Postnatal development of the primate hippocampal formation.

The hippocampal formation is a multicomponent region of the medial temporal lobe preferentially involved in declarative and relational memory processing. Behavioral studies have suggested a protracted functional maturation of these structures in primates, and postnatal developmental abnormalities in the hippocampal formation are thought to contribute to neurodevelopmental disorders, such as autism, schizophrenia, epilepsy and Down syndrome. Despite all that we know about the functional organization of the adult hippocampal formation, notably absent is a systematic study of its postnatal maturation in primates. In this article, we review current knowledge of the structural development of the primate hippocampal formation and present new data on its postnatal neuroanatomical development. We summarize what is known about the neurobiological processes, such as the addition of new neurons, the establishment and elaboration of connectivity, and the neurochemical changes, that underlie the structural development and functional maturation of the primate hippocampal formation. We conclude that there is yet insufficient information to identify distinct developmental windows during which different hippocampal regions undergo specific maturational processes. For this reason, it is currently impossible to determine the ages at which specific hippocampal circuits become structurally mature and potentially capable of supporting defined, age-specific functional processes. Together with work in rodents, systematic studies of the structural development and functional maturation of the monkey hippocampal formation will be necessary to gain insight not only into the types of information processing that it subserves, but also into the specific maturational processes that might be affected in human neurodevelopmental disorders.