Developing vestibular commissural projections display differential patterns with chondroitinase treatment of the hindbrain

Abstract

Chondroitin sulfate proteoglycans have been regarded as molecules that restrict neuronal migration and neurite outgrowth. To assess the role of the chondroitin sulfate moieties on axonal projection in the hindbrain, we chose to perform DiI tracing of vestibular commissural projections following delivery of chondroitinase ABC into the 4th ventricles of rat embryos (E11.5-E13.5) in culture. At E11.5(+1DIV), few outgrowths could be traced. With enzyme treatment, robust outgrowths extended more than half-way towards the midline. At E12.5(+1DIV), the commissural projections assumed fascicles and reached the midline in the controls. In enzyme-treated embryos, the axons remained unfasciculated as they extended normal to the midline. At E13.5(+1DIV), commissural projections were fasciculated and extended beyond the midline in controls. Enzyme treatment did not affect the pioneer axons that had advanced beyond the midline and towards the contralateral vestibular nuclear complex. However, later outgrowths traversed the enzyme-treated matrix as unfasciculated fibres and diverted from the course of the pioneers. These results provide in vivo evidence for contributions of chondroitin sulfates not only to restrict initial axonal sprouts, but also to limit later axonal outgrowths and to foster axonal fasciculation as vestibular neurons project across the midline towards the contralateral target. Supported by HKRGC