Nervous network in larvae of the ascidian Ciona intestinalis.

With the use of the monoclonal antibody UA301, which specifically recognizes the nervous system in ascidian larvae, the neuronal connections of the peripheral and central nervous systems in the ascidian Ciona intestinalis were observed. Three types of peripheral nervous system neurons were found: two located in the larval trunk and the other in the larval tail. These neurons were epidermal and their axons extended to the central nervous system and connected with the visceral ganglion directly or indirectly. The most rostral system (rostral trunk epidermal neurons, RTEN) was distributed bilateral-symmetrically. In addition, presumptive papillar neurons in palps were found which might be related to the RTEN. Another neuron group (apical trunk epidermal neurons, ATEN) was located in the apical part of the trunk. The caudal peripheral nervous system (caudal epidermal neurons, CEN) was located at the dorsal and ventral midline of the caudal epidermis. In the larval central nervous system, two major axon bundles were observed: one was of a photoreceptor complex and the other was connected with RTEN. These axon bundles joined in the posterior sensory vesicle, ran posteriorly through the visceral ganglion and branched into two caudal nerves which ran along the lateral walls of the caudal nerve tube. In addition, some immunopositive cells existed in the most proximal part of the caudal nerve tube and may be motoneurons.