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The phenomena of cell fusion and chromosome pulverization after inoculation with UV-inactivated Sendai virus were studied in mixed suspensions of cell lines from different mammalian species. Two cell combinations were used: the Chinese hamster cell line (Don) and RPMI 8226 (Simpson) cell line of human hematopoietic origin,and the Don and LLC-MK2 (MK) cell lines, the latter derived from monkey kidney cells. Each of the 3 cell lines had a high cell-fusion capacity when infected with the virus in homologous cultures. The heterokaryons of Don and Simpson cells were formed at lower frequency than homokaryons in either Don or Simpson cells, and heterokaryon formation between Don and MK cells was rare. The findings indicated that the cells used maintained some specificity of the cell surfaces. Chromosome pulverization was found in heterokaryons and was similar to that observed in homokaryons. The results confirmed further our presently held concept that cell fusion is a prerequisite for induction of pulverization, and also suggest that the possible role played by the mitotic nucleus with its cytoplasm in pulverization applies to the heterologous as well as to the homologous nuclei.

The process of cellular fusion induced by Sendai virus in Chinese hamster cells (Don line) afforded us the opportunity to study nuclear envelope formation around metaphase sets in the presence of interphase nuclei, when chromosome pulverization failed to occur in such multinucleate cells. Morphologically, the enveloped metaphase chromosomes resembled a normal telophase nucleus, though minor differences prompted us to call it telophase-like. Electron microscopic observations demonstrated that the membranes enveloping the chromosomes appeared to be identical with a normal nuclear envelope. The longer the cells were incubated with Colcemid before fusion, the higher was the number of cells with telophase-like nuclei and the lower the percentage of cells with pulverizations. Furthermore, the number of pulverizations bore a somewhat direct relationship to the ratio of metaphase to interphase nuclei in multinucleate cells, and the number of telophase-like nuclei was inversely proportional to this ratio. A hypothesis is advanced in which a balance between the activities of a chromosome pulverization factor and a nuclear envelope formation factor, the former in metaphase cells and the latter in interphase cells, is decisive as to the nature of morphologic events observed in virus-induced fused cells.

Cultures of a pseudodiploid cell line (Don) of Chinese hamster origin were exposed to varying doses of tritiated thymidine (TdR-(3)H) for relatively long periods of time. In addition to previously observed chromosomal aberrations) such as breaks and reunions, a substantial number of interphasic cells with micronuclei and of metaphases associated with pulverized chromosomes was found; both phenomena were dependent on exposure time to and concentration of TdR-(3)H. The former phenomenon appeared to result from the effects of the beta-emissions originating in the TdR-(3)H. A possible interpretation for chromosome pulverization induction is presented, emphasizing the derivation of the pulverized material from micronuclei in a common cytoplasm with a metaphase nucleus. These observations further substantiate our previously advanced hypothesis regarding the essential role played by substances present in a mitotic cell in the induction of chromosome pulverization and nuclear membrane dissolution.