Nuclear ribonucleoprotein particles were studied in the central nervous system of the rat after fixation by perfusion with hypotonic‐detergent formaldehyde. Beads‐on‐a‐string structures (polyparticles or chains) formed of 14‐nm granules related by a 7‐nm thick filament were found. These polyparticles are positively contrasted by a preferential method for ribonucleoproteins and they are sensitive to RNase. Perichromatin granules are loosened by this fixation and their internal structure is clearly depicted. They are composed of a tangled mass of 3‐nm thick filaments, but lack 14‐nm granules. The internal filaments of the perichromatin granules display frequent continuities with polyparticles. These results show that polyparticles correspond to perichromatin fibrils visualized in nuclei by standard fixation procedures.

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