There are major mechanistic
differences among species in how initial cell fate decisions are made in early
embryos. Mouse trophoblast differentiation involves CDX2 binding to TCFAP2
sites in the Oct4 promoter, shutting
off its transcription. The bovine and human OCT4
promoters lack these sites, suggesting other mechanisms, a concept that is
bolstered by co-immunolocalization of CDX2 and OCT4 in early-stage human
embryos. To gain insights into the molecular mechanisms underlying lineage
allocation in humans, we derived nine cell lines from single blastomeres of
four 8-cell embryos (UCSFB1-9). According to established criteria—marker
analysis, teratoma formation and directed differentiation—they were human
embryonic stem cells (hESCs). We compared their transcriptomes to lines derived
by conventional means from intact blastocysts. The blastomere-derived lines
differentially expressed cell cycle and metabolic regulators. Conventional
hESCs differentially expressed genes that govern developmental processes,
evidence of commitment. Comparative analyses of the two methylomes showed
significantly hypomethylation of the UCSFB lines at loci that controlled
extraembryonic and embryonic development. At a transcriptional level, UCSFB
lines from different embryos were often more closely related than those from
the same embryo. As predicted by these data, immunolocalization of EOMES and T
showed differential expression among blastomeres of 8-12-cell human embryos. As
suggested by the methylation data, the lines formed CDX2-positive progeny that
yielded a human trophoblast stem cell line, the first of its kind. These
results suggested that the UCSFB lines mirror heterogeneity among blastomeres
of early-stage embryos and have features of totipotency. Thus, these hESCs have
unique properties that make them novel models of the initial stages of human
development and potentially valuable tools for cell-based therapies.