Fig. 1

Ontogeny of Pax7^+ve cells in the myotome. A–C,F–J: Lateral flatmounts of embryos stained with α-Pax7 monoclonal antibody (mAb): anterior is to the left and dorsal up. A: Around 30 weakly labeled rounded Pax7^+ve cells nestle between slow muscle nuclei (labeled by Prox1 - red) in the lateral myotome of each somite at 24 hours post fertilization (hpf). B: At the same stage, weak Pax7 immunoreactivity is detected in round cell nuclei located more superficially (arrows) and at much higher levels in the xanthophores, bean-shaped cells that are elongated along the dorsoventral axis (arrowheads). C,D: At 24 hpf, approximately a third of Pax7^+ve cells are differentiating as they co-express Myod (arrows). Numbers based on analysis of three somites in four to seven animals, error bars show standard error. Counts include only weakly stained rounded Pax7^+ve cells and exclude intensely stained elongated Pax7^+ve xanthophores. E,F: At 48 hpf, the xanthophores remain strongly labeled; weaker Pax7^+ve cells (green) are dispersed throughout the somite but several have aligned along the vertical and horizontal myosepta, labeled with anti-Laminin (red). G,H: From 3 days postfertilization (dpf), some Pax7^+ve cells associate closely with myofibers stained with Phalloidin (actin, red). I–M: Pax3/7 cells are surrounded by β-catenin staining in cryostat cross-sections at 6 dpf (arrows), while other myonuclei are not fully surrounded (arrowheads). J,K are magnifications of the boxed area in I. N–Q: Cross-section of muscle fibers labeled with αPax3/7 (red) and Laminin (green) show that Pax3/7^+ve cells are surrounded by laminin staining (arrows) at 6 dpf. Scale bar = 50 μm in A–C,E,F, 25 μm in I–Q.