Sensory axons extend from the chick olfactory epithelium to the telencephalon well before the maturation of their target, the olfactory bulb. During a waiting period of several days, olfactory axons arrive and accumulate outside the CNS while the bulb differentiates beneath them. Semephorin-3A is expressed in the telencephalon during this period and has been proposed to prevent their entry into the CNS. We show that the misexpression of a dominant–negative neuropilin-1 that blocks SEMA-3A-mediated signaling in olfactory sensory axons induces many of them to enter the telencephalon prematurely and to overshoot the olfac tory bulb. These results suggest that chemorepellents can prevent the premature innervation of immature targets.
