Sequential Morphological Alterations in Hepatic Cell Nucleoli Induced by Varying Doses of Actinomycin D

Summary

White female rats were given injections i.v. or i.p. of actinomycin D and the sequence of nucleolar segregation or reorganization in hepatic cells was documented. Subsequent to i.v. injection of 1.25 µg/g, the earliest nucleolar alteration is the accumulation of dense plaques within nucleolar vacuoles and in the region of the nucleolus-associated chromatin. At 10 min these plaques enlarge, and at 20 min they appear to coalesce, with some separation of the fibrillar and particulate components. Nucleolar segregation is more completely developed at 30 min, but it requires between 30 min and 1 hr for complete development. Following i.p. injection of 0.1 µg or 0.3 µg/g, actinomycin D produces only partial segregation, which most frequently appears as a peripheral fibrillar zone with a central particulate area. Restoration of normal nucleolar structure takes place between 105 and 140 min after 0.1 µg/g and begins about 6 hr after 0.3 µg/g although it is not entirely completed until 12 to 24 hr after this dose. During recovery from the 2 lower doses, it appears that the fibrillar component of the nucleolonema is reconstituted first. It is suggested that nucleolar segregation may be brought about by contraction of the intra-and extranucleolar chromatin with a concomitant separation of the attached fibrillar components from the separated particulate components. It is also suggested that the nucleolar particles represent the coiled ends of the fibrils.