The effectiveness of all presently employed cancer therapies is limited by tumour cell heterogeneity. In patients with rhabdomyosarcomas (RMS), differentiation seemed to be associated with poor prognosis and vice versa. In order to confirm the validity of this observation, we have developed in vitro and in vivo models of RMS wherein the degree of tumour cell differentiation could be manipulated. On subcutaneous implantation in nude mice, 4 of 8 alveolar rhabdomyosarcomas (A-RMS), produced tumours. In 3 of these 4 tumours the N- myc gene was amplified. In contrast, 2 embryonal (E-RMS) lacked N-myc amplification and also failed to grow in nude mice. Two cell lines from E-RMS formed tumours in nude mice. In vitro, a reduction in bovine serum concentration or supplementation of the medium with horse serum induced marked differentiation in an A-RMS but not in an E-RMS. In vivo, subcutaneous xenografts of undifferentiated A-RMS could be induced to differentiate by intratumoural injection of horse serum or transplantation into the peritoneal cavity of rats. This dramatic degree of differentiation was detected by histology and immunocytochemical staining using 3 different markers, viz, desmin, vimentin and myoglobin. Somewhat remarkably, when these differentiated A-RMS growing in the peritoneal cavities of rats, were reinplanted into peritoneal cavities of other rats they failed to grow. On the other hand, if they were reimplanted subcutaneously in other rats they grew well and reverted to an undifferentiated morphology. In contrast, the E-RMS could not be induced to undergo differentiation by horse serum and also failed to grow in the peritoneal cavity of rats.

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