The ultrastructural distribution of nuclear ribonucleoproteins (RNP) has been investigated by incubation of thin sections of mouse or rat liver, embedded in Lowicryl K4M or prepared by cryoultramicrotomy, with antibodies specific for RNP. The antibodies were localized by means of a protein A-colloidal gold complex. Anti-small nuclear (sn)RNP antibodies, specific for determinants of the nucleoplasmic snRNP species containing U1, U2, U4, U5, and U6 RNAs, were found associated preferentially with perichromatin fibrils, interchromatin granules, and coiled bodies. This indicates an early association of snRNP with structural constituents containing newly synthesized heterogeneous nuclear RNA. It also suggests a possible structural role of some snRNPs in nuclear architecture. Antibodies against the core proteins of heterogeneous nuclear RNP particles associate preferentially with the border regions of condensed chromatin, and in particular with perichromatin fibrils and some perichromatin granules. These results are discussed in view of recent knowledge about the possible role of nucleoplasmic RNP-containing components in the functions of the cell nucleus.

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