhesus monkey ES cells (line R366.4) were grown on mouse embryonic fibroblast feeder layers for up to 10 days during which time they formed multilayered disc-like colonies with an upper epithelial and a lower mesenchymal cell layer. Processes of epithelialization as well as EMT were studied by transmission electron microscopy, immunohistochemistry combined with confocal laser scanning microscopy, and marker mRNA expression (in situ hybridization, RT-PCR). It was found that under the culture conditions used most of the ES cell colonies developed transitorily a central pit where the epithelial upper layer cells underwent an EMT-like process and appeared to ingress to form the lower, mesenchymal layer, accompanied by appropriate changes of morphology and molecular markers. Similarities and differences in comparison with gastrulation/primitive streak formation in vivo are briefly discussed, as are ethical implications with respect to human ES cells. It is concluded that this rhesus ES cell colony system may be an interesting in vitro model for studies on some basic processes involved in early embryogenesis such as EMT/gastrulation and may open new ways to study the regulation of these processes experimentally in vitro in nonhuman primates.