A Comparative Approach for Quantitative Cell Counting Studies in Widely Different Mammalian Brains

Most neurobiological studies are conducted on laboratory rodents. Despite many similarities across mammalian brains, important differences also exist, which can be misleading in translation. Marked interspecies differences have been found in brain plasticity, particularly neurogenesis. Different neurogenic processes can be prevalent because of evolutionary tradeoffs, displaying variation across brain structures and mammals and shifting the potential for plasticity in divergent species, such as mice and humans. Comparing widely different species raises multiple issues: comparative studies encounter technical difficulties when large-sized brains are involved; the use of heterogeneous experimental approaches by different laboratories can limit the comparison of results; heterogeneity may be related to different time courses of neurodevelopmental processes across mammals, thus adding variables to the comparison. To tackle these limitations, an approach to study the interspecies variation of a population of layer II cortical immature neurons in mammals widely differing for brain size, gyrencephaly, socioecological niche, and age was established. Despite some variables that cannot be fully standardized, a method that combines reduced heterogeneity in collecting brains and the establishment of common anatomical structures as reference points for performing the cell counting on corresponding brain levels is proposed. Data obtained (e.g., cell densities) can be mapped onto phylogenetic trees to reveal evolutionary patterns and analyzed for covariance with neuroanatomical features (e.g., brain size, cortical surface area). This approach has demonstrated remarkable variation in the number of cortical immature neurons between phylogenetic groups and uncovered covariation with brain size. The method can also be used to quantify differences through different developmental stages in the same species and can be extended to other diverse mammals and biological processes to map comparable results that allow for more accurate quantification of different cell populations in adult brains to support plasticity.