The Noncanonical Hippocampus: Shared and Distinct Features of the CA2FCIG Hippocampal System

The hippocampal CA2 region has received considerable interest in recent decades, earning a functional understanding beyond the previously held view as a small transitional zone. However, related hippocampal structures called the fasciola cinereum (FC) and indusium griseum (IG) have been largely neglected to date. While CA2, FC, and IG are typically presented as discontinuous structures on conventional atlases, recent work unveils a surprisingly high degree of overlapping gene expression and anatomical continuity, raising questions of their shared versus distinct functions. Here we review the molecular, anatomical, physiological, and behavioral features of each area, to begin to address whether these regions are subcomponents of a greater hippocampal structure or independent entities with distinct contributions to physiology and disease. Early evidence supports that the CA2-FC-IG axis is indeed a cohesive network unified by shared molecular and anatomical profiles, although regional differences in structure and function need to be taken into account. Notably, transcriptomics has revealed multiple subclusters within this complex of structures with no region-specific cell types, raising the need to understand whether cell types or anatomical subfields are the main functional units of this system. This conceptualization of these noncanonical hippocampal areas as a unified, multifaceted system raises many questions discussed here, along with the need for coordinated investigation across regions in future work.