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 compon ent of the peripheral nervous system of an ascidian showing its three-dimen sional organization and the spatial relationships between nerves and contig uous organs. Nerves of the oozooid and blastozooid of the colonial ascidian Botryllus schlosseri were analyzed using a histochemical method for detect ing 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 blas tozooid of the colony possesses an individual nerve plexus which is not in continuity with that of adjacent zooids. The innervation of the mantle, ora l and cloacal siphons, branchial basket, heart, and gut are described. Most organs possess a complex network of nerves often with multiple origins fro m different pathways. A sophisticated pattern is described for the first ti me in the gut, which receives innervation from the endostyle, the roof of t he branchial basket, and the posterior mantle. Dilated axonal regions but n o obvious cell bodies were recognized in the peripheral nervous system reve aled by cholinesterase activity. The localization of nerves is discussed to gether with a consideration of their physiological role.