Network structure of projections extending from peripheral neurons in the tunic of ascidian larva.

In ascidian Ciona intestinalis, a subset of trunk epidermal neurons were shown to possess external network of neural projections. To characterize a more complete network in naturally hatched (chorionated) larvae, we visualized the structure with a confocal laser scanning microscope. High resolution images revealed the huge network consisting of several subnetworks in whole-larval tunic. We named this network the ASNET (ascidian dendritic network in tunic). The ASNET was dynamically generated and collapsed during larval stages. Interestingly, one of the subnetworks found around apical trunk epidermal neurons was bilaterally asymmetric. In caudal epidermal neurons, transmission electron microscopy revealed that 9+2 axonemes were accompanied by a vesicle-containing mass in the ASNET arbor, but the distal end of the arbor contained only the vesicle-containing fibrous mass and no 9+2 axonemes. The characteristics of the ASNET suggest that it forms a unique outer body network in the ascidian larval tunic.