Clonal analysis of the developmental potential of 6th and 7th day visceral endoderm cells in the mouse

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Abstract
Single visceral endoderm cells from the embryonic regions of 6th and 7th day conceptuses were cloned by blastocyst injection using the genetically determined presence or absence of cytoplasmic malic enzyme activity as an in situ cell marker. In the 6th day cell injection experiments wild-type donor cell clones were readily discernible in the extraembryonic membranes of more than half the midgestation null host conceptuses; a much lower cloning efficiency was encountered with 7th day cells. With one exception, the clones appeared to be confined to the extraembryonic endoderm, most occurring in the parietal endoderm only. By means of the in situ marker, the morphology and arrangement of donor cells in the parietal endoderm could be compared with that of host cells, thereby demonstrating that they had undergone an appropriate phenotypic change after colonizing this tissue. Control experiments indicated that the procedures used to dissociate and select donor cells for injection were likely to have ensured that a representative sample of visceral endoderm cells had been transplanted. Hence on the 6th day of development, a high proportion of cells in the visceral endoderm seem to retain primitive endodermal characteristics, but these appear to be lost by the 7th day, when markers of visceral endoderm differentiation have first been demonstrated.