Anatomical studies of simple invertebrate synapses utilizing stage rotation election microscopy and densitometry.

The radial nerve cords of starfish and the central ganglia of a gastropod mollusc were examined for the presence of chemical synapses. No structures with the degree of specialization of synapses in the vertebrate CNS were observed. Presumed chemical synapses, which possessed slight but variable paramembrane densities, were examined with a tilting stage. This showed that such densities were frequently due to overlap of vesicle and axon membrane within the section, which were resolved at the correct angle of tilt. Many structures resembled demonosomes. The necessity for care in interpreting the structure of chemical synapses in vertebrates is emphasized.