Artificial heterokaryons of animal cells from different species.

A virus, inactivated by ultraviolet light, was used to fuse together cells from different species of vertebrate, and the resulting heterokaryons were examined by autoradiographic and cytological techniques. Heterokaryons could be made with both differentiated and undifferentiated cells: HeLa and Ehrlich ascites cells were studied as examples of undifferentiated cells; rabbit macrophages, rat lymphocytes and hen erythrocytes as examples of differentiated cells. These last three cells were chosen because in them, in varying degrees, the process of differentiation has resulted in suppression of the synthesis of DNA or of both DNA and RNA. This suppression was in all cases found to be reversible: the dormant nuclei could be induced to resume the synthesis of RNA or DNA or both when the differentiated cells were fused with a cell which normally synthesizes RNA and DNA. Observations on heterokaryons in which differentiated cells were fused with HeLa cells and with each other permitted certain general conclusions to be drawn about the regulation of nucleic acid synthesis in the heterokaryon. It was found that if either one of the parent cells normally synthesized RNA, RNA synthesis took place in both types of nuclei in the heterokaryon. If either of the parent cells normally synthesized DNA, DNA synthesis took place in both types of nuclei in the heterokaryon. If neither of the parent cells synthesized DNA, no DNA synthesis took place in the heterokaryon. In all cases where a cell which synthesized a particular nucleic acid was fused with one which did not, the active cell initiated the synthesis of this nucleic acid in the inactive partner. In no case did the inactive cell suppress synthesis in the active partner.