The work which I have been asked to review began a little over a year ago with an experiment which I made in collaboration with Dr J. F. Watkins (Harris & Watkins 1965). We showed that an animal virus, killed by irradiation with ultraviolet light, could be used to fuse together cells derived from mouse and man to produce artificial man-mouse hybrid cells. The idea of using viruses in this way has its origins in observations which go back for more than a century. (For a review of the relevant literature see Harris, Watkins, Ford & Schoefl 1966). Many diseases have long been known to be associated with lesions in which multi-nucleate cells are found. In the medical literature of the nineteenth century there is a protracted and vigorous controversy about the mode of formation of these cells. Multinucleate cells are commonly found in the lesions produced by certain pathogenic viruses and, during the last decade, it has become clear that in at least some cases the virus produces the multinucleate cell by fusing single cells together. It was thus a very small step to attempt to see whether a virus could be used to fuse together cells of different kinds, and whether the resulting hybrid cells, if they were formed, would survive. And since the survival of the hybrid cells might be jeopardized by infection with a living virus, the virus was killed before the cells were treated with it. In the event, it turned out that viruses, inactivated by ultraviolet light, could be used to provide a general method for fusing together both differentiated and undifferentiated cells from different species and even different orders of vertebrate. The resulting interspecific hybrid cells survived for long periods and proved in some cases to be capable of multiplication. They thus offered interesting possibilities for the study of nucleo-cytoplasmic relationships and lent themselves to experiments of a kind which had not hitherto been feasible. It is these experiments which I propose to discuss. The virus used in this work was the ‘Sendai’ virus, a member of the para-influenza group of myxoviruses; other members of this group of viruses have since been used by other workers. Sendai virus was chosen because it had been shown by Okada (1958, 1962) that animal tumour cells in suspension could be rapidly fused together by high concentrations of this virus. The virus was irradiated with doses of ultraviolet light which reduced its infectivity by at least 10
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; but the dead virus retained its ability to fuse cells together. The two cell types studied in the first instance were the HeLa cell (a cell of human origin which has been grown for many years in artificial culture) and the Ehrlich ascites cell (a tumour which grows as a cell suspension in the peritoneal cavity of the mouse). These two cell types were chosen for a number of technical reasons, but mainly because their nuclei were easily distinguishable on morphological grounds. When a suspension containing a mixture of the two cell types is treated with the dead virus under appropriate conditions the cells clump together, and, during the ensuing 20 to 30 min, at 37 °C, the cell surfaces at the points of contact between the cells undergo dissolution. This process results in the progressive coalescence of the cytoplasms of neighbouring cells, so that multinucleate cells containing varying numbers of nuclei are formed.
A self-exciting point process to study multicellular spatial signaling patterns
