研究セミナー 3D analysis of axon guidance in transparent vertebrate embryos: of mice and men

In all vertebrates, commissural axons project across the midline of the nervous system to targets on the opposite side. The appearance and modification of commissural circuits has accompanied the emergence of key neurobiological features in vertebrate evolution, such as depth perception, hearing and limb-derived locomotion. Therefore, while vertebrate brains share a common overall architecture, many neuroanatomical differences can readily be observed, as well as differences in their ability to perform specific tasks. Robo receptors play a major role in the control of axon guidance at the midline of the central nervous system. In vertebrates, a divergent Robo family member Robo3 is expressed by commissural axons of the mouse spinal cord and hindbrain before and during crossing of the ventral midline (the floor plate), and many commissures fail to develop in mice and humans lacking Robo3. We found that the Robo3 receptor fundamentally changed its mechanism of action during mammalian evolution. Interestingly, we found that the Netrin-1 receptor DCC does not exist in all vertebrates. Overall, our results suggest that commissural axon guidance mechanisms may be more diverse across species than previously appreciated. This illustrates how adaptive changes in an axon guidance receptors can lead to fundamental changes in its function and distinct neuronal circuits, helping to understand the emergence of specific sensory, motor and cognitive functions and why they differ between species. To facilitate the analysis of commissural systems in vertebrates, we have developed a new imaging method which combines whole-mount immunostaining, tissue clearing and light-sheet microscopy. I will present new applications of this method to provide the first comprehensive description of development in human embryos during the first trimester of gestation.