The histological appearance of tumours derived from rat embryo cells transformed in vitro spontaneously and after treatment with nitrosomethylurea.

Wistar rat embryo cells were treated in vitro with either 25 μg/ml of nitrosomethylurea (NMU) or phosphate buffered saline. Both groups showed morphological transformation by the 13th passage but their ability to grow in soft agar did not occur until at least passage 23; plating efficiencies indicated that NMU had reduced transformation. However, both control and treated cells gave rise to fibrosarcomata after similar latent periods following inoculation into syngeneic recipients. The fibrosarcomata had "myxoid" and "leiomyomatous" areas, and two resembled haemangiopericytomata; for the most part the tumours were transplantable. Inoculation of cloned NMU-treated cells produced fibrosarcomata with a high proportion of giant cells but only after a very long latent period. No virus particles were detected in tumour samples by electron microscopy.