Mesoangioblasts cultured in polyethylene glycol-fibrinogen (PF) hydrogels. (A,B) Phase-contrast images of mesoangioblasts in 8 mg/ml PF hydrogel, giving rise to a robust three-dimensional myofiber network. (C) Immunofluorescence showing multinucleated muscle fibers; staining is with an antibody against myosin heavy chain (MyHC; red) and nuclei counterstaining with 4,6-diamidino-2-phenylindole (DAPI; blue). (D) Scanning electron microscopy image revealing the presence of differentiating skeletal-muscle fibers (red arrows) within the PF hydrogel. Scale bar: (A) 200 μm, (B) 50 μm and (C) 10 μm.