An ultrastructural study of peripheral neurons and associated non-neural structures in the bivalve mollusc, Spisula solidissima

The ultrastructural morphology of peripheral neurons and associated structures in the bivalve mollusc, Spisula solidissima have been studied in an effort to describe the synaptic topography and to provide anatomical correlates of previous physiological observations. The somata of the peripheral neurons are located within the perineurium at branch points of the siphonal nerves. There are many fiber-fiber synaptic contacts which are characterized by isolated sites of contact with membrane specialization and unilateral accumulation of synaptic vesicles. There are also synaptic contacts involving the somata, both axo-somatic and somato-axonic, the two being distinguishable on the basis of the polarity of vesicle accumulation. All of the observed synaptic profiles were similar in appearance regardless of the neuron loci involved. Much of the non-synaptic soma surface is covered with processes of glial cells. Likewise, in many cases, individual fibers and groups of fibers are encased with glial processes. Unique clusters of membrane bound, pigment containing glial like cells occur throughout the nervous system of Spisula. The heterogeneous appearance of the inclusions and the presence of lysosome-like bodies in the cytoplasm of these cells suggest a possible phagocytic role.