Fine structure of the apical ganglion and its serotonergic cells in the larva of Aplysia californica

The apical ganglion is a highly conserved structure present in various marine invertebrate larvae. Although one of the hallmarks of this ganglion is the presence of serotonergic cells, little is known about the structure and function of these cells. We have examined this ganglion in larvae of the marine mollusc Aplysia with light- and electron-microscopic immunocytochemistry. The results indicate that the cellular composition of the apical ganglion of Aplysia is very similar to that of other opisthobranchs. It consists of three classes of sensory cells (ampullary, para-ampullary, and ciliary tuft cells) and of other nerve cell types. Almost a third of the cells in the apical ganglion of Aplysia are serotonergic, and these can be divided into two classes: three para-ampullary and two interneuronal cells. All of the serotonergic cells extend an axon into the central nervous system. The variety of sensory and serotonergic cell types suggests that each type processes distinct attributes of the sensory environment. We argue that the apical ganglion, by virtue of its serotonergic cells, is well-suited to play important roles in the integration of sensory information to achieve proper motor adaptation to variable seawater conditions.