Functional guidance components and their cellular distribution in retinotectal co-cultures

Recent advances in the study of the developmental processes that direct afferent axons toward their final destinations are beginning to elucidate the cellular and molecular bases for the early development of retinotectal topography. The present review focuses on two guidance components (repulsive and adhesive) and their cellular localization as revealed by a newly established retinotectal co-culture system. These co-cultures enable general outgrowth patterns to be examined when retinal ganglion cell (RGC) axons attempt to traverse cells dissociated from their target nuclei. Additionally, the recording of RGC growth cone responses to single living target cells allows a cellular localization of guidance components that is not possible by other methods. This co-culture system, therefore, complements the repertoire of cell-culture techniques used to investigate the sequence of cellular events that underlie the establishment of retinotopic development. With this new approach, time-lapse micrographs were collected when RGC growth cones from temporal or nasal regions of embryonic chick retinae encountered individual cells dissociated from optic tecta. Temporal RGC growth cones collapsed and retracted with a high probability from neuronal cells dissociated from posterior tecta, indicating that repellent components were enriched on posterior target neurons. The response to non-neuronal cells revealed a separate effect on axonal outgrowth that was less dependent upon the particular region of retina or tectum from which the cells originated: most RGC axons adhered to the edge of non-neuronal cells, without retracting. Together, the localization of repellent and adhesive components suggests a sequence of events that occurs during the early stages of tectal innervation and that results in the rudiments of retinotopic projection, and furthermore, it raises a number of experimentally approachable questions concerning the functional expression of several guidance components.