In vivo magnetic resonance images reveal neuroanatomical sex differences through the application of voxel-based morphometry in C57BL/6 mice

Abstract

Behaviorally relevant sex differences are often associated with structural differences in the brain and many diseases are sexually dimorphic in prevalence and progression. Characterizing sex differences is imperative to gaining a complete understanding of behavior and disease which will, in turn, allow for a balanced approach to scientific research and the development of therapies. In this study, we generated novel tissue probability maps (TPMs) based on 30 male and 30 female in vivo C57BL/6 mouse brain magnetic resonance images and used voxel-based morphometry (VBM) to analyze sex differences. Females displayed larger anterior hippocampus, basolateral amygdala, and lateral cerebellar cortex volumes, while males exhibited larger cerebral cortex, medial amygdala, and medial cerebellar cortex volumes. Atlas-based morphometry (ABM) revealed a statistically significant sex difference in cortical volume and no difference in whole cerebellar volume. This validated our VBM findings that showed a larger cerebral cortex in male mice and a pattern of dimorphism in the cerebellum where the lateral portion was larger in females and the medial portion was larger in males. These results are consonant with previous ex vivo studies examining sex differences, but also suggest further regions of interest.

Keywords: Atlas-based morphometry; Magnetic resonance image; Sexual dimorphism; Voxel-based morphometry.

Fig. 1

Tissue probability maps. Tissue probability maps for gray matter (top), white matter (middle), and cerebrospinal fluid (bottom). Coronal (left), sagittal (middle), and axial (right) sections through each map represent the probability that a given voxel belongs to that class of tissue. Maps were created using T2-weighted magnetic resonance images from 30 male and 30 female adult C57BL/6 mice.

Fig. 2

Localized sex differences in gray matter volume. VBM revealed regions that were significantly (p ≤ 0.05, FDR corrected) larger in females (red) or males (aqua). Maximum intensity projections are overlain on the Mortimer Space Atlas standard glass brains where the dotted line represents bregma. Surface views of significant regions are shown over a 3D rendering of the mean template. Coronal sections (bottom row) demonstrate regions larger in females (anterior hippocampus, basolateral amygdala, and lateral cerebellar cortex) or larger in males (cerebral cortex, medial amygdala, and medial cerebellar cortex). Blue lines on the sagittal section indicate the position of the coronal slices from left to right respectively.

Fig. 3

Minimum deformation atlas with region delineations. Surface views of cerebral cortex (blue), anterior hippocampus (yellow), posterior hippocampus (red), and cerebellum (green) delineations are overlaid on the minimum deformation atlas created from the T2-weighted magnetic resonance images of 30 male and 30 female C57BL/6 mice used in this study.

Fig. 4

Atlas-based morphometry supports VBM results. (A) Whole brain volume showed a statistical trend towards being larger in males (aqua) than females (red) (p = 0.07). (B) Cortical volume was larger in males (p = 2.96 × 10⁻⁶). (C) No statistically significant sex difference was found in cerebellar volume. (D) Whole hippocampus volume was larger in males (p = 2.47 × 10⁻³). (E) Anterior hippocampus volume was larger in females (p = 1.71 × 10⁻⁴) while (F) Posterior hippocampus volume was larger in males (p = 8.25 × 10⁻⁷). All p-values are FDR-corrected for multiple comparisons.