FUNCTIONAL GUIDANCE COMPONENTS AND THEIR CELLULAR-DISTRIBUTION IN RETINOTECTAL COCULTURES

Recent advances in the study of the developmental processes that direc t afferent axons toward their final destinations are beginning to eluc idate the cellular and molecular bases for the early development of re tinotectal topography. The present review focuses on two guidance comp onents (repulsive and adhesive) and their cellular localization as rev ealed 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 re sponses to single living target cells allows a cellular localization o f guidance components that is not possible by other methods. This co-c ulture system, therefore, complements the repertoire of cell-culture t echniques used to investigate the sequence of cellular events that und erlie the establishment of retinotopic development. With this new appr oach, time-lapse micrographs were collected when RGC growth cones from temporal or nasal regions of embryonic chick retinae encountered indi vidual cells dissociated from optic tecta. Temporal RGC growth cones c ollapsed and retracted with a high probability from neuronal cells dis sociated from posterior tecta, indicating that repellent components we re enriched on posterior target neurons. The response to non-neuronal cells revealed a separate effect on axonal outgrowth that was less dep endent upon the particular region of retina or tectum from which the c ells originated: most RGC axons adhered to the edge of non-neuronal ce lls, without retracting. Together, the localization of repellent and a dhesive components suggests a sequence of events that occurs during th e early stages of tectal innervation and that results in the rudiments of retinotopic projection, and furthermore, it raises a number of exp erimentally approachable questions concerning the functional expressio n of several guidance components.