Knockdown of Lbx2 activity results in malformation of slow muscle fibers. A-D: Control embryos. E-H: lbx2-MO injected embryos. I-L: lbx2 mRNA injected embryos. (A-B, E-F, I-J) Embryos labeled with the slow muscle marker, F59 (red). (B, F, J) Higher magnification. Rostral-caudal extension of slow fibers is severely affected by lbx2 knockdown (B, F, arrows). M: Statistical analyses of mean filament rostral-caudal length (arrows) and dorsoventral thickness (B, F, arrowheads). Analysis by ANOVA demonstrates significant differences in length (P < 0.01) and thickness (P < 0.05). (C, G, K, N) Embryos labeled with Prox1 (green, nuclear slow muscle and muscle pioneer marker), and 4D9 (red, Eng, muscle pioneer marker,). Green indicates slow muscle cells and yellow shows muscle pioneers (MP). (ctrl: n = 10, lbx2-MO: n = 6, lbx2-mRNA: n = 4). No apparent differences can be detected between controls and embryos injected with lbx2-MO. N: Analysis by ANOVA demonstrates no significant differences. The data represent the average ± s.e.m. (D, H, L) Transverse sections of a 24 hpf embryo labeled with the slow muscle marker, F59 (green) and Hoechst to mark nuclei. Slow muscle cells migrate properly to the superficial layer. (A-C, E-G, I-K) Lateral views, rostral toward the left, dorsal toward the top; (D, H, L) dorsal toward the top. Scale bar: (A, C-E, G-I, K-L) 50 μm; (B, F, J) 25 μm.