This proposal describes an anatomical investigation of the addition of new neurons and the resultant reorganizations of synaptic connectivity which occur during postembryonic growth of teleost fish. The focus is on two principle topics: changes in retinal cytoarchitecture resulting from ontogenetic differences between rods and cones and changes in the sites of termination of individual retinal fibers in the growing optic tectum. There are three specific aims. 1.) To characterize the population of photoreceptors in goldfish of different sizes, especially with regard to the accumulation of rods in the growing retina, and to determine whether structural changes occur in individual photoreceptors with age. 2.) To identify the progenitor cells which generate new rods. These dividing cells are located in the outer nuclear layer, amongst mature rod nuclei; they have been identified only with thymidine radioautography but have never been described in ultrastructural studies. The role of rod progenitors will be assessed in the normal retina (do they only add to the rod population or are some of their progeny replacements for rods lost through cell death?) and in the regenerating retina (are they able to generate retinal neurons other than rods under those circumstances?). The ontogenetic history of rod progenitors will be investigated in larval retinas to determine where these unique cells come from. 3.) To chart the tectotopic positions of individual retinal terminals over time using long-lasting neuroanatomical markers. Because retina and tectum grow along non-equivalent dimensions, it has been suggested that systematic shifts occur in the retinotectal map, and it is this hypothesis which is to be tested. The anatomical studies of photoreceptor development in teleost retina will provide information about how rods and cones differ that may be significant for understanding disease processes which preferentially affect specific photoreceptors. The analysis of retinotectal projections may provide evidence for naturally-occuring changes in synaptic connectivity that would contribute to our understanding of development and regeneration of neuronal connections.