Loss of type I fibres in canine pectineus muscle hypotrophy

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Abstract
The total number of fibres, as well as, the number of fibres per fibre type were determined by the indirect fibre-counting method in 32 pectineus muscles from 16 dogs of mixed sexes. Eight pairs of muscles from 8 dogs were judged to be hypotrophic, while the other 8 pairs from another 8 dogs were judged to be normal. The hypotrophic muscles had extremely small muscle fibres, particularly type II fibres. They also had apparently higher percentages of type II muscle fibres within a section. The apparently higher percentage of type II fibres usually observed in the histochemical examination of the sections of hypotrophic pectineus muscles did not result from failure of type II fibres to transform to type I fibres. It was rather due to too few type I fibres being present in these muscles as compared to the normal muscles. It was not because there were more type II fibres present in them than in the normal muscles. The fewer type I fibres resulted most likely from loss of already differentiated type I fibres. The loss may be of neural origin.

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