Mono- and oligo-vesicular synapses and their connectivity in a Cnidarian sensory epithelium (Coryne tubulosa).

The spherical end-knobs of the tentacles of capitate hydropolyps are an evolutionarily early paradigm of a chemo- and mechanosensory epithelium composed of four types of sensory cells and one type of chemo-mechanosensitive nematocytes (stinging cells), all separated by supporting cells. The epithelium discriminates sites and compositions of stimuli and induces various kinds of behavior. Recent electrophysiological studies demonstrated rapid chemo-synaptic signal transmission of nematocytes and mechanosensory hair cells, graded in amplitude and duration. The present electron microscopic work, applying serial sectioning, analyses the ultrastructural basis of signal transmission and efference control in the tentacular spheres of Coryne tubulosa, a species also used in preceding studies. Neurites of sensory cells and of proximal ganglion cells constitute a nerve plexus at the base of the ectodermal cells. No ganglion cells are located within the spheres. Chemical synapses of the usual configuration connect neurites or are efferent to nematocytes and hair cells. Each of these synapses contains only 3-10 clear and/or dense-core vesicles of 70-150 nm diameter (oligo-vesicular synapses). For the graded afferent signal transmission of nematocytes and hair cells, the only candidates are regularly occurring zones of neurite contacts at the base of these cells. At their presynaptic side, mostly one (more seldom two to four) large vesicles (160-1100-nm-diameter magno-vesicles) are attached to a surface membrane density. In order to reconcile structural and functional data, a transient fusion and partial depletion of stationary vesicles is considered for the release of transmitter in mono-vesicular synapses, similar to recent findings for vertebrate endocrine secretion. The same principle is discussed for the usual oligo-vesicular synapses of Cnidaria.