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An electron microscopic study of nucleolus‐organizing regions at some stages of the cell cycle (G0 period, G2 period, mitosis)

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.