The two faces of embryonic stem cells

Embryonic stem (ES) cells - those karyotypically normal immortal cell lines that can give rise to all other cell types of that organism - are believed to hold great promise for therapeutic applications.

But before we find a way to apply ES cells, we need to know precisely what they are. And new study from researchers at the University of Edinburgh has found that ES cells are actually a mix of two types of stem cells.

The researchers used a fluorescent reporter cell line to visualise individual stem cells based on the markers they express and were able to identify two different groups of cells.

While both were found to have all the properties of ES cells, when encouraged to differentiate, they each had a strong lineage bias, with some going down the line expected of ES cells, while the others becoming primitive endoderm cells - which are involved in the creation of the placenta.

It appears that, in their natural environment, ES cells respond to signals from surrounding cells, which inform them on which type of cell to become, quickly fixing to become only one type.

However, in the laboratory, these signals are absent, and the cells remain unfixed.

"This study changes our view of what embryonic stem cells are and how they behave," said Dr Josh Brickman, from Edinburgh University's Medical Research Council Centre for Regenerative Medicine.

"Knowing that embryonic stem cells can switch between different founder cell types could help us isolate cells at a point in time when they are primed to become specific cells. This could improve the ability to produce specific cells in the laboratory."

The study was published online in PLoS Biology, doi:10.1371/journal.pbio.1000379.