ULTRASTRUCTURE OF NEURONS AND SYNAPSES IN THE TENTACLE EPIDERMIS OF THE SEA-ANEMONE CALLIACTIS-PARASITICA

The anatomical organization of neurons and synaptic pathways in tentac les of sea anemones is poorly understood. Transmission electron micros copy of serial thin sections was carried out on various regions of ten tacles of the sea anemone Calliactis parasitica in order to locate and characterize typical epidermal neurons and synapses. Both surface-ori ented sensory cells with ciliary cones and basally located ganglion ce lls lacking a cilium have Golgi-derived granular or faintly cored vesi cles. Similar vesicles are present at synaptic loci on some ganglion a nd muscle cells. The synaptic contacts on the longitudinal muscle cell s are generally en passant rather than terminal. They vary from single neuromuscular synapses to pairs of neurites innervating the same musc le cell or one neurite innervating two or more muscle cells. Both two- way and one-way interneuronal synapses with vesicles aligned at paired synaptic membranes with dense material in a 14-20-nm-wide cleft are p resent in the epidermal nerve plexus. The vesicles average from 50 to 80 nm in diameter and vary from electron lucent to faintly cored. The results of this study demonstrate the presence of a complex system of epidermal neuronal pathways with specific synaptic loci in this modern representative of a first-evolved nervous system. (C) 1997 Wiley-Liss , Inc.