Categorization of micronuclei by size and measurement of each ratio in cytokinesis-block and conventional cultures of human lymphocytes exposed to mitomycin C and colchicine.

Micronuclei (MN) assays are very useful tests for monitoring human exposure to mutagens and carcinogens. We investigated the effects of the culture method (either conventional or cytokinesis-block) and exposure time (48 or 72hr) on the frequency and size distribution of MN in human peripheral lymphocytes exposed to mitomycin C (MMC) or colchicine. To quantitatively analyze the effects of the agents, methods and exposure times, we categorized MN by size into small (MN-1), medium (MN-2), and large (MN-3). MN-1 were less than one fifth, MN-2 one fifth to one third, and MN-3 larger than one third of the diameter of the main nucleus.Both MMC and colchicine induced dose-related increases in the frequency of MN. The number and distribution of the size-categorized MN were influenced by the agent, method and exposure time. The conventional culture method was useful for detecting the induction of MN-1 by MMC, whereas the cytokinesis-block method was useful for detecting the induction of MN-1 by colchicine. The ratios of MN in the various size categories reflect the different mechanisms of MN induction by MMC and colchicine.These findings suggest that categorization of MN by size can allow one to differentiate between a clastogen and an aneuploidogen, and that the ratios of MN in the three size categories may provide a good index for estimating the type of MN induction for human monitoring.