Fig. 1. Different states of the skeletal muscle satellite cell. (A) Pax7-positive satellite cells (red) at the surface of a single fiber isolated from the extensor digitorum longus (EDL) muscle of an adult mouse. Nuclei are stained with DAPI (blue). (B) During embryonic development, a Pax7-positive population of myogenic progenitors contributes to myogenesis and gives rise to satellite cells. Cryosection through a masseter muscle from an E14 mouse embryo. Differentiated muscle fibers are stained with an antibody against myosin heavy chain (cytoplasm, blue) and an antibody against myogenin (nuclei, red). Pax7-positive cells (nuclei, green) are located between the newly formed fibers. (C) Transverse cryosection of a transplanted mouse tibialis muscle, showing that transplanted YFP-positive satellite cells can fuse with host muscle fibers and contribute to the host stem cell niche. Grafted cells are labeled with an anti-GFP antibody (green) and anti-Pax7 antibody (red). Muscle fiber basal laminas are labeled with a laminin antibody (white) and nuclei with DAPI (blue). Arrow shows a grafted YFP-positive Pax7-positive cell in a satellite cell position. Images in A and B are courtesy of Fabien Le Grand; image in C is courtesy of S. Kuang.