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Fig. 3 Muscle morphogenesis in fn morphant embryos is more disrupted than in aei/deltaD mutant embryos. A: Summary of experimental design. Muscle development in the posterior of aei/deltaD mutant embryos, where initial somite boundaries did not form, was compared with muscle development in the posterior of 3-ng fn1+3 morphant embryos, where initial somite boundaries did form. Three phenotypes were assessed: uncoupling of fast and slow fiber lengths, absolute fast and slow fiber lengths, and anisotropy (a quantitative measure of organization, see Fig. 4). B-G: Fast-twitch and slow-twitch fiber lengths are more frequently uncoupled in fn morphant embryos than in aei/deltaD mutant embryos. Side views, anterior left, F59 (green) denotes slow-twitch muscle and phalloidin (red) denotes fast-twitch muscle. B,C: Fast-twitch and slow-twitch fibers are approximately the same length at 69% of boundaries in fn1+3 morphant embryos, and at 92% of boundaries in aei/deltaD morphant embryos. D,E: When slow-twitch fibers exhibit disruptions such as gaps, fast-twitch fibers are longer than slow-twitch fibers. F,G: Fast-twitch fibers are sometimes longer than slow-twitch fibers when slow-twitch fibers either appear normal or show only minor disruptions. H: Fast-twitch fibers are longer in fn1+3 morphant embryos than in aei/deltaD mutant embryos. H1-H2: Fibers were interactively traced in Axiovision and the major axes calculated. Note that some fibers are unusually long (white arrowhead). H3,H4: Average fast-twitch and slow-twitch fiber lengths are significantly longer in fn morphant embryos than in either control or aei/deltaD mutant embryos (P < 0.01). H5: The ratio of fast-twitch to slow-twitch fiber lengths is higher in fn1+3 morphant embryos. This increase is indicative of the more frequent uncoupling of fast-twitch and slow-twitch fiber lengths in fn morphant embryos.