Regional and temporal differences in the parietal endoderm of the midgestation mouse embryo.

The morphological appearance of parietal endoderm (PE) cells from ninth to fourteenth day mouse embryos was examined by light and scanning electron microscopy, and their behaviour in vitro was studied using time-lapse video recordings. At all ages, the PE cells proximal to the placenta had a predominantly blebby phenotype whilst, in all but the youngest embryos studied, the distal cells were smooth and rounded. In the older embryos, the cells in the middle and distal regions had numerous short filopodia and were sometimes very large and flattened. Time-lapse studies on tenth day preparations showed intense motile activity (though little net movement) of the PE cells in the proximal region, with relatively little activity in more distal regions. The cell activity was generally less in eleventh day preparations, though it was still significantly greater in the proximal zone than elsewhere. The significance of these observations for understanding the development of the parietal endoderm is discussed.