Slow muscle fiber formation is reduced in sepN morphants. (A–F) Somitic muscle in midtrunk region of 24-hpf WT and sepN morphant embryos. (A and B) S58+ slow muscle fibers and (C and D) 4D9+ slow muscle pioneer fiber nuclei are present in reduced numbers in sepN morphants. Staining for dystrophin expression (E and F) reveals “u-shaped” somites in sepN morphant embryos. (G–J) Expression of myogenic lineage genes in adaxial cells of three-somite stage embryos. myoD (G and H) and α-cardiac actin (aca; I and J) are expressed discontinuously and at reduced levels in the adaxial cell population of sepN morphants. (A–F) Lateral views, rostral left. (G–J) Dorsal views, rostral left.
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