Possible reasons for spontaneous incidence of micronucleus in rodents.

Micronucleus is similar to the nucleus, but smaller, formed in the cell due to damage of chromosomes by external factors. Incidence of micronuclei is scored in different cell lines as a parameter to evaluate the genotoxicity of chemicals and radiation. Bone marrow micronucleus assay in rodents, particularly in mice, has been the most popular technique used to screen the chemicals for their toxic potential. It is not that only exposed subjects show micronuclei, even untreated ones also show some of them. Hence, the incidence of micronuclei is compared between treated and untreated. Untreated buccal mucosa cells of man showed an average of 0.41% spontaneously induced micronuclei 0). Cole et al (2) reported that 0.16% of micronucleated cells are seen among the polychromatic erythrocytes (PCEs) in bone marrow of mice. In rats, 0 to 6.00% of micronucleated PCEs have been found in untreated controls (3). Furher, it was found that males are more sensitive than females In showing spontaneously induced micronuclei (4). Hence the present study was carried out to examine the possible role of exogenous factors i~fluencing the occurrence of micronuclei in control animal models that are frequently employed in micronucleus assay. Six mice and 12 rats were used in this study. Smears of bone marrow were prepared from femora (5), with a slight modification where 5% bovine albumin in phosphate buffered saline (PH7.2), was used as suspending medium (6). Smears were stained in May-Gruenwald-Giemsa at P" 6.8. Slides were screened under oil immersion objective. Two thousand PCEs/animal were scored for the incidence of micronuclei. Normochromatic erythrocytes (NCEs) and micronucleated NCEs (MNNCEs) met during analysis were also recorded.