ABSENCE OF TOPOGRAPHY IN PRECOCIOUSLY INNERVATED TECTA

The retinotectal map in Xenopus forms very early: retinal axons are to pographically ordered along the dorsoventral axis of the tectum by sta ge 39, as they first arrive, To test whether topographic cues are pres ent even earlier, we forced retinal axons to innervate the tectum prem aturely by transplanting stage 28 eye primordia into stage 20 hosts, t hen assayed dorsoventral topography using focal injections of lipophil ic dye into dorsal and ventral retina at donor stages 39-40, Unoperate d and isochronic control projections showed normal dorsoventral orderi ng both in the optic tract and in the tectum, In contrast, projections from heterochronically transplanted eyes were ordered in the tract, b ut spread out upon entering the tectum and did not show significant do rsoventral ordering, Individual axons entering the tectum precociously often made abnormally abrupt and topographically incorrect turns, Thu s, the topographical cues normally expressed in the tectum at stage 39 appear to be absent a few hours earlier. However, this lack of cues i s only temporary, since heterochronic transplants allowed to survive t o donor stages 45-46 showed normal topography, The absence of tectal t opography at a stage when retinal axons can navigate to the young tect um strongly suggests that the molecules that provide tectal topographi cal signals are distinct from those used for pathfinding in the dience phalon and target recognition at the tectum.