Three-dimensional structure of bidirectional, excitatory chemical synapses in the jellyfish Cyanea capillata.

Neurons in the ectoderm of the perirhopalial tissue of the jellyfish Cyanea capillata were exposed and fixed for electron microscopy under conditions designed to minimize exocytosis of synaptic vesicles. The structure of the bidirectional chemical synapses that connect neurons was examined and the three-dimensional organization of these synapses was determined from reconstructions of serial sections. Synapses were characterized by the accumulation of a relatively few, large synaptic vesicles. These lie in a single layer against the terminal membrane of each terminal. The cytoplasmic side of the vesicles in any one terminal was covered by a single, large, perforated cisternal sheet. In addition, there were numerous smaller, bulbous cisternae that intermingled with the vesicles in the terminal. The structure of any one terminal was mirrored by that of the opposite terminal of the pair. The organization of these synapses is discussed from the viewpoint of cnidarian synapses in general.