Ultrastructure of neurons and synapses in the tentacle epidermis of the sea anemone Calliactis parasitica.

Abstract

The anatomical organization of neutrons and synaptic pathways in tentacles of sea anemones is poorly understood. Transmission electron microscopy of serial thin sections was carried out on various regions of tentacles of the sea anemone Calliactis parasitica in order to locate and characterize typical epidermal neutrons and synapses. Both surface-oriented sensory cells with ciliary cones and basally located ganglion cells lacking a cilium have Golgi-derived granular or faintly cored vesicles. Similar vesicles are present at synaptic loci on some ganglion and muscle cells. The synaptic contacts on the longitudinal muscle cells are generally en passant rather than terminal. They vary from single neuromuscular synapses to pairs of neurites innervating the same muscle 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 present 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.