Alternative strategies in muscle genotype and phenotype studies. A model of intrafusal muscle fibre type differentiation.

: The development and regeneration of muscle fibres start from myoblasts of embryos or adult animals. The resulting phenotype is a combination of genetically fixed properties of myoblast cell lineages and of extrinsic, primarily neurogenic factors. Intrafusal fibre types of muscle spindles differ from each other and from extrafusal fibres by their ultrastructure, by the presence of both sensory and motor innervation, and by the content of specific myosin heavy chain (MHC) isoforms. Differentiation of these distinctions depends on the morphogenetic influence of primary afferent neurones. It is, however, not known, whether the intrafusal phenotype can be induced in any myotube regardless of its cell line origin or only in a special predetermined intrafusal lineage(s) committed to differentiate into intrafusal muscle fibres. The aim of our studies was to define the contribution of intrinsic myogenic properties of muscle cell lineage and extrinsic neurogenic factors by the sensory and the motor innervation on the differentiation of intrafusal phenotypes using ultrastructural analysis and immunocytochemical determination of MHCs under various experimental conditions. The presented minireview is based on the results of our previous findings, and preliminary experiments indicate that new important results may be obtained in studies of myogenesis and muscle regeneration.