Mosaicism in the mouse trophectoderm

The issue of mosaicism in the mouse trophectoderm is examined by reviewing two sets of evidence: one arguing for a mosaic, the other for a non-mosaic character. Evidence for mosaicism includes documented cellular contribution from the inner cell mass to the trophectoderm, and data that reveal the gradual pace of the allocation process that separates the inner cell mass and trophectoderm lineages. Evidence suggesting a non-mosaic character for the trophectoderm is based on the polarization process undergone by exterior cells in the eight-celled embryo, the heritability of the changes brought about by this process, and the formation of gap junctions between the resulting apolar, trophectoderm progenitor cells. Since inner-cell-mass cells are developmentally labile, spatially heterogeneous and translocate to the polar trophectoderm, it is concluded that the polar trophectoderm is a mosaic tissue.