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

초록

The morphology of the brain, especially the cortex, is diverse among species, although it arises from a common structure, epithelial tissue. The cortex deforms dynamically as neural stem cells within the epithelial tissue proliferate, migrate, and differentiate according to the genetic program during brain development. Recent 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 in evolution using atomic force microscopy (AFM) (1) (2). AFM measurements revealed a spatiotemporal stiffness gradient along the apicobasal axis during brain development. Non-mammalian brains such as chicks and turtles showed a monotonic increase in stiffness, whereas mammalian brains such as mice and ferrets showed a stiffness gradient in the late stage of brain development (2). In particular, the intermediate zone (IZ), an axon-enriched region, showed a remarkable peak of stiffness in the mammalian brain (1). Therefore, we performed the perturbation assay to identify the factor responsible for changes in stiffness during brain development. We found a correlation between the fasciculation of tubulin-rich axon bundles and the changes in stiffness in the mouse brain. Laser ablation of the axon bundle in the IZ showed a significant decrease in its stiffness. Taken together, our results suggest that species-specific stiffness patterns generated from different components of the cortical tissue during development may contribute to the formation of cortical structures with unique morphology.