Different types of chromosome aberration were observed in mouse bone-marrow cells after treatment with 4-bromo-N,N-diethyl-5,5-dimethyl-2,5-dihydro-1,2-oxaphosphol-2-amine 2-oxide (Br-oxaphosphole, Br-oxph) in a previous study. The aim of the present study is to perform a comparative analysis of these chromosomal damages observed with light microscopy (LM) and by means of atomic force microscopy (AFM). The kinds of aberrations scored by LM were substantially corrected by images at the ultrastructural level. The AFM analysis excluded 29.0% of gaps and 33.3% of fusion-type aberrations. On the other hand, AFM revealed the presence of aberrations that were not visible under the LM. This indicates that only AFM images would provide precise information about the real nature of chromosomal damages. The results of our study revealed that the 'real gaps' represented about 50% of all the gaps visible under LM. Excluded 'false gaps' were detected via AFM as breaks or decondensed chromosome regions. These results would support the statement that gaps must be included when testing genotoxicity. The ultrastructural analysis also confirmed the validity of using LM in the mouse bone-marrow chromosome aberration test, as a tool for detecting genotoxicity of chemicals in routine studies. When there is a need for precise evaluation of chromosome damage, only AFM images can provide information on specific genotoxic effects.
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