Postnatal growth and differentiation of muscle fibres in the mouse. II. A histochemical and morphometrical investigation of dystrophic muscle.

Postnatal development of three hind legs muscles, the soleus, plantaris, and gastrocnemius, of dystrophic mice (ReJ 129) was investigated with histochemical and morphometric methods. The results were compared with normal postnatal development. Especially during the second week postnatally, there was severe fibre necrosis with no apparent preference for any particular fibre type. This period of necrosis was shortly followed by a wave or regeneration during the third week that could not, however, compensate for the loss of fibres. In dystrophic animals of 4-5 months of age, the number of fibres was reduced by 40-70%. Cross sectional areas of dystrophic muscles rarely, if ever, exceeded values for normal animals 14 days of age, while body weights were also drastically reduced. Growth and differentiation of the nonaffected fibres proceeded almost normally during the first month. During the second month, the "slow' fibres in the soleus muscle, and the "fast-oxidative-glycolytic' fibres in the plantaris muscle were hypertrophied, while, incidentally, some "fast-glycolytic' fibres showed hypertrophy; but in this case the average size of the fibre type was not increased. After two months, a general fibre atrophy was observed. The fate of the regenerated fibres was difficult to trace, especially in muscles older than one month. It is assumed that a number of them were capable of developing into "adult' fibre types histochemically. During the course of the disease the percentage of "intermediate' fibres increased markedly, whereas nearly all "fast-glycolytic' fibres disappeared. Because of these shifts in fibre profiles, the plantaris and the gastrocnemius muscles obtained a rather "juvenile' and "oxidative' aspect. Changes in the histochemical character of the soleus muscle were less spectacular. In dystrophic muscles, no new fibre types were found, compared with normal muscles. Rather, fibre types were present at the wrong moment, or occurred in quantities unusual for the age concerned.