rRNA synthesis in proerythroblasts and hepatocytes after cyclosporin A administration in chick embryo.

The ribosomal RNA (rRNA) synthesis in the proerythroblasts and hepatocytes of the chick embryo was examined using the nucleolar test during embryonic days 5-10 after intraamniotic administration of single doses of cyclosporin A (CsA). Administration of the 0.75 or 7.5 g of CsA 5th day of embryonic development caused a significant decrease in proerythroblasts and hepatocytes numbers with active rRNA synthesis. Five days after the CsA administration, the percentage of proerythroblasts with biosynthetically active nucleoli already did not differ from the control while the decreased numbers of hepatocytes actively synthetizing rRNA remained statistically significant. Our results indicate, that CsA inhibits the rRNA biosynthesis in embryonic immature cells. Depending on the cell type, the effect of CsA and its dynamics may be apparently different.