Cleared extrachromosomal domain (CED): a nuclear domain enriched in nuclear matrix filaments is a common structure in...

The cell nucleus is divided into chromosome territories and the extrachromosomal domain. The latter includes several structural and functional compartments involved in RNA processing and transport. Morphological and cytochemical analyses of the cell nucleus in sturgeon podocytes revealed the existence of a previously uncharacterised nuclear compartment. It appears as a cleared nucleoplasmic area of variable size within the extrachromosomal domain. Conventional light and electron microscopy revealed that this compartment, here referred to as cleared extrachromosomal domain (CED), appears free of chromatin and RNA-containing organelles and is closely surrounded by heterochromatin masses. Cytochemical and immunogold electron microscope studies indicated that CEDs lack DNA, RNA and glycoconjugates. The ultrastructural examination of Lowicryl-embedded sections showed that CEDs are formed by a fibrillar network. In resinless preparations, this network appears as a dense mesh of crosslinked nuclear matrix filaments. The density of nuclear matrix filaments within the CED is remarkably higher than that observed in the rest of the nucleus. Our results indicate that the CED is a single, distinct extrachromosomal domain of the nuclear matrix of sturgeon podocytes. The absence in the CED of detectable DNA and RNA, as well as the lack of chromatin and RNA-containing structures, suggests that transcription and RNA processing do not occur in this novel nuclear compartment. The volume occupied by the CED may preserve the volume-to-cytoplasm ratio in the podocyte and contribute to modulate the spatial organisation and the volume occupied by the chromosomal territories.