Fig. 11. Model for the effect of the vasculature on muscle mass separation. (A) The endothelial cells (blue) delineate the future cleavage site in the muscle mass, which is composed of myogenic cells (red) and muscle connective tissue cells (green). (B) At a later stage, the muscle masses are separated. (C) The PDGFB secreted by the endothelial cells acts in a paracrine manner on muscle connective tissue cells, which express PDGFR. In response to PDGFB, connective tissue cells (expressing PDGFR) increase the secretion of extracellular matrix by producing collagen I. This promotes the accumulation of connective tissue cells and the formation of a new muscle membrane, allowing muscle mass separation. Concomitantly, muscle differentiation will be inhibited as a consequence of the accumulation of muscle connective tissue and/or directly by PDGFB acting on muscle cells, which express PDGFRß.