DEVELOPMENT OF MICROGLIA IN THE POSTNATAL RAT HIPPOCAMPUS

During the prenatal development of the hippocampus, microglial cell pr ecursors progressively occur in all subfields in accordance with known ontogenetic gradients of the region (Dalmau et al., J.. Comp. Neurol. 1997a;377:70-84). The present study follows the regional distribution of these microglial cell precursors and their morphological different iation in the rat hippocampus from birth to postnatal (P) day 18. The results demonstrate that the cellular differentiation and the subregio nal distribution of microglia follow the specific developmental gradie nts of the different parts of Ammon's horn and the dentate gyrus. Micr oglial cell distribution in the dentate gyrus is thus delayed compared with that in Ammon's horn. The appearance of microglia in the hippoca mpal subregions and differentiation of cell precursors into adult micr oglia occur earlier at temporal levels than at septal levels. Distribu tion of microglial cells follows an outside-to-inside pattern from the hippocampal fissure to the main cell layers in either Ammon's horn or the dentate gyrus. Meanwhile, the resident microglial cells located i n the stratum oriens and dentate hilus at birth also increase in numbe r and gradually disperse throughout the whole tissue of the two layers with age. In Ammon's horn, microglial differentiation occurs earlier in CA3 than in CA1. In the dentate gyrus, microglia appear earlier in relation to the external limb than to the internal limb, largely follo wing a lateral-to-medial gradient. The differentiation and appearance of microglia in the various hippocampal and dentate subregions often c orrespond to the developmental stage of intrinsic and extrinsic affere nt nerve fiber projections. Finally, in both Ammon's horn and the dent ate gyrus, cells resembling reactive microglia are also observed and, in particular, in the perforant path projections from P9 to P18, sugge sting their participation not only in phagocytosis of dead cells but a lso in axonal elimination and/or fiber reorganization. (C) 1998 Wiley- Liss, Inc.