A comparison of diameters of micronuclei induced by clastogens and by spindle poisons.

To compare the size of micronuclei induced by clastogens and by spindle poisons, bone-marrow smears were prepared 24 h after a single intraperitoneal injection of triethylenemelamine (TEM) (0.3 mg/kg) or vincristine (VCR) (0.125 mg/kg) into male mice. 100 micronucleated erythrocytes (MNEs) were randomly selected from each group and photomicrographed. Diameters of the cytoplasm (D) and the micronucleus (d) of each MNE were measured on the photographs. Relatively large micronuclei (d greater than or equal to D/4) were frequent (74%) in the VCR-treated group, and infrequent (2%) in the TEM-treated group. The frequencies of MNEs resulting in d greater than or equal to D/4 were determined for several other mutagens. All clastogens tested could be classified as TEM type, and all spindle poisons as VCR type. These findings indicate that it is possible to analyze the action site of micronucleus-inducing agents on the basis of the relative sizes of micronuclei.