EXPRESSION OF MYOSIN HEAVY-CHAIN ISOFORMS AND MYOGENESIS OF INTRAFUSAL FIBERS IN RAT MUSCLE-SPINDLES

This review concerns the pattern of expression and regulation of myosi n heavy chain (MHC) isoforms in intrafusal fibres of rat muscle spindl es detected by immunocytochemistry. The three types of intrafusal fibr es-nuclear bag(1), nuclear bag(2), and nuclear chain fibres-are unique in co-expressing several MHCs including special isoforms such as slow tonic and alpha cardiac-like MHC and isoforms typical of muscle devel opment, such as embryonic and neonatal MHC. The distinct intrafusal fi bre types appear sequentially during rat hind limb development, the nu clear bag, precursors being first identifiable at 17-18 days in utero as the only primary myotubes expressing slow tonic MHC. Sensory innerv ation is required for the expression of ''spindle-specific'' MHC isofo rms. Motor innervation contributes to the diversity in distribution of the different MHCs along the length of the nuclear bag fibres. It is suggested that unique populations of myoblasts are destined to become intrafusal fibres during development in the rat hind limb muscles and that the regional heterogeneity in MHC expression is related both to s ensory and motor innervation and to the properties of the myoblast lin eages. These distinct features make intrafusal fibres an attractive in situ model for investigating myogenesis, myofibrillogenesis, and the mechanisms regulating MHC expression. (C) 1995 Wiley-Liss, Inc.