A freeze-fracture study of the skate electroreceptor.

The sensory epithelium lining the ampulla of Lorenzini in the skate was examined by the freeze-fracture technique. Anastomosing tight junctions (zonula occludens) completely encircle the apex of each receptor cell joining it to neighbouring support cells. The tight junctions separate two distinctly different regions of the receptor-cell surface. The apical P-face has numerous large particles while just below the tight junctions of the lateral surface have many smaller particles. On its basal surface each receptor cell makes several evaginating ribbon synapses with an afferent nerve. Three regions of the synaptic evagination can be distinguished on the basis of membrane specializations: 1. At the tip of the evagination a regular array of large particles is found on the P-face of the receptor cell directly opposite a similar regular array of large particles on the P-face of the afferent nerve; 2. just above the tip at a narrow constriction, below which vesicles are not found, a population of large particles on the P-face of the receptor cell opposes a well-defined strip of large particles that cleaves with the E-face of the nerve fibre; 3. at the arch of the synaptic evagination randomly occurring dimples are found on the P-face and protrusions on the E-face of the receptor cell. The density of these protrusions increased in skates that were electrically stimulated. We suggest that the co-extensive arrays of particles at the tip of the ribbon synapse is an intercellular junction; that the active zone of the synapse is at or above the constriction; and that membrane retrieval occurs in the synaptic arch region.