Lamina-specific cues guide outgrowth and arborization of retinal axons in the optic tectum

In the chick, retinal axons enter the optic tectum through a superficial lamina, then branch into distinct deeper retinorecipient laminae, where they arborize and form synapses. To study factors that guide this laminar selectivity, we devised an organotypic culture system in which a transverse tectal section is overlaid with a retinal explant large enough to allow unimpeded access to all tectal laminae. Outgrowth, branching, and arborization patterns of retinal axons on tectal slices were lamina-selective, indicating the existence of localized cues that guide retinal axons. Further studies suggested that some of these cues are: (1) associated with cell membranes or extracellular matrix (because axons grew selectively on chemically fixed tectal sections); (2) intrinsic to the tectum (because axons grew selectively on tectal sections prepared from enucleated embryos); (3) distinct from topographic cues (because axons from nasal and temporal retina behaved similarly on anterior tectal slices); and (4) selective for retinal axons (because axons growing from other explants exhibited different laminar preferences).