Figure 5: Laminin deposition precedes but is not a requirement for fast-twitch skeletal muscle myofibrillogenesis. Zebrafish embryos were immunolabeled for laminin ((a): green; (b), (c), (d): red) and -actinin ((a): red; (b), (c), (d): green) at 20 (a), 24 (b), (d), and 48 (c) hpf. (a) A mature laminin matrix has formed at the somite/myotome boundaries (arrowheads) and along the surface of the myotome prior to fast-twitch skeletal muscle myofibrillogenesis. By this time, slow-twitch skeletal muscle has migrated to its superficial location within the myotome (arrow). As fast-twitch skeletal muscle myofibrillogenesis progresses, laminin persists at the somite boundary at 24 (b) and 48 (c) hpf. In response to obscurin A depletion (d), laminin accumulates normally at the irregularly localized and incomplete MTJs (arrowheads). Fast-twitch skeletal muscle myofibrillogenesis is delayed in the regions lacking laminin deposition but does eventually proceed. Scale bars are 50 μm (a), (b), (d) and 30 μm (c).