Functional Anatomy, Development, and Pathology of the Hippocampus

The hippocampal formation is comprised of a group of cortical regions, including the dentate gyrus, hippocampus, subiculum, presubiculum, parasubiculum, and entorhinal cortex. The hippocampal formation plays a central role in the brain network essential for memory function. Paradoxically, the hippocampus is also the brain structure most sensitive to hypoxic-ischaemic episodes. In addition, abnormalities in hippocampal structure and functions have been reported in a number of neuropathological and neurodevelopmental disorders. This chapter provides a basic description of the structural organization of the primate hippocampal formation. This chapter also discusses recent findings regarding its normal postnatal development, which provide an essential framework to consider the aetiologies of different neurodevelopmental disorders affecting hippocampal structure and function. In particular, a developmental decrease in astrocytic processes and functions may be the critical factor underlying the selective vulnerability of CA1 to hypoxic-ischaemic episodes in adulthood. It also provides an explanation for the relative resistance of this brain structure to hypoxia in the perinatal period, especially during the birth process. In contrast, a major benefit that derives from decreased astrocytic coverage is the regulation of synaptic efficacy leading to an increase in synaptic selectivity advantageous for learning. In addition, the relatively high astrocytic coverage of newborn synapses might also play a central role in the generation of febrile seizures. Finally different periods of postnatal development during which pathogenic factors might impact the structural and functional maturation of distinct hippocampal regions were identified, as is observed in autism, schizophrenia, epilepsy, and Down syndrome