Figure 2: Obscurin A depletion results in abnormalities in the tissue architecture of skeletal muscle. Control (a)–(c) and obscurin A morphant (d)–(h) embryos were injected with a mRNA encoding for lyn-GFP and fixed at 18 (a), (d), (g), 24 (b), (e)–(h), and 48 (c) hpf. (a) Lyn is incorporated into the membrane and accumulates along the somite boundaries as they form (arrowheads). (b) As the myocytes mature, they transition from rounded morphology (arrowhead) and elongate (arrow) to span the length of the developing myotome to insert into the ECM at the MTJ. (c) By 48 hpf, all the fast-twitch skeletal myocytes have elongated. (d) In response to obscurin depletion, only rare somite boundaries (arrowhead) were noted. (e) In a more severely affected obscurin morphant embryo, no nascent somite boundaries are detected and the myoblasts remain rounded. (f) As development proceeds in a mildly affected obscurin morphant embryo, patchy development of nascent MTJs reveals a close relationship between apical lyn accumulation and cell elongation (arrowhead). (g) In the more mature somites, myocyte elongation proceeds in the absence of apical lyn clustering (arrowheads). (h) In a younger somite, myoblast elongation (arrowhead) preferentially occurs at the site of apical lyn clustering while other cells remain rounded (arrow). Scale bars are 50 μm (a)–(f) and 10 μm (g), (h).