DEVELOPMENT OF MICROGLIA IN THE PRENATAL RAT HIPPOCAMPUS

The distribution and appearance of microglial cell precursors in the p renatal hippocampus were examined in embryonic day 14 (E14) to E21 rat s by nucleoside diphosphatase histochemistry. For comparison, the diff erentiation of astroglial cells was analyzed from E17 by vimentin and glial fibrillary acidic protein immunohistochemistry. Based on morphol ogic features, nucleoside diphosphatase-positive microglial cell precu rsors were classified as ameboid microglial cells and primitive ramifi ed microglial cells. Ameboid microglia were present in the hippocampal primordium on E14. As the hippocampus developed, however, ameboid mic roglia gradually transformed into primitive ramified microglia, first recognized at E19. Microglial cell precursors, often related to nucleo side diphosphatase-labeled blood vessels, were particularly observed n ext to the pial surface on days E14 and E17 and in the highly vascular ized area around the hippocampal fissure from E19. Within the brain pa renchyma, the microglial cell precursors tended to be located within t he differentiating cell and neuropil layers rather than in the germina tive zones. The late developing dentate gyrus remained almost devoid o f microglial cell precursors before birth. Vimentin-positive astroglia l processes with radial orientation were observed throughout the hippo campal subregions from E17. In contrast, glial fibrillary acidic prote in-positive, radial processes were barely discernible in the fimbria a nd the dentate gyrus before E19. The results are discussed in relation to the possible interactive role of microglial cells in central nervo us tissue development and histogenesis. Regarding the origin of hippoc ampal microglial cell precursors, the present observations support the view that these cells may well originate from different mesodermal so urces depending on time and localization. (C) 1997 Wiley-Liss, Inc.