Immunocytochemical Analysis of Vimentin Expression Patterns in Porcine Embryos Suggests Mesodermal Differentiation from Day 9 after Conception

The expression of the intermediate filament proteins vimentin and keratin in porcine embryos was studied by whole‐mount immunocytochemistry between day 7 and day 11 after conception. Expression of vimentin was first detected in the inner cell mass of about 50% of the 9‐day‐old embryos. In elongated 11‐day‐old embryos, cells expressing vimentin were observed in the epiblast (after disappearance of Rauber’s membrane) and in cells migrating from the epiblast between the trophoblast and the underlying hypoblast layer. A keratin‐positive response was observed in trophectoderm cells at all stages. These findings suggest that inner cell mass cells in the pig start differentiating into mesodermal cells not later than day 9 after conception. While the delamination of the mesodermal germ layer is known to correlate with the loss of pluripotency of the inner cell mass cells, the early onset of mesodermal differentiation in the porcine embryo, characterized by vimentin expression and in contrast to the mouse, could in part be responsible for the lack of success in establishing pluripotent embryonic stem cell lines in this species. Our results suggest that further attempts to isolate inner cell mass‐derived pluripotent cells should be attempted well before day 9 after conception.