Abstract
A small population of retinal ganglion cells express the photopigment melanopsin and have dendrites that stratify in the inner plexiform layer of the retina. These melanopsin ganglion cells are critical for visual system development as recent studies have demonstrated they are key players in the propagation of retinal wave activity and the development of the image-forming visual system. Early in development some of these melanopsin ganglion cells defy conventional lamination patterns and extend dendrites from the inner retina into the outer retina, and are closely apposed to s-opsin expressing cone photoreceptor pedicles. These outer retinal dendrites may contribute to setting the laminar stratification pattern of s-opsin cones as dark-rearing studies reduce the number of melanopsin ganglion cell outer retinal dendrites while increasing the number of displaced s-opsin cones. Though they are developmentally regulated, many outer retinal dendrites remain in the adult retina and there is anatomical evidence for synaptic transmission between cone photoreceptor terminals and outer retinal dendrites. This is some of the first evidence in a mammalian retina of a photoreceptor directly coupled to a ganglion cell and a potential novel mechanism for melanopsin ganglion cells to shape the development of the visual system.