DEVELOPING HUMAN HIPPOCAMPAL FORMATION: FROM CELL TO VOLUME

The human hippocampal formation (HCF) undergoes substantial geometric transformations during development; however, the relationship between hippocampal folding geometry and cellular-level structural organisation remains poorly understood. Using a multimodal approach, we characterise human HCF's rotation, translation and cytoarchitectural development from 14-24 gestational weeks (GW). Five paraformaldehyde-fixed human fetal brains were sagittally cryosectioned at 20 μm, stained for Nissl substance, and immunostained for calcium-binding and calcium-modulating proteins. Digitised Nissl sections were manually annotated to delineate dentate gyrus (DG), CA fields, and the subicular complex. The HCF was 3D-reconstructed from the Nissl stack to assess changes to its shape and geometric properties. At 14GW, the HCF aligns nearly parallel to the dorsal-ventral axes along the medial telencephalic wall. By 24GW, it rotates in the temporal lobe to an intermediate position observed in the adult brain. Cytoarchitecturally, the DG shows compaction of the granule cell layer by 17GW and polymorphic layer formation by 21GW. CA fields showed limited differentiation at 14 GW; differentiation was observed at 16 GW and beyond. Most hippocampal cells appear rounded in morphology until 24GW, when pyramidal neurons are observed deep in the cortical plate of CA1 and CA2. The subicular complex differentiates into subiculum, presubiculum, and parasubiculum by 14GW, with characteristic presubiculum cell islands and lamina dissecans apparent by 16GW. In addition, the presubiculum displays complementary molecular modularity, with alternating calbindin and PCP4 expression in cellular islands as early as 21 GW. Thus, we provide a cellular-level cytoarchitectural map and 3D histological reconstruction of the second-trimester human hippocampal formation.