The fine structure of the parietal retinas of Anolis carolinensis and Iguana iguana.

An electron microscopic examination of the parietal retinas of Anolis carolinensis and Iguana iguana demonstrated within each retina (1) two distinct populations of neurons, (2) two populations of glia, and (3) a population of photoreceptors which could not be subdivided. A small population of very electron-dense cells, in many respects similar to photoreceptors, was also found in the iguana. Correspondingly dark processes were found in the plexiform layer of each retina. Parietal photoreceptors generally resemble cones of the lateral eye. Glial cells were sub-classified on the basis of the location of their somata and the disposition of their processes. Neurons were identified by virtue of their cytology and their reception of axosomatic ribbon synapses from unidentified plexiform layer processes. Neuronal subtypes were located on opposite sides of the plexiform layer. Neurons distal to that layer were found to project the initial segments of their processes into the plexiform layer parallel to its long axis, while neurons central to the plexiform layer projected axons centrally and dendrites radially into the plexiform layer. The existence of at least two neuronal populations and of interphotoreceptor synapses suggests that photosensory processing within the parietal retina may be more complex than previously assumed.