Topographical organization of the entorhinal projection to the dentate gyrus of the monkey.

The topographic organization of the projections from the entorhinal cortex to the dentate gyrus in the macaque monkey was studied with anterograde and retrograde tracing methods. Injections of WGA-HRP or the fluorescent retrograde tracers, Fast blue and Diamidino yellow, were placed at various levels along the rostrocaudal axis of the dentate gyrus and hippocampus. In 5 experiments the fluorescent dyes were injected at 2 rostrocaudal levels of the same dentate gyrus. Labeled neurons were observed mainly in layers II and III of the entorhinal cortex, though some were also seen in layers V and VI. The labeled layer II cells resulting from each of the tracer injections were located throughout much of the rostrocaudal extent of the entorhinal cortex, though they tended to have a more limited distribution in the transverse or mediolateral axis. Injections of retrograde tracers located caudally in the dentate gyrus resulted in a rostrocaudally oriented zone of labeled cells that was situated laterally in the entorhinal cortex adjacent to the rhinal sulcus. The zone of labeled cells was not oriented strictly parallel to the rhinal sulcus since at caudal levels it extended medially to encompass the full transverse extent of the most caudal portion of the entorhinal cortex. When injections were placed more rostrally in the dentate gyrus and hippocampus, the rostrocaudally oriented zone of labeled cells was situated more medially in the entorhinal cortex. Anterograde tracing experiments using 3H-amino acid injections into different rostrocaudal and mediolateral positions of the entorhinal cortex confirmed the organization demonstrated by the retrograde tracers and further indicated that the entorhinal fibers terminate in the outer two-thirds of the molecular layer of the dentate gyrus. Unlike in the rat, where the entorhinal termination zone in the molecular layer is clearly bilaminate, projections from all portions of the entorhinal cortex appeared to terminate more diffusely throughout the outer two-thirds of the molecular layer. The results of the present study indicate that rostrocaudally oriented zones of cells that cut across several cytoarchitectonic subdivisions of the entorhinal cortex give rise to topographically organized projections to the dentate gyrus. Cells located laterally in the entorhinal cortex project to caudal levels of the dentate gyrus, whereas progressively more medially situated cells project to progressively more rostral parts of the dentate gyrus.