SHIGENO, S.; AGATA, K.; YAMAMOTO, M.; Evol. Regeneration Biol. Group, RIKEN Center for Devel. Biol.; Evol. Regeneration Biol. Group, RIKEN Center for Devel. Biol.; Ushimado Marine Lab., Okayama Univ.: Neural network formation in the embryonic brain of decapodiform cephalopods: evidence for conservative molluscan orthogon and evolutionary origin of higher integration centers

The cephalopods have evolved elaborate brains independently of vertebrates. We studied brain development in decapodiformes using neural specific markers and by TEM. First, we examined the embryonic brain of the pygmy squid Idiosepius paradoxus, a useful model cephalopod for developmental studies. The early formed axonal scaffold was simple in structure, composed of a pair of longitudinal tracts connected with three distinct commissures in the subesophageal mass (SBM), and an arched tract crossing transversely the region just over the oral ingrowth in the supraesophageal mass (SPM). The learning and memory centers such as the frontal-vertical lobe system arose along a pair of longitudinal tracts (future cerebro-buccal tracts) in SPM. Next we examined the brain development in other cephalopods (Sepia, Euprymna, Todarodes, and Sepioteuthis). The results showed that the orthogonal network pattern and developmental order of brain lobes were conserved among these decapodiformes. Comparison of the early cephalopod brains with primitive molluscan nervous systems indicates that the ladder-like tract such as the orthogonal scaffold in SBM is an essential structure in molluscan brains. The fact that the cerebro-buccal tracts in SPM provided basis for construction of the frontal-vertical lobe system suggests that these tracts played an important role in starting evolution of this system.