Predetermined Sequential Chromosome Changes in Serial Transplantation of Rous Rat Sarcomas

The early chromosomal progression in vivo was studied in 5–6 parallel series of passages of 2 primary Rous rat sarcomas (RSV‐SR). Both sarcomas had a normal diploid stemline, no sideline and no deviating cells. During transplantation all series of both tumours showed a heteroploid transformation. There were indications that the initial change in the heteroploid evolution was predetermined with a specificity comprising not only the direction of the heteroploid pathway (hyperdiploidy), but also the karyotype. Thus, among all series of passages derived from the same primary sarcoma, the earliest deviating cells had exactly the same karyotype. In both tumours the early heteroploid evolution displayed the same non‐random and sequential characteristics: gains of chromosome types t, st3 and st5, respectively. In the course of the heteroploid evolution the normal diploid cells decreased by a non‐linear process with a slow initiation and a rapid fall. The chromosomal progression from a normal diploid stemline to a heteroploid stemline was related to a histological dedifferentiation.