Nuclei from different cell types in plants and animals show many features of differentiation; they differ in shape, volume, structure, ultrastructure and in the distribution of nuclear components. Using the filamentous caulonema of the moss Funaria hygrometrica Hedw. this study records the changes in cytoplasmic organization alongside the reorganization of the interphase nucleus, Events taking place in the meristematic cells at or near the lip of the advancing caulonemal filaments (e.g. acquisition of polarity, tip growth, nuclear and cell division, side branch initiation] are associated with haploid nuclei (1C DXA amount 0.5 pg) that are spherical or slightly oval, with no blocks of condensed chromatin, and a large central nucleolus with a large granular component. Maturation of the caulonemal cells involves wall thickening and pigmentation concomitant with suspension of elongate plastids in linear arrays along endoplasmic strands. Many cells become highly polarized with the majority of the organelles at their apical ends. These eytoplasmic changes are associated with endoreduplication of the genome to about 8C, endoreduplication occurs by amplification of the 1C genome to give nuclei with IC-SC DNA amounts. There is no evidence of differential amplification of the genome. The amplification in the copy number of ribosomal RNA genes is associated with the heterochromatinisation of the genes within the nucleolus. At the same time the nucleolus reduces in volume owing to a diminution of the granular component and all components of the nucleolus become spatial separate. There is an increased nuclear volume associated with endoreduplication and the nucleus elongates causing an increase in the surface area of the nuclear envelope. These major nuclear reorganizations are associated with a stable distribution of the 'D' polypeptide involved in pre-mRNA splicing. Scrutiny of published data suggests that similar differentiation events might be encountered commonly in other organisms. The changing nuclear morphology probably reflects the changing activity of the nucleus and the cell. It might be that nuclear reorganization changes the balance of genes or gene products and the spatial distribution of the component pans to enable the new nuclear functions. These results suggest that nuclear differentiation is a fundamental feature of cell differentiation.
Keywords: endoreduplication; plant development; rDNA; spliceosome.