The stability of the differentiated state of skeletal muscle cells in culture. II. Evidence of specific traits and modulation of phenotypic expression in mononucleated myogenic cells.

In vertebrate skeletal muscle cultures the authors observed mononucleated striated cells, with very abundant cytoplasm, which are extremely stretched so as to mime the aspect of new formed fibers; these myoblasts have been considered as myogenic cells differentiated at the mononucleated stage. The peculiarity described was typically observed when the starting muscles were at an early stage of development and gave origin to cultures containing mainly fibroblasts; when myogenic cells were repeatedly subcultured up to 20 to 25 days, giving origin to populations very rich in fibroblasts; when, as a consequence of fusion inhibition by 5 bromodeoxyuridine treatment, the culture became once again very rich in fibroblasts. The differentiated myoblasts stopped synthesizing DNA and began to multiply, but they could probably dedifferentiate and resume cell division to finally form multinucleated fibers, if they are trypsinized and subcultured in presence of 'fresh' myoblasts coming directly from muscle tissue. Thus for skeletal myoblasts too, it is reasonable to postulate a modulation of the differentiated state. A probable contact inhibition of DNA synthesis occurring at confluence in cultures in which the dilution of myoblasts did not allow fusion to occur, caused the differentiation of mononucleated myogenic cells. Moreover, taking fusion capacity as a differentiated character specific for skeletal myoblasts, it was possible to observe a modulation of this character in culture; once again this modulation was mediated by the interactions with fibroblasts and particularly with the collagen they produced.