Development of Axonal Projections in Cocultures of the Hippocampal Formation Visualized with β-Actin-gfp Transgenic Slices

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Abstract
Coculture of immature brain tissue is a widely used model to study the formation of connections and neuronal differentiation under experimental conditions. Very often, the labeling of fiber outgrowth is incomplete and accompanied by tissue damage due to the application of the tracer. Here we used organotypic slice cocultures from neonate wild-type mice and β-actin-gfp transgenic mice, which allowed us to prelabel the vital axons and to monitor the complete axonal projection. In cocultures from entorhinal cortex and hippocampus, gfp-labeled entorhinal axons terminate in their correct hippocampal layers. Cocultures between the CA3 region of a gfp transgenic mouse and a CA1 slice from wild-type mouse show green fluorescent Schaffer collaterals in the stratum radiatum of the CA1 target slice. β-Actin-gfp-expressing cells often migrated into the wild-type part of the coculture and developed processes. From our data, we demonstrate that this chimeric coculture approach is appropriate in tracing entire developing projections and may serve as a tool to directly observe the navigation and axonal elongation of living axons.