Morphological Changes of the Human Hippocampal Formation from Midgestation to Early Childhood

Abstract

Cytoarchitectonic layers of the human hippocampal formation are formed by the 24–25th fetal weeks. Consequently, cell formation in the hippocampal ventricular zone is occasional after the 24th week. Cell formation in the hilar region is continuous perinatally, but neuronal cell formation and cell death occur at a very low rate. Cell migration from the proliferative zones lasts until the 32–34th fetal weeks in Ammon’s horn. Immature cells accumulate in the hilus of the dentate gyrus throughout the first year, when the subgranular hilar zone appears cell-free, suggesting that immature cells from the hilus migrate to the granule cell layer and differentiate into granule cells. Dendritic development and synapse formation last for several years. Light microscopic changes of dendrites of the hippocampal neurons have been reported until the fifth postnatal year. Although the hippocampal formation of newborn infants has the necessary synaptic connections for memory formation, the number of postnatal morphological changes suggests a significant modification of hippocampal circuits between the neonatal period and adulthood.