Surface expansion regionalization of the hippocampus in early brain development

The hippocampus is implicated in a myriad of crucial functions, particularly centered around memory and emotion, with distinct or multiple subdivisions fulfilling specific roles. However, its heterogeneity is multidimensional, given that the functional connectivity and gene expression-based parcellation along its long axis differs from histology-based parcellation along medial-lateral axis. The rapid nonuniform surface expansion of the hippocampus during early development reflects underlying changes of microstructure and functional establishment, providing important clues. Furthermore, the thin and convoluted properties bring out hippocampal maturity largely in the form of expanding surface area. We thus unprecedentedly explore the development-based surface area regionalization and patterns of the hippocampus by leveraging 513 high-quality longitudinal MRI scans during the first two postnatal years. Our findings imply two discrete hippocampal developmental patterns, featuring one pattern of subdivisions along anterior-posterior axis (head, regions 1 and 5; body, regions 2, 4, 6, and 7; tail, region 3) and the other one along medial-lateral axis (subiculum, regions 4, 5, and 6; CA fields, regions 1, 2, and 7). The resulting 7 subdivisions exhibit region-specific and nonlinear spatiotemporal surface area expansion patterns. These results provide important references for exploring fine-grained organization and development of the hippocampus and its intricate cognitions.