Chromatin condensation in isolated rat hepatocyte nuclei induced by ribonuclease treatment

The architectural organization of nuclear metabolism

Nucleic acid metabolism is structurally organized in the nucleus. DNA replication and transcription have been localized to particular nuclear domains. Additional domains have been identified by their morphology or by their composition; for example, by their high concentration of factors involved in RNA splicing. The domain organization of the nucleus is maintained by the nuclear matrix, a nonchromatin nuclear scaffolding that holds most nuclear RNA and organizes chromatin into loops. The nuclear matrix is built on a network of highly branched core filaments that have an average diameter of 10 nm. Many of the intermediates and the regulatory and catalytic factors of nucleic acid metabolism are retained in nuclear matrix preparations, suggesting that nucleic acid synthesis and processing are structure-bound processes in cells. Tissue-specific and malignancy-induced variations in nuclear structure and metabolism may result from altered matrix architecture and composition.

Nuclear growth and chromatin relaxation-condensation cycle in hepatocytes during the proliferative activation of rat liver

In order to quantify the changes in nucleolar and nuclear volumes and in chromatin condensation produced during proliferative activation we have carried out morphometric studies on hepatocyte nuclei during rat liver regeneration using electron microscopy. To minimize the artefactual effects produced by fixation on subcellular structures we have fixed the livers by perfusion with glutaraldehyde. The mean values for the nucleolar and nuclear volumes were progressively increased until 28 h after 66% partial hepatectomy. The maximum values raised for the nuclei and nucleoli at this time were 3 and 4.28 times, respectively, those of controls. Later, nuclear and nucleolar volumes progressively declined. Two waves of diminution in nuclear electron-dense material were produced after hepatectomy. The first occurred between 0 and 12 h, with minimum values 1.34 times lower than those from control animals at 8 h. The second occurred between 12 and 28 h, with minimum values 2.56 times lower than those from control rats at 24 h. These two waves in chromatin relaxation correlate very well with the transcriptional changes described by other authors during the pre-replicative, replicative and mitotic phases of liver regeneration.

Microfluorometric investigations of chromatin structure. I. Evaluation of nine DNA-specific fluorochromes as probes of chromatin organization

The ability of the highly condensed chromatin of small thymocyte nuclei and the more loosely organized chromatin of hepatocyte nuclei to interact with nine DNA-specific fluorochromes was assessed by microfluorometry. Although the results obtained with five of the fluorochromes - mithramycin, 7-aminoactinomycin D, Hoechst 33258, DAPI, and propidium iodide - were found to be virtually unaffected by differences in the degree of condensation of the chromatin, the values obtained with the remaining fluorochromes - proflavine, quinacrine mustard, berberine sulfate, and pyronin Y - appeared to be affected significantly by organizational differences of the chromatin. All of the latter "structural probes," except quinacrine mustard, produced fluorescence values which were higher in the 2c nuclei of hepatocytes than in the nuclei of small thymocytes. Quinacrine mustard yielded higher values in thymocyte nuclei; and in the hepatocyte polyploid series (2, 4, and 8c), it did not produce the expected multiples of the 2c value. Pretreatment of the two types of nuclei with RNase affected their total fluorescence in unpredictable ways. While RNase extraction lessened the differences between thymocyte and 2c hepatocyte nuclei stained with propidium iodide, Hoechst 33258, proflavine, and berberine sulfate, it increased the differences between nuclei stained with mithramycin, quinacrine mustard, pyronin Y, and 7-aminoactinomycin D. The ability of RNA-depleted chromatin to interact with various types of fluorochromes might be a useful parameter in subsequent studies of chromatin organization.