Cell differentiation as assayed by the topography and number of ribosomal genes.

In situ hybridization, Ag-staining and electron microscopy were used to study the distribution of ribosomal genes in isolated nuclei of rat cerebellar cells and the correspondence of the ribosomal genome topography to the nucleolar structure. rDNA-DNA autoradiography revealed clusters of silver grains, as well as diffuse groups and rows. The cluster frequencies corresponded to the frequencies of nucleoli on Ag-stained slides. Competitive hybridization in situ using unlabelled rat rRNA and hybridization with a nonspacer rDNA fragment showed that the diffuse groups and rows of grains also correspond to the ribosomal genes. Spatial organization of the ribosomal genome in the Purkinje cells differs from that in the other cerebellar neurons and glial cells. A 1.5-fold redundancy of the ribosomal genes was found in some Purkinje cells, while most of these as well as microneurons contained the diploid value of the genes.