REGULATION OF THE RIBOSOMAL-RNA GENE-EXPRESSION IN FIBROBLASTS ISOLATED FROM DIFFERENT HUMAN OR MURINE TISSUES

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