The chromosomes of fifty primary Rous rat sarcomas
The chromosomes of 50 primary Rous sarcomas (RSV–SR) in the rat have been studied in direct fixations. About 80% of the sarcomas had a normal diploid stemline, and half of these had one or more sidelines. Hyperdiploidy was the most important heteroploid pathway, and within this group the trisomic number predominated. The second common progressional pathway was pseudodiploidy, whereas hypodiploidy and polyploidy were of minor importance. The karyotypic changes in the development of the heteroploid stem- and sidelines were non-random. There were strong indications of sequential, predetermined karyotypic changes in early tumour progression. Chromosomal analysis of different tumour regions of the sarcomas provided evidence of a monoclonal evolution of the tumours. There was no relationship between the latent period and the chromosomal findings. Sarcomas with heteroploid stemlines had about twice as long a tumour age as the sarcomas with normal diploid stemlines. With increasing tumour age the normal diploid cells decreased non-linearly. The progression from normal diploidy to heteroploidy was associated with a histologic dedifferentiation. The chromosomal pattern of the Rous rat sarcomas was discussed in relation to RSV-induced tumours of the mouse and the Chinese hamster, and in relation to other experimental rat neoplasms.