Cone synapses with Golgi-stained bipolar cells that are morphologically similar to a center-hyperpolarizing and a center-depolarizing bipolar cell type in the turtle retina.

Two types of Golgi-stained bipolar cells have been examined by light and electron microscopy to determine the ultrastructure of their synapses with photoreceptors and the spectral type of photoreceptor with which they connect in the retina of the turtle (Pseudemys scripta elegans). We have chosen bipolar cells that correspond in morphology to a center-hyperpolarizing type (H-bipolar) and a center-depolarizing type (D-bipolar) shown in Marchiafava and Weiler's paper (Proc. R. Soc. B 208:103-113, '80). The latter authors recorded intracellular responses and marked their cells with the fluorescent dye Procion yellow so that we have a clear picture of their morphology. These bipolars have been called B4 and B6, respectively, according to our recent classification scheme of Golgi-stained cells. Serial section electron microscopy of two B4 bipolars shows that they made wide-cleft, striated basal junctions with red and green cone pedicles. They connected to six or seven different cone pedicles. The three B6 bipolar cells studied made narrow-cleft, semi-invaginating basal junctions with cone pedicles. The dendrites of B6 bipolars did not become central elements at ribbon synapses although they invaginated toward the synaptic ribbon. Serial section electron microscopy indicated that B6 bipolars contacted three to five cone pedicles also of a red or green cone type. We suggest that in the case of these two varieties of bipolar cell in turtle retina, sign-conserving synapses with cones are wide-cleft basal junctions while sign-inverting synapses are narrow-cleft, semi-invaginating in morphology.