The central nervous system, its cellular organisation and development, in the tadpole larva of the ascidian Ciona intestinalis

From its numerical composition, the central nervous system (CNS) of the ascidian larva is one of the simplest known nervous systems having a chordate plan. Fewer than 350 cells together constitute a caudal nerve cord, an interposed visceral ganglion containing motor circuits for swimming and, rostrally, an expanded sensory vesicle containing major sensory and interneuron regions of the CNS. Some cells are ependymal, with ciliated surfaces lining the neural canal, while others are clearly either sensory receptors or motoneurons, but most are distinguishable only on cytological grounds. Although reassignments between categories are still being made, there is evidence for determinacy of total cell number. We have made three-dimensional cell maps either from serial semithin sections, or from confocal image stacks of whole-mounted embryos and larvae stained with nuclear markers. Comparisons between the maps of neural tubes in embryos of successive ages, that is, between cells in one map and their progeny in older maps, enable us to follow the line of mitotic descent through successive maps, at least for the caudal neural tube. Details are clear for the lateral cell rows in the neural tube, at least until the latter contains ~320 cells, and somewhat for the dorsal cell row, but the ventral row is more complex. In the hatched larva, serial-EM reconstructions of the visceral ganglion reveal two ventrolateral fibre bundles at the cau-dalmost end, each of 10–12 axons. These tracts include at least five pairs of presumed motor axons running into the caudal nerve cord. Two pairs of axons decussate. Complementing this vertebrate feature in the CNS of the larval form of Ciona, we confirm that synapses form upon the somata and dendrites of its neurons, and that its motor tracts are ventral.