Conditions for monolayer cultures of chick skeletal muscle in a rich medium showed that non-muscle contaminants were only 20% of the population. Although the calcium content of this rich medium was 0·95 mm, multinucleate and long fibres were observed after 8 days in culture.

Either in rich or in restricted media, gelatin organized the pattern of cell and fibre development in the Petri plates. Cells grown outside the gelatin boundaries looked fibroblast-shaped and many were vacuolated. Gelatin did not fully prevent the growth of fibroblast-like population.

Calcium in restricted media appeared to be very important for the acquisition of a definite elongated shape.

The possibility of the existence of myofibro-myoblasts was supported by the finding of multinucleated fibroblast-like cells during culture in restricted medium. Epigenetic factors, such as different media in a given culture or the origin of a serum batch utilized as a component of those media, affected the fate of the cultures and might also explain the myoblastic variance observed in this and other studies reported.

The capacity of phenotypic expression during the modulation change of muscle cells, in vitro, depends not only upon their genotypic origin but also on parameters such as the cell stage during this process, the type of nutrient media used and the interplay of both parameters; in vivo it depends upon specific cytological interactions.

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