Early development of amphioxus nervous system with special reference to segmental cell organization and putative sensory cell precursors: a study based on the expression of pan-neuronal marker gene Hu/elav. 
The development of the nervous system of amphioxus was investigated at the cellular level based on the expression of the pan-neuronal marker gene Hu/elav.  In situ hybridization analysis showed that an amphioxus Hu/elav homolog (AmphiHu/Hel) was expressed in individual cells within the neural plate, but the cells exhibited no obvious arrangements in early embryos without distinct somites.  However, in neurulae with somites, AmphiHu/Hel-positive cells were clustered along the D-V axis in close register with the boundaries of somites, resulting in reiterated cell arrangements that became evident along the neuraxis.  Furthermore, AmphiHu/Hel-positive cell clusters appeared one by one along with the development of underlying somites.  Double-staining in situ hybridization analysis with the islet gene revealed that the cell clusters contain presumptive motoneurons.  In addition, AmphiHu/Hel expression was also observed outside the CNS, probably in the epithelial ectoderm, suggesting that amphioxus has a large number of putative sensory cell precursors as early as the early neurula stage.  Taking recent gene expression studies and anatomical studies into consideration, we discuss ontogenetic and phylogenetic features of the amphioxus nervous system.