Lineage tracing demonstrates that blastomeres of early cleavage-stage human pre-embryos contribute to both trophectoderm and inner cell mass.

We injected a fluorescent lineage tracer (Texas Red-lysine-dextran) into individual blastomeres of donated human diploid 2- to 8-cell pre-embryos and cultured them to blastocysts. Once pre-embryos reached the expanded blastocyst stage, they were fixed and examined in a scanning confocal microscope to identify the location of fluorescent tracer. In successfully injected pre-embryos that developed to expanded blastocysts, we found that randomly injected blastomeres formed both trophectoderm (TE) and inner cell mass (ICM). More labelled progeny were found in TE than in ICM. Our results show that individual early blastomeres are not yet committed to form either TE or ICM but instead can form both rudiments.