Development of the central and peripheral nervous systems in the lamprey.

Vertebrate brains are highly organized structures that show remarkable diversity throughout the animal groups. Among the vertebrates, the agnathan animals, which diverged from the gnathostomes early in the evolution of the vertebrates, occupy a key phylogenetic position in order to clarify the origin and evolution of the brain. We found that the lamprey brain has the basic molecular mechanisms necessary to form brain compartments. Conversely, the telencephalon and cerebellum display gnathostome-specific developmental mechanisms. We also propose that, in contrast to those of gnathostomes, the maxillary ramus of the trigeminal nerve and buccal ramus of the nerve on the anterior lateral line are not fused in the developing lamprey. Thus, the development of the central nervous system and the framework of the peripheral nerve around the oral region are thought to have improved in the course of the agnathan-gnathostome transition.