A method is suggested for isolation of highly purified mouse centromeric heterochromatin. Treatment of mouse liver nuclei with decreasing concentrations of Ca2+ resulted in the gradual unraveling of chromatin in the nucleus and at 0.1 mM Ca2+ electron microscopy revealed several dense particles per nucleus, surrounded by decondensed chromatin. These particles, assumed to represent centromere regions of interphase chromosomes by in situ hybridization with radioactive mouse satellite DNA and by differential staining for centromere heterochromatin, were isolated in preparative amounts and their DNA and protein composition was analyzed. The preparation represented practically pure mouse centromere heterochromatin, since more than 90% of its DNA was satellite DNA.

The quantitative characteristics of chromosomal nucleolus-organizing regions (NORs) and some other nucleolar components were studied on ultra-thin sections of pig embryo kidney cells (PK cells). It was shown that: 1) nucleoli-per-cell volumes were 3 times smaller in the G0 period than in the G2 period; 2) the number of fibrillar centers (FCs) per cell in the G0 period, the G2 period, and at metaphase was equal to 7, 33.7, and 8, respectively; 3) mean volumes of individual FCs in the G0 period (0.033±0.005 μm3), G2 period (0.014±0.001 μm3), and at metaphase (0.04±0.05 μ3) were significantly different; 4) the total volumes of FCs calculated per haploid set of chromosomes were practically the same in the G0 (0.105 μm3) and G2 (0.107 μm3) periods, but were twice as large as those at metaphase (0.04–0.05 μm3). These data show that partial activation and inactivation of ribosomal genes in interphase PK cells are not accompanied by a considerable change in the total volume of FCs and may be due to the fragmentation and fusion of individual FCs. Complete inactivation of ribosomal genes in mitosis results in a decrease of total volumes of FCs per cell; 5) in G0 and G2 periods the total volume of the dense fibrillar component per nucleolus is practically proportional to the nucleolus volume (r = 0.99);6) in the G2 period, the nucleolus volume is also proportional to the number of FCs (r = 0.99;7) the volume of the dense fibrillar component within individual fibrillar complexes is not a constant one. This is indirect evidence for a different level of NOR functional activity in different NO-chromosomes at interphase.

Nucleolus-organizing regions (NORs) at some stages of the cell cycle (G0 period, G2 period and mitosis), differing in the level of NOR transcription activity, were studied on ultra-thin serial sections of pig embryo kidney cells. Particular attention was given to the spatial organization of fibrillar centers (FCs) in the nucleoli and to comparing the number of FCs with that of NORs. It was shown that within nucleoli, FCs may be located singly or form groups surrounded by a dense fibrillar component. The overall number of FCs exceeds that of NORs 1.7-fold in the G0 period, and 4.6-fold in the G2 period. At prophase, with progressive condensation of chromosomes and NOR inactivation, the number of FCs decreases and at metaphase corresponds to the number of silver-stained NORs. The data thus obtained make it possible to conclude that in active nucleoli, the number of FCs is higher than that of NORs. Proceeding from these numerical and ultrastructural data, it is suggested that a single NOR of pig embryo kidney cells corresponds to one "fibrillar complex", i.e., to an association of a different number of FCs with the dense fibrillar component. These "fibrillar complexes" contain 1 to 3 FCs in the G0 period and 1 to 12 FCs in the G2 period.