Post-mitotic assembly of the nucleolus. I. The internal matrix network is a recruitment site for processing nucleolar components in prenucleolar bodies.

The aim of this work was to investigate whether the nuclear matrix could provide the nucleation sites for dispersed parental nucleolar components to form post-mitotic prenucleolar bodies (PNBs). For this purpose, nuclear matrices from asynchronous populations of onion cells were fractionated, and the distribution of the insoluble components of the nucleolar processing complexes in the matrices were analysed by fibrillarin immunolabelling. The ultra-structural organization of the nuclear matrix of cells from late telophase to late G1, corresponding to the period of nucleolar reassembly and activation, was also analysed. Our results demonstrate that PNBs are structural components of the telophasic nuclear matrix and that this structure provides recruitment and assembly sites for the components of the nucleolar processing machinery, and suggests that the telophasic matrix network is involved in the early steps of post-mitotic nucleologenesis.