Background: Ascidian larval epidermal sensory neurons (ESNs) each extend a single cilium into the outer body structure, or tunic, to form a unique external sensory network, the ASNET (ascidian dendritic network in tunic). The functions of ESNs and the ASNET are unknown, though it has been suggested that they play important roles in swimming larvae. In this study, we used transgenic larva expressing the photoconvertible protein Kaede pan-neuronally to identify the orientation and morphology of single ESN external processes within the ASNET. Results: When individual ESN cell bodies were UV-irradiated, the photoconverted Kaede protein diffused into the external processes that projected to the edge of the tunic, indicating that these processes are cytoplasmically connected to the cell bodies of ESNs. We were, therefore, able to comprehensively catalog the morphology and orientation of each ESN external process. Most trunk ESNs appeared to extend neurites to the palp, but no processes were observed to emanate from palp neurons. ESN processes differed systematically in their morphology and orientation, suggesting that the ASNET is formed in a regulated, non-random fashion. Conclusions: This study reveals the organization of ESN sensory fields in the ascidian larval tunic.
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