Developmental fate in chimeras derived from highly asynchronous murine blastomeres.

The developmental fate of single blastomeres from eight-cell murine embryos reaggregated with intact two-cell embryos was evaluated after culture. Fluorescein isothiocyanate was used to follow developmental fate in preblastocyst chimeric embryos. Expression of stage-specific embryonic antigen 3 was used to assay developmental fate at the blastocyst stage, and glucosephosphate isomerase variants were used to assay at the blastocyst stage and after implantation. The results suggest that the descendents of the 1/8 component stay in a patch area and do not selectively migrate to the inner cell mass (ICM). This is in contrast to many studies that indicate that smaller blastomeres, which are more advanced in development, migrate to the ICM. The differences in experimental designs are discussed. Possible mechanisms for this phenomena are that the eight-cell blastomere is physically excluded from the ICM by position or polarization, or that it is differentiating ahead of the two-cell component and becomes trophectoderm.