Systematic variation in myosin expression along extraocular muscle fibres of the adult rat.

Monoclonal antibodies (McAB) specific for fast (C14) and slow (S58) myosin, and a myosin antigenically similar to neonatal/embryonic myosin in mammals (ALD180), were used to characterize the myosin distribution in orbital layer fibres of rat extraocular muscles (EOM) in relation to innervation patterns. The orbital layer is composed of both singly-innervated (SIF) and multiply-innervated (MIF) fibres. The SIFs have the characteristics of twitch fibres, while the MIFs, in addition to possessing many small endings characteristic of tonic fibres, also have an en-plaque-like innervation in the endplate band resembling that of the adjacent SIFs. Myosin expression in MIFs and SIFs is unusual and varies systematically along the length of the fibres. Both SIFs and MIFs label with ALD180, but this labelling is absent in both fibre types in the endplate band region, where all fibres label with C14. Distally and also proximally to the endplate band, SIFs label with both ALD180 and C14, while the MIFs, innervated by many small, superficial endings in these regions, label with ALD180 only. This pattern of myosin expression could also be demonstrated in isolated fibres. The results are discussed in relation to the hypothesis that both populations of orbital layer fibres express constitutively both fast and the neonatal-like myosin, and that superimposed on this constitutive expression twitch or tonic innervation acts locally to selectively suppress either neonatal-like or fast myosin, respectively.