Individual limb muscle bundles are formed through progressive steps orchestrated by adjacent irregular connective tissue fibroblasts during primary myogenesis

Although the factors regulating muscle cell differentiation are well described, we know very little about how differentiating muscle fibres are organised into individual muscle tissue bundles. Disruption of these processes leads to muscle hypoplasia or dysplasia and replicating these events is vital in tissue engineering approaches. We describe the progressive cellular events that orchestrate the formation of individual limb muscle bundles and directly demonstrate the role of the connective tissue cells that surround muscle precursors in controlling these events. We show how disruption of gene activity within or genetic ablation of connective tissue cells impacts on muscle precursors causing disruption of muscle bundle formation and subsequent muscle dysplasia and hypoplasia. We identify several novel markers of the populations of connective tissue cells that surround muscle precursors and provide a model for how matrix-modifying proteoglycans secreted by these cells may influence muscle bundle formation via effects on the local ECM environment.