Background: During human preimplantation development, early blastomeres are believed to be totipotent. It is likely, however, that blastomeres are allocated to a specific lineage prior to any morphological differentiation. NANOG, SOX2 and SALL4 are transcription factors that play a key role in controlling stemness in embryonic stem cells (ESC) and are therefore candidate markers for developmental triggers in early embryos. KRT18, a trophoblast-determining gene, may mark early differentiation. Examining the expression pattern of these genes may inform us about when and in which cells totipotency is lost during early human development.
Methods: Thirtheen oocytes, 124 preimplantation embryos and 7 human embryonic stem cell (hESC) lines were examined for the presence of NANOG, SOX2, SALL4 or KRT18 proteins using immunostaining and confocal microscopy.
Results: All stemness markers were expressed in the hESC, but none of them was specific for totipotent cells during human preimplantation development, and none of them seemed to mark cells allocated to the inner cell mass (ICM) or trophectoderm. After lineage specification, only the nuclear expression of NANOG and SOX2 became restricted to the ICM, at least to some cells because only a subpopulation expressed NANOG. KRT18 expression was seen for the first time during compaction in some outer cells. KRT18 was not expressed in hESC.
Conclusion: We conclude that the protein expression patterns of markers that define stemness in ESC do not identify the totipotent cells in human preimplantation embryos. Assessing the presence of KRT18 proteins implied that the outer cells of compacting embryos have probably lost their totipotent competence prior to any visible differentiation.