**2. Basis of the bone marrow micronucleus test (BmMNt)**

In 1973, the bone marrow micronucleus test (BmMNt) was reported as being a more effective in determining chromosomal damage than the metaphase scoring method used at that time. Nevertheless, limitations of this method included the use of high concentrations of metaphase cells to quantify significant differences, in addition to the animal sacrifice requirement [19]. Then, in 1975 W. Schmid reported the principles of BmMNt, describing that MNi result from a malfunction in the cell division process, mainly in two different ways [5]:

#### *Genomic Instability and Cyto-Genotoxic Damage in Animal Species DOI: http://dx.doi.org/10.5772/intechopen.99685*

Case 1: Acentric or fragment chromosomes do not migrate to the spindle poles in anaphase stage of cell division. Then MNi can be seen in the daughter cells.

Case 2: After one or more mitoses of exposed cells to mutagen agents, if the mitotic spindle is damaged, the nucleus of daughter cells could contain many MNi of a larger size than those produced in case 1.

Schmid manuscript also describes erythrocytes derived from the bone marrow as the best cell type to perform the assay since distinguishing between immature erythrocytes (polychromatic) and mature (normochromic) erythrocytes is possible, considering immature erythrocytes remain in circulation for 24 to 48 hours, while mature erythrocytes remain for about 30 days [5].

#### **2.1 Mammalian bone marrow erythrocyte micronucleus test (MEMT)**

Chromosomal damage, genome instability, and cancer risk assessment are the main objectives of bone marrow erythrocyte micronucleus assay (BmMNt) [20]. Its robustness lies in the fact that it determines in a simple and relatively fast way the clastogenicity or aneugenicity of chemicals [21]. The mammalian erythrocyte micronucleus test (MEMT) has been widely reported and reviewed by different research groups and government agencies. However, since its publication, more than 30 years of evidence was compiled to standardize the procedure to ensure its applicability. In addition, MEMT has been compared with other mutagenicity assays, which include the mutation in mouse lymphoma cells L5178Y, *Salmonella typhimurium*, sister chromatid exchanges, and chromosomal aberrations in Chinese hamster ovary cells [22].

The number of cells required for an appropriate genotoxicity analysis was defined by the statistical analyses of all the techniques used to evaluate MNi formation, including the MEMT technique [23, 24]. On the other hand, the preferred species for this technique are mice, rats, and Chinese hamsters. Therefore, in vivo testing is usually performed on rodent's bone marrow erythrocytes. However, other mammals, in which the spleen does not filter efficiently micronucleated erythrocytes, are accepted if stain accuracy is evaluated [24–26].
