Regional differences in expression of specific markers for human embryonic stem cells

Characterization of human embryonic stem cell (hESC) lines derived from the inner cell masses of blastocysts generally includes expression analysis of markers such as OCT4, NANOG, SSEA3, SSEA4, TRA-1-60 and TRA-1-81. Expression is usually detected by immunocytochemical staining of entire colonies of hESC, using one colony for each individual marker. Four newly established hESC lines showed the expected expression pattern and were capable of differentiating into the three germ layers in vitro. Neighbouring sections of entire colonies grown for 4, 11, 21 and 28 days respectively were stained with different markers to study the regional distribution and cellular co-expression. TRA-1-60 staining defined the hESC territory at all time points analysed. This territory comprised a characteristic OCT4 and NANOG staining often in overlapping subregions. Staining intensity of nuclei varied from strong OCT4 staining to weak or absent NANOG staining, and vice versa. SSEA4 staining was only observed in small clusters or single cells and not confined to the TRA territory. Co-expression of all markers was only detected in small areas. SSEA1 expression was found exclusively outside the TRA territory. In conclusion, pronounced regional differences in the expression of markers considered specific for undifferentiated hESC may suggest the existence of different cell populations.