Cultures of a pseudodiploid cell line (Don) of Chinese hamster origin were exposed to varying doses of tritiated thymidine (TdR-3H) 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-3H. The former phenomenon appeared to result from the effects of the ß-emissions originating in the TdR-3H. 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.

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