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Development of microglia in the prenatal rat hippocampus
1Department of Cell Biology and Physiology, Faculty of Medicine, Autonomous University of Barcelona, Bellaterra, Spain.
Abstract:
The distribution and appearance of microglia 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 diphosphatase-labeled 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.
Insights
Microglial cell precursors appear in prenatal rat hippocampi by embryonic day 14 (E14), transforming from ameboid to primitive ramified forms. Their distribution suggests mesodermal origins and roles in central nervous tissue development.
Area of Science:
- Developmental Neuroscience
- Neurohistology
- Cell Biology
Background:
- Microglia are the primary immune cells of the central nervous system.
- Understanding microglial development is crucial for comprehending brain development and disease.
- The prenatal origins and differentiation of microglia in the hippocampus remain incompletely understood.
Purpose of the Study:
- To investigate the distribution and morphology of microglial cell precursors in the developing rat hippocampus.
- To compare microglial precursor development with astroglial differentiation.
- To explore the potential origins and roles of early hippocampal microglia.
Main Methods:
- Nucleoside diphosphatase histochemistry was used to identify microglial cell precursors from embryonic day 14 (E14) to E21.
- Vimentin and glial fibrillary acidic protein (GFAP) immunohistochemistry were employed to analyze astroglial differentiation from E17.
- Morphological classification distinguished between ameboid and primitive ramified microglial forms.
Main Results:
- Ameboid microglial precursors were present from E14, transitioning to primitive ramified forms by E19.
- Microglial precursors were associated with blood vessels, particularly near the pial surface and hippocampal fissure.
- Precursors were found in differentiating cell and neuropil layers, avoiding germinative zones; the dentate gyrus was largely devoid of them prenatally.
Conclusions:
- Hippocampal microglial precursors exhibit distinct morphological changes and spatial distribution during prenatal development.
- Astroglial differentiation follows a different temporal and spatial pattern compared to microglial precursors.
- Findings support the hypothesis that hippocampal microglial precursors originate from diverse mesodermal sources.