EFFECT OF POLYSIALIC ACID ON THE BEHAVIOR OF RETINAL GANGLION-CELL AXONS DURING GROWTH INTO THE OPTIC TRACT AND TECTUM

We have demonstrated previously that the polysialic acid (PSA) moiety of the neural cell adhesion molecule (NCAM) can regulate peripheral ne rve branching during development. In particular, it was found that spe cific enzymatic removal of PSA from motor axons causes them to form ti ght fascicles that are less responsive to normal guidance cues. In the present study, the role of PSA in the behavior of axons in the centra l nervous system has been examined through an analysis of chick optic axons during development. Unlike peripheral axons, which generally gro w in a PSA-free environment, PSA was found to be present both on retin al ganglion cell axons and their environment in the tract and tectum. Furthermore, the enzymatic removal of PSA from the optic axons caused them to defasciculate in the tract/tectal region. This response was mo rphologically similar to targeting corrections made by these axons at a later stage when PSA levels have decreased, suggesting that the PSA may serve to shield them from responding prematurely to some guidance cues in their target region.