The effect of aging on the results of the rat micronucleus assay.

We conducted the micronucleus assay in 3-, 5-, 7-, 9-, 11- and 13-week-old male rats to determine whether the results varied with age. We administered cyclophosphamide orally at 0 (vehicle), 5, 10 or 20 mg/kg, twice, 24 h apart, to five rats per dosage group and collected bone marrow and peripheral blood 24 h after the second treatment. We observed an age-related decrease in micronucleus-inducing activity in both polychromatic erythrocytes (bone marrow) and reticulocytes (peripheral blood), which we attributed to an age-related decrease in hematopoiesis. In spite of the age-related decrease in sensitivity to the model chemical studied here, all age groups tested showed positive responses. We concluded that the rat is suitable for the micronucleus assay regardless of age.