Fig. 6

Lack of myod alters somite growth. A. Single confocal planes of myod^fh261 mutant and sibling zebrafish expressing plasma membrane-GFP from Tg(Ola.Actb:Hsa.HRAS-EGFP)^vu119. Individual retrospectively-genotyped larvae are shown at comparable levels in somites 16–18 at successive stages in each tryptich in lateral (left), transverse (centre) and magnified transverse (right) views. The somite is outlined (red dots). Bars 10 µm. B. Fibre number per somite in each stage and genotype as indicated in key. Note that sibling fibre numbers at 1 dpf are higher than those in Fig. 1E, perhaps because sequential live scanning means that ‘1 dpf’ fish are on average ~4 h older than in fixed preparations. C. Mean fibre volume = myotome volume/fibre number. D,E. Larvae from myod^fh261 heterozygote incross were stained at 3 and 5 dpf for Pax7, MyHC and DNA and analysed by confocal microscopy. Pax7⁺ cell number/epaxial somite (D) and number of nuclear profiles within the myotome/transverse optical section of epaxial somite 17 (E) were scored from fish genotyped by loss of head myogenesis. F-H. Analysis of nuclear number in fast fibres of myod^fh261 mutant and sibling Tg(Ola.Actb:Hsa.HRAS-EGFP)^vu119 injected with RNA encoding H2B-mCherry to permit counting in live larvae. Fibres analysed (number on columns) are shown in Fig. S5. A cumulative frequency plot (F) reveals the larger number of nuclei in fibres of mutants, which is reflected in an average of 16% increase in nuclear number (G) accompanied by a 63% increase in fibre cross-sectional area per nucleus (H), reflecting an 80% increase in fibre size. Differences tested by ANOVA with Tukey post-hoc (B,C), Kruskall-Wallis (F,G) and t-test (E,H).