Cell formation in the human hippocampal formation from mid-gestation to the late postnatal period.

In the present study cell formation was studied in the human hippocampal formation from the 24th gestational week until the end of the first postnatal year. Proliferating cells were detected with the monoclonal antibody MIB-1. The cytoarchitectonic layers of Ammon's horn are formed before the 24th gestational week. In harmony with this observation, cell proliferation in the hippocampal ventricular zone is minimal after the 24th week. In addition, local cell multiplication in Ammon's horn is occasional and the proliferating cells are glial or endothelial cells. In contrast, cell formation continues in the hilar region of the dentate gyrus even after birth. Immature cells accumulate in the hilus, and at the border between the hilus and the granule cell layer throughout the first eight postnatal months. The subgranular zone of the dentate gyrus becomes a cell sparse area at about the 11th postnatal month, indicating that immature cells from the hilus have already migrated to the granule cell layer and differentiated into granule cells. There is an increase in glial cell proliferation both in Ammon's horn and the dentate gyrus at the 11.5th postnatal month suggesting the onset of myelination by the end of the first year. Our findings indicate that most pyramidal neurons of Ammon's horn are generated in the first half of pregnancy and no pyramidal neurons are formed after the 24th gestational week. In contrast, granule cells of the dentate gyrus proliferate in a decreasing rate during the second half of pregnancy and after birth. Proliferating neuronal precursors occur in a low percentage in the dentate gyrus of 3-, 5- and 11.5-month-old children.