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Fig. 2 Maternal smo promotes motor activity and motoneurogenesis. (A) Wild-type, smo and MZsmo zebrafish embryos were imaged at 24 hpf during spontaneous twitching. Ten overlaid images taken at 100-msecond intervals are shown. (B) Embryos at 24 hpf were observed for the ability to twitch in response to physical stimulus, spontaneously, or in response to applied voltage. (C-N) Wild-type, smo and MZsmo embryos at the 6-somite (12 hpf, dorsal view) and 18-somite (18 hpf, lateral view) stages were stained for isl1 or isl2a transcripts, which mark ventral PMNs and dorsal RB neurons. Anterior is to the left. Arrowheads in I-K indicate non-PMN isl1-labeled interneurons that preclude accurate counting of isl1-positive PMNs at this developmental stage. (O) Average number of isl1-positive or isl2a-positive PMNs per embryo for each genotype and developmental stage. For each condition, 100-200 PMNs were counted. nd, not determined owing to technical challenges in quantifying PMNs at this stage. Error bars represent s.e.m. **, P<0.01; ***, P<0.001. Scale bars: 200 μm.