/. Embryo/, exp. Morph. Vol. 54, pp. 31-46, 1979 37 Printed in Great Britain © Company of Biologists Limited 1979 Immunofluorescence studies on deoxyribonuclease from mouse teratocarcinoma cells during cell differentiation By L. SORIANO 1 AND D. PAULIN 1 From the Department de Biologie Moleculaire, Institut Pasteur, Paris SUMMARY Specific anti-DNase-I IgG have been used to detect deoxyribonuclease in teratocarcinoma cells by an indirect immunofluorescence method. All the cells studied show fluorescence staining. However, the patterns are quite different in embryonal carcinoma cells (amorphous cytoplasmic fluorescence and absence of nuclear staining) as compared to differentiated cell lines (diffuse, bright granular nuclear and fibrillar cytoplasmic fluorescence). It is possible by this method to distinguish different cell types derived from the same origin. Deoxyribo- nuclease from teratocarcinoma cells can therefore be considered as a marker of cell differen- tiation in this system. INTRODUCTION Many of the problems related to early embryonic development are difficult to analyse because of the scarcity of material and its heterogeneity. Today it is possible to overcome at least some of these difficulties by using the mouse terato- carcinoma as a model system for studying some aspects of cell differentiation (Jacob, 1975, 1977, 1978; Martin, 1975). Similarities between embryonal carcinoma cells (EC cells) derived from the teratocarcinoma and uncommitted cells of the early embryo have been shown by different criteria, morphological (Pierce & Beals, 1964), biochemical (Bernstine, Hooper, Grandchamp & Ephrussi, 1973), biological (Kleinsmith & Pierce, 1964) and serological (Artz et al. 1973). Markers of embryonic development have been described, such as LETS protein, which is absent in the earlier stages of embryonic development but begins to be expressed in the inner cell mass of the later blastocyst (Zetter & Martin, 1978), and a family of high-molecular-weight glycopeptides that are present in the EC cells and absent in differentiated cells derived from the EC cells (Muramatsu et al. 1978«). A similar observation has been made during post-implantation embryogenesis of the mouse: the amount of large molecular- 1 Authors' address: Department de Biologie Moleculaire, Institut Pasteur, 75015 Paris, France.