The corpus callosum (CC) is a major central nervous system (CNS) pathway that is morphologically related to most neocortical regions. Previous studies have shown that the CC plays a significant role in the development of other CNS pathways. Specifically, CC input during the first postnatal month in cats is critical to normal visual development, yet CC input after the first postnatal month is unnecessary for normal visual development. This project will focus on determining the morphological substrate for the role of die CC in CNS development by definitively determining the distribution of CC cell bodies and terminals at different stages of development. Previous studies of these distributions were inconclusive because of conflicting results and methodological limitations. This project will utilize a major technical advance in the tracing of neuronal connections. CC neurons will be labeled by in vitro application of a fluorescent carbocyanine dye, DiI. By application to fixed tissue, DiI labels CC neurons whose locations and connections are frozen in development. DiI travels retrogradely and anterogradely, and will extensively label many CC neurons. DiI will be applied to the CC in different aged normal animals to identify the areal and laminar location of CC cell bodies and terminals. In addition, DiI will be used to determine the location and fate of transitory CC projections at different stages of normal development to determine the potential for synaptically-mediated interactions between transitory CC axons and cortical neurons. Finally, tissue labeled by DiI will be processed for immunofluorescence to determine possible neurotransmitters of both cortical neurons that are targets of CC projections, as well as CC neurons. These studies will provide significant new information on the role of the CC in cortical development; the results will provide a model for analysis of the relationship between the morphological and functional development of CNS pathways. In addition, these studies will provide a model for investigating the development of, and analyzing the role of, transitory connections, a pervasive yet poorly understood feature of CNS development.