| Abstract
| - The effect of cytochalasin B (Cyt-B; 3 and 6 μg/ml; for the last 28 h) on micronuclei (MN) was studied in 72-h purified lymphocyte cultures of three male donors. The frequency of MN was much higher in multinucleate cells (mean 100-204 MN per 1000 cells) than in binucleate cells (mean 8.2-21.0 MN per 1000 cells), tetranucleate cells containing more MN than trinucleate cells. The presence of whole chromosomes in the MN was studied in two separate experiments by immunofluorescence using antikinetochore (CREST) serum and by a centromeric alphoid DNA oligomer probe (in situ hybridization, ISH). In the tri- and tetra-nucleate cells produced by Cyt-B, MN were clearly more often kinetochorepositive (K+) (mean 82-86%) and centromere-positive (C+) (mean 73-83%) than in mononucleate cells of cultures containing no Cyt-B (mean 63% for CREST and 50% for ISH), indicating that most of the excess MN in the multinucleate cells were due to whole chromosomes. The binucleate lymphocytes had about as high prevalence of K+ MN (mean 79-84%) as the tri- and tetra-nucleate cells, despite their low MN count. Also in the ISH analysis, the majority of MN in binucleate cells were positively stained (mean 58-62%). If it is assumed that the extra labelled MN are due to Cyt-B, the present findings suggest that Cyt-B could be responsible for ˜45-57% (CREST data) or ˜17-23% (ISH data) of MN in binucleate cells. The presence of whole chromosomes in the majority of human lymphocyte MN is problematic when the assay is used for biomonitoring of exposure to clastogens, where the usually small effects expected may be masked by the high background. The identification of kinetochores or centromeres in MN might help in dealing with this problem.
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