Growth of mdx and WT muscle fibres up to 28 weeks of age, showing the total phalloidin fluorescence per myofibre, as an indicator of fibre volume, plotted against the number of myonuclei in that myofibre, displayed as individual data points and linear best fits across the range of ages as identified by colour. (A) Growth of WT fibres. Myonuclear number, plotted along the X-axis, increases up to week 3 but not thereafter, all further growth being accomplished by increase in fibrous actin content, along the Y-axis, as indicated too by the increasing slope of the best fit lines. (B) The equivalent plot to (A) of mdx muscle fibres using the same scales, to show the marked difference between growth patterns of the two strains. Up to week 4, mdx growth is broadly similar to that of WT but, beyond this point, continues both by a great increase in number of myonuclei per fibre and by increase in F-actin content. The total F-actin per fibre becomes larger than that of WT, in association with fibre branching, but the ratio of fibrous actin per myonucleus is low, as indicated by the shallow slopes of the best fit lines.