Anatomical Characterization of the Fasciola Cinereum

Abstract

The fasciola cinereum (FC) is one of the subregions of the hippocampus. However, the canonical trisynaptic circuit contains only the CA1, CA3, dentate gyrus (DG) and CA2. The anatomical characteristics of the FC and the functions of FC in the hippocampus are unknown. In addition, the anatomical boundary between the FC and CA1 has been controversial. I sought to define the border between the FC and CA1, and examine the anatomical characteristics of the FC in this study. Two types of staining methods have used to delineate the borders between the FC and CA1 in this study. I also injected anterograde and retrograde tracers into the FC, CA1, and DG to estabilish afferent and efferent projections of the FC. As a result, I have found that the afferent projections of the FC originate from the layer II of the lateral entorhinal cortex (LEC), the layer II of the perirhinal cortex (PER), all layers of the postrhinal cortex (POR), and also the supramammillary body (SuM). The efferent projections targeted the septal dentate gyrus (DG). In addition, the FC contained strong intrinsic connections. One of the notable anatomical characteristics of the FC was that the locations of the labelled cells overlapped with the layer II of the LEC labelled after retrobeads (RB) injections in the septal DG with some cells found overlapped with PER and POR as well. Based on these findings and its cytoarchitectonic characteristics, I presume that the FC is composed of granule cells. Injection of colchicine, a DG-specific neurotoxin, lesioned the cells in most FC, suggesting the FC is likely to contain similar cytoarchitectonic characteristics with granule cells in the DG. The results suggest that the FC may be an independent hippocampal subregion. The FC may be involved in encoding new memory by receiving inputs from the PER, POR, and LEC, and sending its outputs to the DG.