CONNECTIONS OF THE HIPPOCAMPAL-FORMATION IN HUMANS .2. THE ENDFOLIAL FIBER PATHWAY

We investigated the anatomical connections of the pyramidal neurons lo cated within the hilar region of the dentate gyrus of the human hippoc ampus, neurons which do not have a rodent equivalent. The myeloarchite ctural patterns of the human hippocampus indicated the presence of a d istinct fiber pathway, the endfolial fiber pathway in the stratum orie ns of the hilus and field CA3. By using the fluorescent lipophilic dye DiI in formalin-fixed human hippocampal tissue, we demonstrated that this is a continuous fiber pathway between the deep hilar region and C A2. This fiber pathway did not enter the fimbria or alveus along the e ntire distance of the traced pathway and ran exclusively in the stratu m oriens of the hilus and CA3. Tracing studies with biocytin in in vit ro human hippocampal slices indicated that the hilar and CA3 pyramidal neurons contributed to this pathway. Out distally in field CA3, the l ong transverse fibers became short and choppy, suggesting that they we re beginning to move out of the plane of the tissue slice. Numerous fi bers from this pathway were seen crossing the pyramidal layer. Based o n comparative studies, we propose that the endfolial fiber system is a component of the hilar Schaffer collateral system in humans. The pres ence of a significant Schaffer collateral system from the pyramidal ne urons in the hilar region would indicate that these neurons are anatom ically related to the CA3 pyramidal neurons. Therefore, we suggest the inclusion of the human hilar pyramidal neurons within Lorente de No's field CA3 and, in particular, within subfield CA3c. (C) 1997 Wiley-Li ss, Inc.