bHLH factors in muscle development: dead lines and commitments, what to leave in and what to leave out.

In recent years, skeletal muscle has become an important model for understanding the mechanisms that regulate tissue-specific gene expression. The formation of skeletal muscle during embryogenesis involves commitment of mesodermal progenitors to the myogenic lineage and subsequent differentiation of skeletal myoblasts into terminally differentiated myotubes. Like many cell types, skeletal myoblasts do not express markers of terminal differentiation until they are forced to exit the cell cycle in response to environmental cues. Growth factor signals play a central role in regulating the program for muscle-specific transcription by maintaining myoblasts in a proliferative state that is nonpermissive for the expression of muscle-specific genes.