Phenotypic expression of 2b myosin heavy chain isoform: a comparative study among species and different muscles.

Myosin heavy chains (MyHCs) are the mainly investigated proteins to study muscle plasticity. These proteins characterize muscle fibre types which have a different expression of MyHC isoforms. In mammals the expressed MyHC isoforms in trunk and leg/arm skeletal muscles are types 1, 2A and 2X while the type 2B is species-specific. The latter isoform is present in marsupials and in laboratory animals but it is lacking in man, dog and bovine although the gene is present. Among large size animals, only the pig shows an expression of the 2B isoform but only in hybrid 2X/B fibres. Interestingly, “special” muscles (laryngeal, extraocular and masticatory) that originate from pre-somitic mesoderm express 2B isoform differently from trunk and leg/arm muscles.