Cytoarchitecture, topography, and descending supraspinal projections in the anterior central nervous system of Branchiostoma lanceolatum.

The central nervous system (CNS) of the chordate amphioxus (Branchiostoma lanceolatum) is divisible into a spinal cord and an anterior portion in some ways equivalent to the brain of craniates. The present study reports on this anterior portion, with respect to general topography, cytoarchitecture, and cells that give rise to descending supraspinal projections. The anterior portion of the CNS is located adjacent to the first four myomeres and rostral to the first giant cell of Rohde-it can be divided into several regions that differ with respect to their cytoarchitecture. The tip of the neural tube is formed by a small anterior vesicle; caudally, there is a much larger region that is intercalated between the anterior vesicle and the first cell of Rohde. This intercalated region, in turn, consists of three subdivisions: an anterior subdivision adjacent to myomere 1, an intermediate subdivision adjacent to myomere 2, and a posterior one adjacent to myomeres 3 and 4. After injections of tracers into the spinal cord a large number of cells were labeled in the intercalated region. The spinally projecting cells were not evenly distributed: their number was decreased in the center of the intermediate subdivision. These subdivisions, which have previously not been noted, may be aligned with the expression domains of regulatory genes (e.g., AmphiOtx, AmphiHox) in larval lancelets. In particular, the center of the intermediate subdivision may correspond to a "nonHox/nonOtx" domain in the CNS of the larva. A similar embryonic domain occurs in the brain of craniates in which it develops into the isthmus cerebri that separates mid- and hindbrain. A close structural and topographical inspection of the corresponding region of adult lancelets reveals, however, that this region is not the homolog of an isthmus, but a uniquely derived, autapomorphic feature of lancelets.