Origin and topography of fibers contributing to the fornix in macaque monkeys

Abstract

The distribution of neurons contributing to the fornix was mapped by placing the retrograde tracer horseradish peroxidase (HRP) in polyacrylamide gels in different medial to lateral locations within the fornix of three rhesus monkeys (Macaca mulatta). The HRP was placed from 3 to 5 mm caudal to the descending columns of the fornix. Additional information came from a series of rhesus and cynomolgus monkeys (Macaca fasciculata) with anterograde tracer injections in the medial temporal lobe. The hippocampal formation, including the subiculum and presubiculum, together with the entorhinal cortex (EC) and perirhinal cortex (area 35) contribute numerous axons to the fornix in a topographical manner. In contrast, the lateral perirhinal cortex (area 36) and parahippocampal cortical areas TF and TH only contained a handful of cells labeled via the fornix. The medial fornix originates from cells in the caudal half of the subiculum, the lamina principalis interna of the caudal half of the presubiculum, and from the perirhinal cortex (area 35). The intermediate portion of the fornix (i.e., that part midway between the midline and most lateral parts of the fornix) originates from cells in the rostral half of the subiculum and prosubiculum, the anterior presubiculum (only from the lamina principalis externa), the caudal presubiculum (primarily from lamina principalis interna), the rostral half of CA3, the EC (primarily 28I and 28M), and the perirhinal cortex (area 35). The lateral parts of the fornix arise from the rostral EC (28L only) and the most rostral portion of CA3. Subcortically, the medial septum, nucleus of the diagonal band, supramammillary nucleus, lateral hypothalamus, dorsal raphe nucleus, and the thalamic nucleus reuniens all send projections through the fornix, which presumably terminate in the hippocampus and adjacent parahippocampal region. These results not only help to define those regions that project via the fornix, but also reveal those subcortical projections to the hippocampal formation most likely to rely entirely on nonfornical pathways.