Cell Types and Laminae of the Vertebrate Retina

The retina is a part of the brain that is displaced into the periphery during embryonic development (see Chapter 2). Because it is relatively accessible, retinal tissue provides a useful experimental model of brain circuitry. Some of the most significant advances in our understanding have come from studies of the retinas of cold-blooded vertebrates, which contain large neurons amenable to intracellular recording and staining. Mud puppies, turtles, frogs, and fish have been particularly rich sources of information. The cells of the mammalian retina are more difficult to study, but much work has been done in primates, cats, rabbits, and rats. Although some general patterns of organization have emerged, it is clear that no retina is exactly like any other, not even among mammals. This must be kept in mind when extrapolating from the wealth of comparative data to mammalian retinas, and particularly primate retinas.

The lamination of the retina is a guide to the locations of the various cell types and to the regions in which they make synaptic contacts. The somata or perikarya of the photoreceptors form the outer nuclear layer (Figure 6.1), and their inner and outer segments lie between this layer and the pigment epithelium. Axonal processes of the photoreceptors extend toward the outer plexiform layer, where they establish contacts with bipolar cells and horizontal cells. The somata of bipolar cells and horizontal cells share the inner nuclear layer with those of amacrine cells, interplexiform cells, and Müller cells (glial cells unique to the retina).