Commissural connections of the dentate area in the rat.

The commissural connections of the area dentata were investigated with the Fink-Heimer silver impregnation method and the commissural terminals in the hilus of the fascia dentata further studied by electron microscopy of anterograde degeneration. The commissural endings in the molecular layer were found to terminate as previously reported by others. In addition it was shown that the hilus also receives a significant commissural input. The commissural afferents to both the molecular layer and the hilus terminate along the full rostro-caudal extent of the area dentata, but with varying densities. The degeneration in the molecular layer is maximal dorso-rostrally and declines in the caudo-ventral direction, whereas the degeneration in the hilus varies inversely. The commissural terminals in the hilus make asymmetrical contacts with dendritic spines and to a lesser extent with dendritic stems. Dark, but otherwise apparently normal terminals with the features of mossy fiber boutons, were encountered in low numbers in both decommissurated and control animals. The commissural projection to the dentate area originates in the opposite hilus and possibly the adjacent part of CA3 (CA3c). Fibers from middle dorso-basal levels of the hilus to the opposite molecular layer are distributed more rostrally than ventrally relative to the level of the source of the fibers.