Scientists identify embryonic stem cells by appear...(ulting mouse contained the markers. ...)

In the original experiment, the researchers took fibroblasts from the tail of this mouse and infected them with a special virus containing four genes (Oct4, Sox2, c-myc, and Klf4) capable of converting the cells to an embryonic state. Genes typically active in embryonic stem cells roared to life, triggering the adjacent foreign DNA to provide antibiotic resistance. Thus only fully reprogrammed cells survived exposure to an antibiotic, which allowed the scientists to isolate them.

When we conducted the original experiment, we noticed that many of the infected cells had already started to change shape before the markers were activated, says Wernig.

So they set up a new experiment to test if visual identification alone would work. Indeed, they were able to separate the reprogrammed cells from ordinary fibroblasts under a microscope, based on several physical differences. Fibroblasts are big and flat. Embryonic stem cells are small, round and form tight colonies.

Weve shown that theres no need to use markers to isolate successfully reprogrammed cells, says Meissner. This significantly simplifies this approach in mice, as we can now work with ordinary fibroblasts.

But another hurdle remains before the technique can be applied to human cells.

We still used viruses containing foreign DNA to introduce the genes that induced the reprogramming, explains Meissner.

The scientists are now working to eliminate the virus from the reprogramming process. Jaenisch believes they will eventually succeed and points out that the technique could eventually yield a bountiful supply of custom human embryonic stem cells for use in therapy.

Meissner and Wernig successfully reprogrammed about 0.5 percent of the fibroblasts. Given that there are millions of cells in a typical skin biopsy (researchers used skin from either the end of the tail or from the ear of the mouse), that translate
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