Fast and slow rat muscles degenerate and regenerate differently after whole crush injury.

The whole-crush injured rat skeletal muscle was used as a model to explore the regenerating potentialities of fast and slow muscles. Laminin was chosen to follow changes in basal lamina and desmin to visualize new muscular elements; they were revealed by immunofluorescence on cryostat sections of either fast (extensor digitorum longus) or slow (soleus) regenerating muscle. Soleus myolysis was rapid, extensive and heterogeneous. Basal laminae were nearly destroyed. In contrast, extensor digitorum longus maintained its basal lamina framework during myolysis. Soleus reconstruction began early, following the pattern of remaining basal laminae as closely as possible, but regeneration stagnated from day 16 and the regenerated muscle was fibrotic. In extensor digitorum longus, reconstruction progressed slower than in soleus, but regularly from the periphery toward the centre of the muscle. The regenerated extensor digitorum longus showed a quasi-normal structure from day 16. At the end of the process, the elimination of old basal lamina was completed in extensor digitorum longus, but was not achieved in soleus. We propose that the old basal lamina should help the initiation of reconstruction. This new model also underlines the importance of the turnover of basal laminae in muscular regeneration, and will be useful to understand the background of the different regenerative response of both muscles.