The use of fertilised mouse eggs in detecting potential clastogens.

Male mice were treated with methyl methanesulphonate (MMS) and then serially mated to females in oestrus, over the whole of the spermatogenic cycle. Chromosome preparations were made from fertilised eggs obtained from the matings and cultured overnight in the presence of a mititic inhibitor. No chromosomally abnormal eggs were found in matings using untreated animals but matings involving MMS-treated males produced a variety of abnormalities. The most sensitive stage in the spermatogenic cycle was 8 days after treatment, corresponding to the testicular sperm stage of spermatogenesis. At this sampling time 97% of the eggs analysed were chromosomally abnormal and the aberrations detected were predominantly 'shattered' male chromosomes. The aberration frequency in the post-meiotic stages decreased steadily up to day 20. No further structural chromosome aberrations were detected, until day 48, when chromosome fragments were detected in 2 eggs (4%) indicating that pre-meiotic damage can be induced and transmitted. The low background frequency obtained with the procedures used in this study enhances the sensitivity of the system for experimentally assessing the effects of clastogenic agents on male and female germ cells.