Formation of layer-specific fiber projections to the hippocampus in vitro.

The factors determining the layer-specific termination of hippocampal afferents are not known. Previous studies have suggested that the laminated termination of afferent fiber systems is caused by their sequential ingrowth during development. Here we have tested this temporal hypothesis of fiber segregation by an in vitro confrontation system in which the sequential arrival of entorhinal and commissural fibers was reversed. However, despite the temporal reversal of ingrowth, both fiber systems terminated in their normal positions. We conclude that the sequence of fiber ingrowth does not determine the lamination of hippocampal afferents.