The peripheral nervous system of an ascidian, Botryllus schlosseri, as revealed by cholinesterase activity

In this study we present the first detailed description of the motor component of the peripheral nervous system of an ascidian showing its three-dimensional organization and the spatial relationships between nerves and contiguous organs. Nerves of the oozooid and blastozooid of the colonial ascidian Botryllus schlosseri were analyzed using a histochemical method for detecting cholinesterase activity in whole-mount preparations and in sections for light and electron microscopy. Except for the neural gland and gonads, all tissues are well innervated by cholinesterase-reactive neurites. Each blastozooid of the colony possesses an individual nerve plexus which is not in continuity with that of adjacent zooids. The innervation of the mantle, oral and cloacal siphons, branchial basket, heart, and gut are described. Most organs possess a complex network of nerves often with multiple origins from different pathways. A sophisticated pattern is described for the first time in the gut, which receives innervation from the endostyle, the roof of the branchial basket, and the posterior mantle. Dilated axonal regions but no obvious cell bodies were recognized in the peripheral nervous system revealed by cholinesterase activity. The localization of nerves is discussed together with a consideration of their physiological role.