Fig. 6

Fibre size and fibre numbers at E15.5 and P0. Double immunolabelling of slow myosin with laminin (a). At E15.5, many myotube clusters are outlined by perlecan immunolabelling. These clusters consist of one primary and one or more smaller secondary myotubes (white arrow) together with a number of secondary myotubes/fibres that have attained their own basement membrane (yellow arrows). Fibre size (area) therefore refers to measurements of either myotube clusters or individual myotubes/fibres that have attained their own basement membrane. There are many single myotubes enclosed within their own basement membrane in the FKRP^KD at E15.5 which are less prominent in the wild type (yellow arrows). At P0, a population of smaller myotubes/fibres were still enclosed within the primary myotube basement membrane, this was more evident in the FKRP^KD at P0 (red arrows). This was reflected by a significant reduction in fibre size in the TA muscle in the FKRP^KD compared to wild type (b). Fibre number (includes both myotube clusters and myotubes/fibres with their own basement membrane) at E15.5 was not significantly different between FKRP^KD and wild type as was indicated in Fig. 5 where different sections along the mid-third of the muscle length were analysed (c). A comparison of the size distribution graph clearly shows a shift towards smaller clusters in FKRP^KD relative to wild type at E15.5 attributed to a population of smaller secondaries attaining their own basement membrane (e). At P0, in sections taken at the midpoint region of the limb fibre counts showed a significant increase in fibre number in the TA but not in the EDL of the FKRP^KD compared to wild type at P0 (d). The size distribution graph showed this to be due to an increase (p = 0.0547) in number of smaller fibres in the FKRP^KD at P0 compared to wild type in the TA (f)