The distribution and appearance of microglial cell precursors in the prenatal hippocampus were examined in embryonic day 14 (E14) to E21 rats by nucleoside diphosphatase histochemistry. For comparison, the differentiation of astroglial cells was analyzed from E17 by vimentin and glial fibrillary acidic protein immunohistochemistry.
Based on morphologic features, nucleoside diphosphatase-positive microglial cell precursors were classified as ameboid microglial cells and primitive ramified microglial cells. Ameboid microglia were present in the hippocampal primordium on E14. As the hippocampus developed, however, ameboid microglia gradually transformed into primitive ramified microglia, first recognized at E19. Microglial cell precursors, often related to nucleoside diphosphataselabeled blood vessels, were particularly observed next to the pial surface on days E14 and E17 and in the highly vascularized area around the hippocampal fissure from E19. Within the brain parenchyma, the microglial cell precursors tended to be located within the differentiating cell and neuropil layers rather than in the germinative zones. The late developing dentate gyrus remained almost devoid of microglial cell precursors before birth. Vimentin-positive astroglial processes with radial orientation were observed throughout the hippocampal subregions from E17. In contrast, glial fibrillary acidic protein-positive, radial processes were barely discernible in the fimbria and the dentate gyrus before E19.
The results are discussed in relation to the possible interactive role of microglial cells in central nervous tissue development and histogenesis. Regarding the origin of hippocampal microglial cell precursors, the present observations support the view that these cells may well originate from different mesodermal sources depending on time and localization.