Differential responsiveness to the chemorepellent Semaphorin 3A distinguishes ipsi- and contralaterally projecting axons in the chick midbrain

In the chick dorsal mesencephalon, the optic tectum, the developing axons m ust choose between remaining on the same side of the midline or growing acr oss it. The ipsilaterally projecting axons, forming the tectobulbar tract, course circumferentially toward the ventrally situated floor plate but befo re reaching the basal mesencephalon, the tegmentum, gradually turn caudally . Here, they follow the course of the medial longitudinal fasciculus (MLF), located parallel to the floor plate. By in vivo labeling of tectal axons, we could demonstrate that these axons arise primarily in the dorsal tectum. To test the idea that chemorepellent molecules are involved in guidance of the nondecussating axons, we performed coculture experiments employing tec tal explants from various positions along the dorso-ventral axis. Axons ema nating from dorsal tectal explants were strongly repelled by diencephalic t issue containing the neurons that give rise to the MLF whereas ventral tect al axons showed only a moderate response. This inhibitory effect was substa ntially neutralized by the addition of anti-neuropilin-1 antibodies. A simi lar differential response of axons was observed when tectal explants were c ocultured with cell aggregates secreting the chemorepellent Semaphorin 3A ( Sema3A). Sema3B and Sema3C, respectively, did not inhibit growth of tectal axons. In addition, neither the floor plate nor Slit2-secreting cell aggreg ates influenced outgrowth of dorsal fibers. in Sema3A-deficient mice, DiI-l abeling revealed that dorsal mesencephalic axons cross the MLF instead of t urning posteriorly upon reaching the fiber tract, thus behaving like the ve ntrally originating contralaterally projecting axons. A differential respon siveness of tectal axons to Sema3A most likely released by the MLF thus con tributes to pathfinding in the ventral mesencephalon. (C) 2001 Academic Pre ss.