Ag-NOR stainings of the fibroblastic cultures derived from skin, lung and kidney of mice and humans have shown differential regulation of rRNA gene expression. Ag-NOR activities were found to be highest in the fibroblasts derived from the lung and the lowest in the skin-derived metaphases from both mice and humans and they were intermediate in the fibroblasts of kidney from both these species. Our observations indicate further that fibroblasts derived from different tissues are different not only in producing organ-specific growth factors but also in differential expression of ribosomal cistrons. This difference can also explain why orthotopic implantation of human tumor cells into nude mice is preferential compared with ectopic implantation in tumor take and metastasis.

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