Optimum conditions for detecting hepatic micronuclei caused by numerical chromosome aberration inducers in mice

To ascertain an optimum condition for detecting micronuclei in the liver caused by numerical aberration inducers, either carbendazim (125-1000 mg/kg, p.o.), colchicine (0.375-1.5 mg/kg, i.v.), cytochalasin B (2.5-20 mg/kg, i.v.), diazepam (3.13-25 mg/kg, i.v.), noscapine (7.8-62.5 mg/kg, i.v.), paclitaxel (1-100 mg/kg, i.v.) or trichlorfon (18.75-150 mg/kg, i.v.) was administered once to male Slc:ddY mice 1 day before or after partial hepatectomy (PH, Day 1). Five days after PH (on Day 6), hepatic micronuclei were determined in conjunction with classifications of the main nuclei and relative liver weights as a proliferative indicator or a dysfunction marker of cell division. Additionally, hepatocyte proliferation index (HPI) was calculated by using mono-, bi- and multinucleated cell counts. Treatment of mice with six compounds, except for colchicine, after PH showed higher incidence of micronucleated hepatocytes (MNH) than that before PH, and also increases in binucleated and multinucleated cells. Especially for carbendazim, diazepam, noscapine and trichlorfon, the dosing after PH was essential for the detecting numerical aberration. Colchicine evidently increased HPI and decreased relative liver weights without MNH induction on Day 6. On Day 8 when HPI and relative liver weights almost returned to the basal range, a significant increase in MNH was noted. This implied that the strong inhibition of colchicine on hepatocyte proliferation may obstruct the induction of MNH on Day 6. In conclusion, to detect the potential numerical aberration, exposure of mice to test chemicals should be performed 1 day after PH, during which enhanced proliferation of hepatocytes was seen, and it would be better to analyze the liver specimens on Day 6 or more post-PH.