We show an as yet unnoticed feature of mammalian erythrocyte maturation, i.e., the formation of heterogeneous ectopic RNP-derived structures in the nucleus of erythroblasts, occurring in parallel with chromatin condensation. Inside these structures, RNPs are always recognized by specific antibodies, which demonstrates that the protein moieties of RNPs still preserve (at least partially) their native organization. This phenomenon shares extensive similarity with the segregation and clustering of nuclear RNPs occurring during spontaneous apoptosis of rat thymocytes and in several other cell models in which transcription is physiologically arrested.

In order to localize at EM level the sites of transcription of both pre-mRNA and pre-rRNA, we have detected the DNA/RNA hybrid molecules and m3Gcapped structures by means of specific antibodies after short bromo-uridine (BrU) incorporation. In addition, the sections have been stained by a selective RNA stain, terbium citrate. Our data indicate that perichromatin fibrils incorporate BrU and are labeled by the anti-hybrid probe; this supports the idea that they are the pre-mRNA transcription sites. On the contrary, interchromatin granules do not incorporate BrU after short pulses and are not labeled by the anti-hybrid probe. Concerning the nucleolus, anti-hybrid and anti-BrdU antibodies colocalize only on the dense fibrillar component, suggesting that this is the site of rRNA transcription. Interestingly, the dense fibrillar component and the granular component, after specific RNA staining, show remarkable structural similarities, both containing fibrogranular RNA structures.