The muscle phenotype is the result of interaction between intrinsic genetic factors and extrinsic factors including innervation and level of thyroid hormones. Although these factors were intensely studied, the direct comparison of the results is difficult because various models, muscles and animal species have been used. We have therefore introduced a model of heterochronous isotransplantation in order to study in a single model the influence of muscle cell lineage, innervation and level of thyroid hormones on the expression of muscle phenotype of the regenerated muscle grafts. The aim of the present article is to compare the current knowledge of this problem. The expected experimental results will be primarily of the cognitional value, as they will provide a deeper insight into regulatory processes controlling expression of muscle genes. Controlled regulation of muscle differentiation may be, however, of clinical importance for muscle transplantations in man.

        Key words: muscle fibre types, myofibrillar proteins, muscle innervation, thyroid hormones, genetic variability of muscles