Ultrastructural Nucleolar Segregation and Carcinogenicity Among the 4-Nitroquinoline-1-oxides

4-Nitroquinoline-1-oxide (4-NQO) and several related derivatives, both carcinogenic and noncarcinogenic, were studied by electron microscopy (or their ability to produce nucleolar segregation in vitro. 4-NQO and its carcinogenic derivatives induced well-defined nucleolar segregation in most nucleoli examined, but with weakly carcinogenic derivatives the frequency was low. Most nucleoli exposed to weakly carcinogenic derivatives exhibited loss of a well-defined nucleolonema and pars amorpha concurrent with the formation of nucleolar vacuoles and dense plaques throughout the nucleolus. These events were suggestive of the intermediate stages of nucleolar segregation induced by the potent carcinogenic derivatives. Noncarcinogenic derivatives of 4-NQO produced no significant changes in nucleolar morphology from the normal appearance of the nucleoli characteristic of Chang liver cells in tissue culture.