Specific target-directed axonal outgrowth from transplanted embryonic rodent retinae into neonatal rat superior colliculus.

We have investigated whether there is evidence of target-directed growth of retinal axons to the tectum, and whether the surface of the rostral brainstem is an obligatory substrate for growing optic axons by transplanting embryonic mouse retinae to the cerebral aqueduct of neonatal rats. Such retinae emit axons that grow dorsally through the brain parenchyma to reach the superficial layers of the superior colliculus either by running dorsally along the midline, or by following a dorsally directed, radially arching course through the brain parenchyma. Studies of the early outgrowth pattern from transplanted retinae, and comparison studies of the axonal outgrowth from similarly placed cortical grafts suggest that outgrowth of retinal axons is target-directed and specific to optic axons.