/. Embryol. exp. Morph. Vol. 54, pp. 277-295, 1979 277 Printed in Great Britain © Company of Biologists Limited 1980 Growth and differentiation of an embryonal carcinoma cell line (C145b) By Y. E. PAPAIOANNOU, 2 E. P. EVANS, 2 R. L. GARDNER 2 AND C. F. GRAHAM 1 From the Department of Zoology and the Sir William Dunn School of Pathology, University of Oxford SUMMARY Several cell and tumour lines were isolated from a single-embryo-derived teratocarcinoma and their karyotypes and differentiation in adult hosts recorded. The majority of cells contained normal karyotypes by banding. The cells were injected into blastocysts and although they sometimes colonized the yolk sac, they never colonized the embryo. Thus the possession of a normal karyotype is not a sufficient condition for embryo colonization. The loss of growth capacity was investigated by studying differentiation and tumourigenicity in a variety of circumstances. The change in appearance from an EC cell morphology to a big flat cell in culture leads to retardation of growth in adult hosts. When EC cells are injected into a blastocyst, the ability to grow progressively both in culture and in adult hosts is lost. INTRODUCTION Transplantable teratocarcinomas can frequently be obtained by grafting normal mouse embryos, at particular stages of embryogenesis, into extra- uterine sites (e.g. Stevens, 1967, 1970; Solter, Skreb & Damjanov, 1970; lies, 1977). These tumours contain embryonal carcinoma (EC) cells, grow continuously, and eventually kill their host. Single EC cells form embryoid bodies, the ascites form of teratocarcinoma, can differentiate into a wide range of tissue types in the animal, and they can also form transplantable terato- carcinomas. Thus they can be the sole stem cells of teratocarcinomas (Kleinsmith & Pierce, 1964). There are several reasons for thinking that the differentiated cells derived from EC cells cannot form transplantable teratocarcinomas. For example, the differentiated cells which form on embryoid bodies are unable to form trans- plantable growths (Pierce, Dixon & Verney, 1960; Pierce, 1974). EC cells also differentiate into a wide range of tissue types when injected into the blastocyst and, with the exceptions mentioned below, the chimaeric animals rarely develop 1 Author's address: Department of Zoology, University of Oxford, South Parks Road, Oxford, 0X1 3PS. U.K. 2 Authors' address: Sir William Dunn School of Pathology, University of Oxford, South Parks Road, Oxford, 0X1 3RE, U.K.