The Difficulties and Possibilities of Classifying Muscle Fibres in Distinct Nonoverlapping Types
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There are three properties which differ from fibre type to fibre type: MHC isoforms, TnT isoforms, and metabolic enzymes. As already mentioned in the previous chapter, the MHC isoforms are suitable markers of fibre types. MHCs are discretly distributed among normal adult muscle fibres. Mostly, only one or predominantly one isoform is expressed per fibre in normal adult muscles. In contrast to MHC isoforms, the TnT isoforms are distributed less discretly. Among the fast fibres, “myriad” TnT isoforms exist, often with multiple isoforms in each fibre (Kelly and Rubinstein 1994). Furthermore, there is no clear correlation of TnT isoforms and MHC isoforms. TnT isoforms are not suitable as markers for practical fibre typing. The levels of metabolic enzymes characterize different fibre types. However, these classifications do not correlate with the MHC classification; considerable overlapping exists. Here, it should be emphazised: Muscle fibres can be typed according to several classification systems. The different criteria which underlie different classification systems are not obviously interchangeable with another. The intermingling of different systems have led to some confusions in the literature about muscle fibre types. In the following, an overview about the usual muscle fibre types is given, considering both literature and our own results. Special fibre types which occur, e.g. in extraocular muscles and avian skeletal muscles, are described in Chap. 12.
KeywordsATPase Activity Fibre Type Muscle Fibre Type Succinate Dehydrogenase Activity Fast Fibre
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