Involvement of fast and slow twitch muscle fibres in avian muscular dystrophy.

The extent of differential fibre type involvement in chicken muscular dystrophy can be assessed quantitatively by the statistical parameters of fibre area, nuclei content and nuclei distribution in the individual fibre types. Two muscles, the posterior latissimus dorsi (PLD) and the serratus metapatagialis (SMP), were found to have similar overall fibre type composition, although the latter contains two subtypes of type I fibres, one of which has not previously been recognised in avian muscle. In both muscles, type IIB fibres are most affected by the progressive pathology. Nuclear proliferation is one of the histopathological features which can be measured, and in the PLD, the mean number of total nuclei in type IIB fibre cross-sections (Nt) is increased from 2.23 in normal chickens to 3.70 in dystrophic chickens, by 60 days. The corresponding values for Nt in type IIB muscle fibres of the SMP at 50 days are 1.74 and 5.10. Likewise, statistical analyses of the distribution of the fibre areas and their variability demonstrate that the incidence of abnormality in chicken dystrophy is greatest in type IIB fibres in both these muscles. Although type I fibres in the PLD are resistant to dystrophic change, it is noteworthy that in the SMP the type I fibres, also, are severely affected from an early stage, by these quantitative criteria. On the other hand, all fibres in a tonic muscle, the metapatagialis latissimus dorsi, are unaffected, as is true of all other tonic muscles previously studied. It is concluded that any twitch fibre type can, in principle, be affected by the actions of the gene concerned, and that this expression can be greatly modified in individual muscles by various physiological features, for example their natural pattern of use or relative disuse.