Gaps in the body wall (arrows) form in an embryo lacking MyoD-producing epiblast cells.

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 ended up in the somites that give rise to muscle throughout the body. The researchers then destroyed the MyoD cells in the epiblast and followed the developmental consequences. The loss reduced the amount of skeletal muscle, weakening the trunk muscles so much that organs bulged through the body wall.

Implanting beads saturated with Noggin into the embryos corrected most defects. Noggin's restorative ability indicates that the MyoD-carrying cells exert their impact mainly by spurring other cells to form muscle. The researchers plan to determine whether these influential muscle progenitors have other functions during embryonic development.