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

Little is known about gastrodermal neurons and synapses in the tentacl es of sea anemones. Using transmission electron microscopy of serial t hin sections of Calliactis parasitica, we have identified both a senso ry cell and a ganglion cell with granular vesicles originating from th e Golgi complex and have identified four types of synapses in the tent acular gastrodermal nerve plexus. The sensory cell has a recessed apic al cilium with a basal body and a perpendicularly oriented centriole, below which are several strands of striated rootlets surrounded by mit ochondria. The ganglion cell lacks a cilium and resembles a bipolar ne uron, with oppositely directed processes lying parallel to the basally located circular smooth muscle. Both one-way and two-way interneurona l synapses are present with 60- to 90-nm granular vesicles of various densities aligned at the paired electron-dense membranes and fine cros s filaments in the intervening 13-nm cleft. Two types of neuroeffector synapses have been located. Dense granular vesicles are present at ne uromuscular synapses, whereas less dense vesicles are present at neuro glandular synapses. Most of the synaptic vesicles range from 60 to 120 nm in diameter. Tao types of nerve cells and a variety of synaptic lo ci provide morphological substrates for the spontaneous SS2 conduction pulses in the tentacular gastrodermis of C. parasitica. (C) 1997 Wile y-Liss, Inc.