Brain segmentation and trigeminal projections in the lamprey; with reference to vertebrate brain evolution

Vertebrate brains exhibit remarkable diversity in each animal group, reflecting evolutionary changes at the molecular-level developmental program of the nervous system that vertebrates have experienced. We focused on the developmental morphological plan of the brain to understand the evolutionary scenario that led to the above diversity. By comparing the organization of the brain of non-vertebrate chordates, cyclostomes and gnathostomes, a step-wise modification of brain patterning programs becomes apparent. Furthermore, by labeling the lamprey oral region, the somatotopic projections of the trigeminal nerve that enter into the hindbrain become visible. Finally, by combining the knowledge on rhombomere segments and neuronal projections, the evolutionary relationship between somatotopy and brain segmentation are discussed.