Fate of the inner cell mass in mouse embryos as studied by microinjection of lineage tracers

We microinjected horseradish peroxidase and rhodamine-conjugated dextran into single inner cell mass (ICM) cells of preimplantation mouse embryos to study their fate in culture. Simultaneous iontophoresis of both lineage markers allowed immediate localization of the injected cell by epifluorescence, followed by microdrop culture of individual embryos. After 24 hr in culture, labeled descendants were found in the polar trophectoderm, ICM, and parietal endoderm, providing direct evidence that the ICM contributes descendants to the trophectoderm and the endoderm in the intact mouse embryo. Our results substantiate the totipotency of the ICM during the expanding blastocyst stage and further demonstrate that the ICM is a stem cell population from which cells are recruited into these tissue lineages during growth of the blastocyst.