Differential chromatid staining by in vivo treatment as a mutagenicity test system

DIFFERENTIAL staining of sister chromatids can be demonstrated with Hoechst 33258 fluorochrome1 and with Giemsa after special pretreatment2, even if 5-bromodeoxyuridine (BUdR) is present during the first S phase only3. These techniques permit the use of the phenomenon of the sister chromatid exchange (SCE). The average spontaneous rate of 10–14 SCEs per metaphase seems to be fairly constant in all cultures from several species4,5. A variety of mutagenic factors (that is, ultraviolet light and alkylating agents) produce very high frequencies of SCE even at concentrations far below the level necessary for the induction of chromosomal breakage, whereas others, like X rays or certain chemicals, hardly influence the rate of SCE6. Because of the high turnover of BrdU in the liver and the physiologically available thymidine, differential chromatid staining has so far only been produced in in vitro systems (cell cultures) and in the chicken embryo.