Asymmetries of multiple components of the rodent cerebrum have been described at various levels of organization. Yet, despite its ubiquitous nature, many confusing and sometimes contradictory reports regarding structural asymmetries in the rodent brain have been published. There is a need, therefore, for a whole-brain imaging analysis technique for asymmetry studies that is both accurate, reproducible and robust. To this end, a comprehensive three-dimensional examination of differences in brain structure in an inbred mouse strain was undertaken. The goal of this study was thus to use high-resolution magnetic resonance imaging to assess structural asymmetries in the adult C57Bl/6J mouse brain. Fixed brain T2-weighted images of 20 male C57Bl/6J mice were acquired on a 7T scanner at 32 microm isotropic resolution. We used voxel-based analyses to examine structural asymmetries throughout the whole mouse brain. The striatum, medial-posterior regions of the thalamus, and motor, sensorimotor, and visual cortex were found to be asymmetrical. The most significant asymmetry was found in the hippocampus and, specifically, the dentate gyrus. In each case, the left region was larger than the right. No other regions of the mouse brain showed structural asymmetry. The results in the dentate gyrus were confirmed using stereology, revealing a correlation of r=0.61 between magnetic resonance and stereological measures. Hippocampal, along with cortical asymmetry, has been discussed repeatedly in the literature, yet a clear pattern of directionality, until this point, has not been described. The findings of asymmetry in the striatum and absence of asymmetry in the rest of the brain are novel and show the advantage of using the whole-brain three-dimensional techniques developed herein for assessing asymmetry.
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