Frequency and size of micronuclei induced in gill cells of medaka fish (Oryzias latipes) after whole-body exposure to clastogenic chemicals

Medaka fish (Oryzias latipes) were whole-bodily treated with various doses of mitomycin C (MMC), ethylmethanesulfonate (EMS), cyclophosphamide (CP), diethylnitrosamine (DEN), or colchicine (COL) for 24?h, and the frequency of micronucleated cells (MNCs) was measured in the gills at 24 and 48?h after treatment. In the present experiments, MMC, CP, and DEN were recorded as efficient inducers of micronuclei at both sampling times, and none of the MNC frequencies recorded with these agents at 24?h significantly exceeded the corresponding frequency at 48?h. For EMS and COL, positive responses were recorded only 48?h after treatment. By comparison with the time-course data reported for radiation-induced MNCs in the same MN assay system, the clear responses observed at the 48-h time point for all the chemicals used were regarded as evidence of their delayed effects on micronucleus (MN) formation. The mean sizes of micronuclei induced after exposure to COL was significantly larger by a factor 2 as compared with that induced by X-irradiation, whereas those determined for the other four chemicals were almost equal to that induced by X-irradiation. These results demonstrate that the medaka gill-cell MN assay can detect chemically-induced chromosome damage, either directly or after metabolic activation, and spindle malfunction, and provide a basis for further development of the present assay system for testing cytogenetic activities of chemical agents.