Autoradiographic studies of the commissural and ipsilateral association connections of the hippocampus and dentate gyrus. I. The commissural connections

Abstract

The commissural connections of the two major cytoarchitectonic fields of the hippocampus (fields CA 1 and CA 3) and of the dentate gyrus of the rat have been studied autoradiographically following injections of 3 H‐leucine into the contralateral hippocampal formation. The commissural projection to the dentate gyrus has its origin in the hilar region of the gyrus of the opposite side (probably from the pyramidal cells of subfield CA 3c) and the majority of its fibers terminate upon a restricted segment of the granule cell dendrites (about 30 μ wide) in the inner one‐third of the molecular layer. Like the ipsilateral association pathway to this region, the commissural projection has an extensive distribution along the septo‐temporal axis of the gyrus. The commissural connections of field CA3 appear to be organized homotopically in the sense that each of its subfields seems to be related principally (if not exclusively) to the corresponding subfield on the opposite side. However, along the septo‐temporal axis of the hippocampal formation the commissural projection seems to be much more widespread, so that isotope injections into this field at any one level always give rise to transported label over a considerable rostro‐caudal extent on the opposite side. Within field CA 3 the commissural terminals are confined to the region of the basal dendrites in the stratum oriens, and the middle portion of the apical dendrites in the stratum radiatum. Field CA1 does not appear to contribute to the commissural system: this is certainly true of its anterior portion which projects into the dorsal fornix rather than the fimbria, and may also hold for the posterior part of the field. Most, if not all, of the commissural fibers to field CA 1 have their origin in field CA 3 of the opposite side. After crossing the midline in the ventral hippocampal commissure, these fibers follow an intrahippocampal course which appears to parallel that of the Schaffer collateral system. Within field CA 1 the crossed fibers terminate in relation to the basal dendrites and the proximal three‐fourths of the apical dendrites in the stratum oriens and the stratum radiatum.