Long-distance aberrant heterotopic connectivity in a mouse strain with a high incidence of callosal anomalies.

Corpus callosum dysgenesis (CCD) is a developmental brain condition in which some white matter fibers fail to find their natural course across the midplane, reorganizing instead to form new aberrant pathways. This type of white matter reorganization is known as long-distance plasticity (LDP). The present work aimed to characterize the Balb/c mouse strain as a model of CCD. We employed high-resolution anatomical MRI in 81 Balb/c and 27 C57bl6 mice to show that the Balb/c mouse strain presents a variance in the size of the CC that is 3.9 times higher than the variance of normotypical C57bl6. We also performed high-resolution diffusion-weighted imaging (DWI) in 8 Balb/c and found that the Balb/c strain shows aberrant white matter bundles, such as the Probst (5/8 animals) and the Sigmoid bundles (7/8 animals), which are similar to those found in humans with CCD. Using a histological tracer technique, we confirmed the existence of these aberrant bundles in the Balb/c strain. Interestingly, we also identified sigmoid-like fibers in the C57bl6 strain, thought to a lesser degree. Next, we used a connectome approach and found widespread brain connectivity differences between Balb/c and C57bl6 strains. The Balb/c strain also exhibited increased variability of global connectivity. These findings suggest that the Balb/c strain presents local and global changes in brain structural connectivity. This strain often presents with callosal abnormalities, along with the Probst and the Sigmoid bundles, making it is an attractive animal model for CCD and LDP in general. Our results also show that even the C57bl6 strain, which typically serves as a normotypical control animal in a myriad of studies, presents sigmoid-fashion pattern fibers laid out in the brain. These results suggest that these aberrant fiber pathways may not necessarily be a pathological hallmark, but instead an alternative roadmap for misguided axons. Such findings offer new insights for interpreting the significance of CCD-associated LDP in humans.