Immunocytochemical characterization of nuclear ribonucleoprotein fibrils in cells of the central nervous system of the rat.

Nucleoplasmic structural constituents observed in partially decondensed nuclei of the central nervous system of the rat were analyzed by postembedding immunoelectron microscopy using antibodies specifically recognizing heterogenous nuclear ribonucleoprotein (hnRNP) and small nuclear ribonucleoprotein (snRNP) complexes and DNA. Fibrogranular RNP structures (polyparticles) were found in close proximity to DNA containing fibrillar areas resulting from partial dispersion of compact chromatin. The polyparticle-type fibrils are labeled by antibodies recognizing hnRNP core proteins as well as snRNPs (Sm antigen or 70 kDa protein of U1snRNP) or the m3G-cap structure of snRNAs. These observations suggest that such polyparticle-type fibrils correspond to extended perichromatin fibrils. Partially decondensed perichromatin granules are rarely labeled by anti-snRNP or snRNA antibodies. When labeling occurs it is restricted to the periphery of the granules. However, anti-hnRNP antibodies frequently label these granules. Our results favor the idea, previously proposed for Balbiani ring granules, that perichromatin granules are formed by the folding of hnRNP containing perichromatin fibrils (polyparticles) in the process of splicing, and that mature perichromatin granules contain already spliced messenger RNA.