ALTERNATIVE STRATEGIES IN MUSCLE GENOTYPE AND PHENOTYPE STUDIES - A MODEL OF INTRAFUSAL MUSCLE-FIBER 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 ext rinsic, primarily neurogenic factors. Intrafusal fibre types of muscle spindles differ from each other and from extrafusal fibres by their u ltrastructure, by the presence of both sensory and motor innervation, and by the content of specific myosin heavy chain (MHC) isoforms. Diff erentiation of these distinctions depends on the morphogenetic influen ce of primary afferent neuroses. It is, however, not known, whether th e intrafusal phenotype call be induced in ally myotube regardless of i ts cell line origin or only in a special predetermined intrafusal line age(s) committed to differentiate into intrafusal muscle fibres. The a im 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 intra fusal phenotypes using ultrastructural anslysis and immunocytochemical determination of MHCs under various experimental conditions. The pres ented minireview is based on the results of our previous findings, and preliminary experiments indicate that new important results may be ob tained in studies of myogenesis and muscle regeneration.