THE IMPORTANCE OF TIMING DIFFERENTIATION DURING LIMB MUSCLE DEVELOPMENT

Background: Skeletal muscle of trunk, limbs and tongue develops from a small population of cells that originates from somites. Although prom oters and inhibitors of muscle differentiation have been isolated, not hing is known about how the amplification of the muscle precursor pool is regulated; this amplification provides muscle mass during developm ent. Furthermore, little is known about how cells accumulate in the pr e-muscle masses in the limbs. We investigated the role of bone morphog enetic proteins (BMPs) and Sonic hedgehog (Shh) during proliferation, differentiation and positioning of muscle. Results: The proliferation of muscle precursors in limbs was linked to Pax-3 expression. Ectoderm removal downregulated Pax-3 expression, arrested proliferation and pr ematurely initiated muscle differentiation which exhausted the muscle precursor pool and prevented further muscle growth. BMP-2, BMP-4 and B MP-7 had a dose-dependent effect on pre-myogenic cells: low concentrat ions maintained a Pax-3-expressing proliferative population, substitut ing for ectoderm-derived proliferative signals and delaying differenti ation, whereas high concentrations prevented muscle development, proba bly by inducing apoptosis. In the limb, Shh upregulated Bmp-2 and Bmp- 7 expression which delayed muscle differentiation, upregulated Pax-3, amplified the muscle precursor population and stimulated excessive mus cle growth. Conclusions: These data indicate that embryonic muscle gro wth requires muscle differentiation to be delayed. Muscle differentiat ion may occur through a default pathway after cells escape proliferati ve signals. Positioning of muscle is regulated by high concentrations of BMPs, thus a single type of signalling molecule can determine cruci al steps in muscle development: when and where to proliferate, and whe n and where to differentiate. (C) Current Biology Ltd ISSN 0960-9822.