Does the synthesis of ribosomal RNA take place within nucleolar fibrillar centers or dense fibrillar components?

By means of immunocytochemistry performed on cryosections of cultured cells, RNA polymerase I was localized mainly to nucleolar fibrillar centers. The labelling of nucleolar dense fibrillar components was low and depended on the cell type. In contrast, DNA topoisomerase I and RNP complexes containing U3 snRNA were enriched in dense fibrillar components, their occurrence in fibrillar centers being usually much less.