Intraretinal grafting reveals growth requirements and guidance cues for optic axons in the developing avian retina

To study environmental factors controlling the growth and navigation of optic axons in the eye, grafts of retinal, optic disc, optic tectum, and floor plate tissue were transplanted into organ-cultured embryonic chick or quail eyes. The growth of axons into and out of the graft was studied in cross sections of the cultured eyes and by DiI tracing in retinal whole mounts. Based on the location and trajectory of axons and based on the quantity of axons that entered and exited the grafts, several requirements for axonal navigation were established: (1) Axonal growth is restricted to an approximately 10-microm-thick layer at the vitreal surface of the retina. (2) The retinal neuroepithelium prior to axogenesis is nonpermissive for neurite outgrowth. This nonpermissive quality is transient and recedes peripherally as the differentiation of the retina progresses. (3) Embryonic axons are able to grow into neonatal and adult retinal grafts, demonstrating that older retina remains permissive for axonal growth. (4) The trajectory of axons into and from retinal grafts that had been rotated in their peripheral-central orientation showed that the retina has an inherent polarity that permits axon growth toward and away from the optic disc, but does not allow axon growth perpendicular to this direction. This centroperipheral cue operates locally rather than by long distance. (5) The optic disc provides an exit for the axons from the retina, but has no detectable neurotropic activity. Finally, optic axons from the host retina readily enter grafts of their target tissue, the optic tectum, but few axons are able to leave tectal transplants.