STRIATAL PRECURSORS ADOPT CORTICAL IDENTITIES IN RESPONSE TO LOCAL CUES

One of the early steps in the regionalization of the CNS is the subdiv ision of the forebrain into dorsal and basal telencephalic ventricular zones. These ventricular zones give rise to the cortex and striatum r espectively, in the mature brain. Previous work suggests that while ne ural precursors are able to move within both the dorsal cortical and b asal striatal ventricular zones, they are unable to cross the boundary area between them. To determine if the regional identities of the cel ls in these ventricular zones are restricted, cells from the basal str iatal ventricular zone were either transplanted back into their origin al environment or into the dorsally adjacent cortical ventricular zone . Use of in vitro explants of mouse telencephalon demonstrated that st riatal precursors are able to integrate heterotopically within 12 hour s of being placed onto the surface of cortical ventricular zone. To ex amine whether heterotopically placed neural precursors have phenotypes appropriate to their host or donor environment, in vivo transplants i n rats were performed. Striatal ventricular zone cells transplanted to a striatal environment adopt morphologies and axonal projections char acteristic of striatal cells. In contrast, striatal ventricular zone c ells transplanted in vivo to a cortical environment acquired morpholog ies and axonal projections specific to cortex. These findings suggest that within forebrain, position-specific cues play an instructive role in determining critical aspects of regional phenotype.