The anatomical organization of the rat fascia dentata: new aspects of laminar organization as revealed by anterograde tracing with Phaseolus vulgaris -Leucoagglutinin (PHAL)

The rat fascia dentata is characterized by a simple cytoarchitecture and characteristic lamination of afferents. Entorhinal afferents are believed to terminate exclusively in the outer two thirds of the molecular layer, whereas commissural fibers are believed to terminate exclusively in the inner molecular layer of the fascia dentata. A sharp border divides these two major afferent fiber systems and is regarded as the main boundary of the fascia dentata. This concept of a highly laminated brain structure has made the fascia dentata attractive for studies analyzing normal or pathological processes of the brain. Recently, entorhinal as well as commissural fibers have been identified which do not follow the classical lamination of the fascia dentata. Using anterograde tracing with Phaseolus vulgaris-Leucoagglutinin, an entorhino-dentate projection to the molecular layer, granule cell layer, and hilus of the fascia dentata was described. With the same technique, GABAergic commissural fibers to the outer molecular layer of the fascia dentata were revealed and a previously unknown heterogeneity of the commissural projection was demonstrated. These previously unknown fiber systems complicate the interpretation of lesion effects in this brain region and have to be taken into account as possible sources of sprouting fibers following the partial denervation of the fascia dentata.