A MyoD positive embryonic cell (red) keeps its identity even in the foreign environment of the heart.

There's always one or two kids in every class who are already certain what they want to be when they grow up. Gerhart and colleagues now report on page 649 that certain cells of the early embryo are no different.

The team is interested in how cells commit to becoming skeletal muscle, which requires the expression of a transcription factor called MyoD. This expression was thought to start in the somites. But George-Weinstein's group discovered that they could detect MyoD mRNA in the blastocyst—a whole day before somites form, when cells are thought to be still pluripotent.

MyoD-expressing cells isolated from the blastocyst were capable of differentiating into skeletal muscle in culture. This ability did not mean, however, that the cells were already committed to muscle differentiation. It...

The Rockefeller University Press
2007
The Rockefeller University Press
2007
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