Organisation of complex nuclear domains in somatic mouse cells

The number and associations of heterochromatin chromocenters, nucleoli, centromeres and telomeres were studied in the nucleus of different somatic cells of Mus domesticus. Fibroblasts of the cell line 3T3, kidney cells (primary culture), and bone marrow cells were used. The above mentioned nuclear and chromosome markers were identified by DAPI/actinomycin D, indirect immunofluorescence with anti-centromere antibodies, silver impregnation for nucleolar proteins and fluorescence in situ hybridisation (FISH) with telomeric probes. The quantitative analysis of the nuclei showed that the pericentromeric heterochromatin is organised in about 18 chromocenters per nucleus in the 3T3 cells, and about seven in kidney and bone marrow cells, having generally a peripheral distribution in the nucleus of all the studied cells. Several aggregated centromeres were participating in each of the chromocenters, about four centromeres per 3T3 cell and about six centromeres per kidney and bone marrow cells. Some of the chromocenters were also in close association with nucleoli. The number of telomeric labels per nucleus was as expected for each chromosome set (2n = 68-70 and 2n = 40). About half of the telomeric signals were loosely aggregated within the heterochromatic blocks while the rest were distributed in the nucleus as unrelated units not bound with chromocenters. The three cell types have complex nuclear territories formed by different chromosomal domains: the pericentromeric heterochromatin, centromeres, proximal telomeres and nucleoli. With the exception of some bone marrow cells, we have not found a nuclear polarisation of the analysed chromosomal markers compatible with the Rabl configuration. However, Rabl anaphasic polarisation allows the contact of centromeric regions making possible that centromeric associations arise. If in addition, associative elements such as constitutive heterochromatin or nucleoli are close to the centromeric regions, like in Mus domesticus chromosomes, then the associations might be consolidated and persist until the interphase. These associations may be the origin of the nuclear domains described here for Mus domesticus somatic cells.