Figure 1.

Schematic representation of the spatial arrangement of myogenic cells during muscle formation. Muscle morphogenesis produces the definitive shape and arrangement of each individual muscle and precedes the formation of secondary myotubes. From their first appearance, primary myotubes (shaded rectangles) span forming muscles in a parallel end-to-end arrangement. During muscle morphogenesis, a substantial part of the muscle mass contains undifferentiated myoblasts (circles), and primary myotubes are arranged in parallel bundles of three to seven. When morphogenesis is completed, primary myotube bundles dissociate, and single primary myotubes serve as scaffolds along which secondary myotubes (white rectangles) fuse and elongate. Note that due to the particular arrangement of the primary myotubes, muscle splitting and shaping may in principle be regulated by controlling the position and local elongation of primary myotube end regions. The latter process may involve selective fusion of myoblasts to primary myotube end regions.