Spatial rearrangement and enhanced clustering of kinetochores in interphase nuclei of dorsal root ganglion neurons in vitro: association with nucleolar fusion.

Interphase nuclei of several cell types display distinct, nonrandom arrangements of specific chromatin domains. It has been suggested that this arrangement is associated with the functional commitment of the cell and results from compartmentalization of specific DNA sequences to transcriptionally competent sites. In a test of the hypothesis that such topological organization is established during cellular differentiation, the spatial distribution of centromeres was determined, in dorsal root ganglion neurons in vitro, using immunocytochemistry in conjunction with fluorescence microscopy, confocal laser microscopy, and ultrastructural immunogold techniques. Kinetochores occurred as clusters, in association with nucleoli and with the nuclear envelope. Neurons at different stages of differentiation, as determined by nucleolar distribution, exhibited a distinct, stage-specific, spatial organization of kinetochores. Morphometric analyses, together with serial reconstruction, indicated that progressive clustering of kinetochores accompanies differentiation and that such clustering occurs in association with nucleolar fusion. The data presented indicate that the chromatin organization observed in the fully differentiated state may be the result of controlled rearrangements of specific chromatin domains during differentiation and that the mechanism governing such rearrangement and the process of cellular differentiation may be linked.