Getting the embryo into shape

Some stem cells in the embryo have the same effect as a personal trainer, show Gerhart et al. on page 283. Although they are candidates for forming muscle cells themselves, they instead coax others to build the muscle. Muscle-promoting cells might thus emerge earlier in development than many researchers thought.

When a chick embryo is only a two-layered disk, the upper layer called the epiblast already houses a few cells that have turned on the gene for MyoD, one of the supervisors for muscle development. But the cells also manufacture another muscle stimulator, known as Noggin. In the culture dish, these cells prod unprogrammed cells to form muscle.

To investigate the cells' function in bulking up the embryo, Gerhart et al. tracked them with antibodies. Most of the MyoD producers...