Nuclear lamins comprise the nuclear lamina, a scaffold-like structure that lines 
the inner nuclear membrane. B-type lamins are present in almost all cell types, 
but A-type lamins are expressed predominantly in differentiated cells, 
suggesting a role in maintenance of the differentiated state. Previous studies 
have shown that lamin A/C is not expressed during mouse development before day 
9, nor in undifferentiated mouse embryonic carcinoma cells. To further 
investigate the role of lamins in cell phenotype maintenance and 
differentiation, we examined lamin expression in undifferentiated mouse and 
human embryonic stem (ES) cells. Wide-field and confocal immunofluorescence 
microscopy and semiquantitative reverse transcription-polymerase chain reaction 
analysis revealed that undifferentiated mouse and human ES cells express lamins 
B1 and B2 but not lamin A/C. Mouse ES cells display high levels of lamins B1 and 
B2 localized both at the nuclear periphery and throughout the nucleoplasm, but 
in human ES cells, B1 and B2 expression is dimmer and localized primarily at the 
nuclear periphery. Lamin A/C expression is activated during human ES cell 
differentiation before downregulation of the pluripotency marker Oct-3/4 but not 
before the downregulation of the pluripotency markers Tra-1-60, Tra-1-81, and 
SSEA-4. Our results identify the absence of A-type lamin expression as a novel 
marker for undifferentiated ES cells and further support a role for nuclear 
lamins in cell maintenance and differentiation.