Sex and species differences in mouse and rat forebrain commissures depend on the method of adjusting for brain size

Abstract

Sex differences in the forebrain commissures (corpus callosum, hippocampal commissure, and anterior commissure) were examined in B6D2F2 hybrid mice and Sprague-Dawley rats. Twenty-four male-female littermate pairs of mice were perfused at each of 21, 42 and 63 days of age and the midsagittal area of the commissures was measured from en bloc stained tissue. Twenty-two male-female littermate pairs of rats were examined at 110 days of age using the same methods. Male mice had larger bodies than females but no sex differences were found for mouse brain weight or commissure areas. In contrast, a significant sex difference was found for rat body, brain, corpus callosum and hippocampal commissure sizes. Four methods were used to adjust for differences in brain size (ratio, geometric, linear regression, and allometric). When the two species were analysed separately, neither mice nor rats showed significant sex differences in commissure areas relative to brain size if regression or allometric adjustments were made. Even when data from mice and rats were combined into one large group with a wide range of values, no species or sex differences were apparent after adjustments were made for brain size with either the regression or allometric methods. The use of ratios to adjust for differences in overall size is not recommended, especially because this method does not effectively remove the influence of brain size from commissure size; a substantial correlation is often present between the ratio and brain size.

Survey of 21 inbred mouse strains in two laboratories reveals that BTBR T/+ tf/tf has severely reduced hippocampal commissure and absent corpus callosum

“…As shown in Fig. 3, areas of all three commissures were strongly and linearly related to brain weight [2]. Because this study included such a wide range of strains, linear regression equations were based on the 368 mice that clearly had normal commissures, rather than using equations from a previous study done in Edmonton [9].…”

Loss of Neuronal Potassium/Chloride Cotransporter 3 (KCC3) Is Responsible for the Degenerative Phenotype in a Conditional Mouse Model of Hereditary Motor and Sensory Neuropathy Associated with Agenesis of the Corpus Callosum

“…Two-month-old male and female mice were perfused as described previously , and the brains were dissected out, weighed, and split in the midsagittal plane. The gold chloride staining of myelinated axons was performed as described previously (Bishop and Wahlsten, 1999;Wahlsten et al, 2003). The sagittal cutting face of right hemispheres of all brains was photographed using a Leica DFC 320 camera mounted on a Leica MZ 12.5 stereomicroscope at the same magnification.…”

“…The length of the corpus callosum (CCL) and the surface area of the anterior commissure (AC) were quantified using ImageJ software (Abramoff et al, 2004) by tracing anterior commissure and the corpus callosum. Because the size of the corpus callosum correlates with brain weight, the length was adjusted to the brain weight average in that group using the linear equation YЈ ϭ Y Ϫ b (X Ϫ M ), where YЈ was the adjusted size, Y was the size, b was the slope, X was the brain weight, and M was the average brain weight (Bishop and Wahlsten, 1999). The splenium was defined as the posterior 20% of the total corpus callosum length, which was established as the boundary between axons from the visual and parietal cortices in rat (Kim et al, 1996;Kim and Juraska, 1997).…”

Wheel running behavior is impaired by both surgical section and genetic absence of the mouse corpus callosum

“…Brains were removed from the skull and allowed at least 1 further week in fixative, and then weighed. Each brain was bisected at the midsagittal plane and stained en bloc with gold chloride for myelin [4,28], and then commissure areas were measured with the JAVA image analysis program from Jandel Scientific (San Rafael, CA, USA). To assess the extent of transection of the CC during surgery, brains of B6D2F 2 mice in both the surgery and sham surgery groups were sectioned coronally at 60 μm throughout the rostrocaudal extent of the CC, and every fourth section was stained with gold chloride.…”

“…Genetic group (B6D2F 2 versus RI) was effect coded (+ 1/2, -1/2), whereas CC area was centered by taking the difference from the overall group mean CC size [4]. The third term in the regression equation was the interaction coded as the product of group and centered CC area.…”