Nucleolar lesions induced by ribonuclease in living cultured cells

The nucleolar lesions provoked by the action of ribonuclease (RNase) on living chick embryo fibroblasts were studied by means of microcinematographic analysis and at the ultrastructural level using oxidized diaminobenzidine as a differentially contrasting stain for nucleic acids. This study has shown that the induction of nucleolar dispersion by RNase was only the beginning of a series of discrete steps. The following sequences are described: dispersion of the nucleolus into fragments, their reassembly, and the emission of spherules which appear of chromatin origin. At that step nucleoli are typically segregated. The alteration of the nucleolar associated chromatin seemed to be primordial in these processes. Moreover, the large mass of heterochromatin intimately associated with the nucleolus and which has been considered to be a part of the nucleolar organizer region apparently plays a chief part in the reassembly of the nucleolar fragments into a segregated nucleolus. Ribonuclease is compared to other drugs known to act on nucleolar DNA.