Myosin heavy chain composition of the human sternocleidomastoid muscle

In addition to the MyHC isoforms, characteristic of adult limb muscles, a very low percentage of muscle fibres (0.2-2.7%) expressed MyHC-neo, which is normally not found in adult limb muscles. Only two samples exhibited MyHC-neo at a rather higher percentage (6.3% and 7.5%) of muscle fibres. The high share of hybrid fibres and the presence of MyHC-neo in the SCM muscle differ from that of adult limb muscles where hybrid fibres are rare and the expression of immature MyHC isoforms occurs only in pathological or experimental conditions. Since the SCM muscle shares the same embryogenic potential as limb muscles, its distinct MyHC expression appears to be associated with twin innervation and with the intrinsic specialisation to perform multiple functions.