Comparative analysis of interspecies diversity in cortical stiffness using atomic force microscopy

초록

The morphology of the brain is diverse among species, although it arises from a common structure, epithelial tissue. The brain, especially the cortex, deforms dynamically as neural stem cells within the epithelial tissue proliferate, migrate, and differentiate according to the genetic program during cortical development. Previous studies have shown that tissue deformations driven by cellular dynamics generate distinct mechanical properties of the tissue and provide topographical cues. However, it is still unclear how the mechanical interaction between tissue and cells generates species-specific diversity in cortical morphology. To address this question, we developed a method to measure the mechanical property "stiffness" of cortical tissue from representative species using atomic force microscopy (AFM). AFM measurements revealed a spatiotemporal stiffness gradient along the apicobasal axis during cortical development. Non-mammalian cortex such as chicks and turtles showed a monotonic increase in stiffness, whereas mammalian cortex such as mice and ferrets showed a stiffness gradient in the late stage of cortical development. In particular, the intermediate zone (IZ), an axon-enriched region, showed a remarkable peak of stiffness in the mammalian cortex. Therefore, we performed the perturbation assay to identify the factor responsible for changes in stiffness during cortical development. We found a correlation between the fasciculation of tubulin-rich axon bundles and changes in stiffness in the mouse. Laser ablation of the axon bundle in the IZ showed a significant decrease in its stiffness. Recent studies have suggested that the volume, aspect ratio, and orientation distribution of axon fibers influence the mechanical properties of the brain. Taken together, our results suggest that species-specific stiffness patterns generated by axon bundles in cortical tissue during development may contribute to the formation of cortical structures with unique mechanical properties.