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[Fibrillar centers in the nucleoli of a PEK cell culture after administration of actinomycin D].

A study was made of the fine structure of the light zone (fibrillar centres) that appear in the interphase nucleoli of PEK cell culture after the action of actinomycin D. The fibrillar centres contain densely packed homogeneous fibrils whose diameter ranged between 7-10 nm. Analysis of serial sections suggests that fibrillar centres may represent spherical zones within the nucleolus, displaying structural continuity with the intra- or perinucleolar chromatin and the perinucleolar nucleoplasm. The origin and some properties of fibrillar centres are discussed.