Positional specificities of retinal growth cones in the mouse superior colliculus

In the developing retinotectal system, repulsive topographic tectal cues have been demonstrated to contribute to the final mapping. Here, we describe a novel response of nasal axons to growth-promoting cues expressed by anterior tectal cells. In in vitro experiments, contact of fibres from the nasal (but not temporal) pole of the mouse retina with anterior (but not posterior) tectal membranes leads to their adopting very elongated and filopodial morphologies, and to increase their growth rates. As previously demonstrated, fibres from the temporal pole of the retina are collapsed by posterior tectal membranes in vitro. In addition, a study of retinal growth cone morphologies in vivo, at early stages of target invasion, shows that growth cones of nasal fibres have streamlined morphologies, usually indicative of active elongation growth modes, in the anterior part of the embryonic mouse tectum, and more elaborate morphologies posteriorly. Vice versa, temporal fibres have mainly elaborate growth cones anteriorly, and collapsed growth cones posteriorly. These experiments demonstrate that nasal retinal fibres respond preferentially to permissive or growth-promoting cues in the embryonic mouse tectal environment, both in vitro and in vivo. This phenomenon might contribute to ingrowth of retinal fibres in their target area, and to promote the homing of nasal fibres towards the posterior aspect of the tectum, which is their normal target region.