Floor plate, glia and other support cells in the anterior nerve cord of amphioxus larvae

Serial electron micrograph reconstructions and interval series were used to examine the support cells, including glia, of the anterior nerve cord in 6-, 8- and 12.5-day larvae of Branchiostoma floridae and one newly metamorphosed juvenile. The floor plate begins immediately behind the infundibular cells. It consists for the most part of a single file of midline cells, but adjacent lateral floor plate cells occur in some places. The floor plate is interrupted at one point, in the posterior part of the cerebral vesicle above the tegmental neuropile. A class of early developing axons crosses the midline at this point, which suggests that the floor plate may have a developmental role in axon guidance. The structural integrity of the cord is maintained by ependymal and ependymoglial cells that attach to its sides. Two other glial cell types were found in larvae. Both appear to originate adjacent to the floor plate and hence are referred to here as midline glia. Those in somites 1 and 2 remain connected to the central canal; they appear to be a mixed population that may include precursors of midline support cells which are present later in the juvenile. Those caudal to somite 3 detach early from the central canal and develop an extensive network of axial processes; they are referred to here as axial glia and treated as a subcategory of midline glia. Based on their site of origin and the absence of glial filaments, their closest counterpart among vertebrate glia may well be the oligodendrocyte. To our knowledge, this is the first report of a possible amphioxus homologue of this important vertebrate cell type.