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 was possible that the MyoD mRNA was nonfunctional. Indeed, no MyoD protein was detectable at this stage.

To investigate whether MyoD-positive blastocyst cells were stably committed, the team introduced them into early cardiac muscle and nervous tissue of the embryo. MyoD-negative cells adopted the same fate as their surrounding cells, whereas MyoD-positive cells did not. A small number of MyoD-positive cells also ended up in these foreign environments as part of normal development. Such immigrants continued to express MyoD, although they remained undifferentiated, as the necessary inductive signals for skeletal muscle development were presumably missing.

When MyoD cells were removed from the foreign tissues and cultured, they promptly started turning into skeletal muscle. Seems you can take the MyoD cell out of the muscle, but you can't take the muscle out of the embryonic MyoD cell.