Visualization in situ of extended DNA filaments in nucleolar chromatin of rat hepatocytes

Using the osmium-ammine DNA tracer (Cogliati, R & Gautier, A, Compt rend acad sci ser D 276 (1973) 3041 1]), we were able to study the structure of intranucleolar chromatin in thin sections of rat hepatocytes fixed in situ after partial hepatectomy and cortisol treatment. Three levels of chromatin organization were observed: (1) highly compact clumps; (2) fibres with a thickness varying from 11 to 25 nm; (3) loose agglomerates of extended DNA filaments with a thickness of 2–3 nm. Both the clumps and the fibres of chromatin showed nucleosomal configuration which was absent in the loose agglomerates. These latter structures appeared to be peculiar to nucleolar chromatin.