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Our results show that when myoblasts, isolated at different stages during muscle development, were cultured they formed myotubes expressing distinct phenotypes. An early phenotype was expressed by myoblasts isolated from 4-5 day old embryos and this phenotype could be modulated according to the culture conditions, i.e. seeding of the myoblasts as isolated cells or as reaggregated clumps of cells. An intermediate phenotype was expressed by myoblasts isolated from 7-8 day old embryos and it was independent of culture condition. A late phenotype was expressed by myoblasts isolated from embryos older than 9 days. In this case again, it could be modulated by culture conditions but, this time, modulation was brought about by subculturing the cells before they differentiate. These various phenotypes do not result from environmental differences in the culture but reflect the existence of distinct classes of myoblasts present at these different stages. This was demonstrated by isolating homogeneous clones of myoblasts at these stages and by showing that they express the corresponding phenotype.

Rhabdomyosarcomas were observed at birth in two premature infants. The evolution of these tumors was both rapid and fatal. One case was a botryoid sarcoma of the bladder and the second was a cervical alveolar sarcoma of Riopelle and Thériault. A cytological differentiation of the tumor was observed in the first case spontaneously and in the second following chemotherapy. In the second case histology, immunocytochemistry and a novel biochemical analysis of the contractile proteins demonstrated this differentiation of the tumor. The cellular reactivity varies according to the degree of muscular differentiation that can be defined either by studying the type of intermediate filaments present or the expression of specific muscle contractile proteins.