Distribution and morphology of GFAP-positive astrocytes in the human fetal brain at second trimester

Background: Various studies have shown that a close physical and functional relationship exists between astrocytes and microglia during development. The maturation of astrocytes can be followed by their specific expression of glial fibrillary acidic protein (GFAP). Although there have been several reports on the expression of GFAP in the developing brain, these are mainly descriptive and have not defined clearly the regional distribution patterns of these cells during fetal development. Aim: This study set out to analyse the regional distribution and morphology of GFAP-expressing astrocytes during the latter half of the second trimester (19-23 gestational weeks) within five normal fetal brains, with a view towards comparing the findings with the microglial distribution and differentiation previously recorded throughout the same period. Results: A clear account of the morphology and regional distribution of GFAP-positive astrocytes is presented. Differentiating astrocytes (including radial glia and their processes) were found in the germinal layers, corpus callosum and cavum septum pellucidum, within neural tracts and surrounding the basal ganglia - areas known to be populated by microglia during the same period. The differentiation of astrocytes was predominant within the subplate and intermediate zone towards the end of the second trimester. Importantly, the differentiation of astrocytes within these regions followed a similar spatially interspersed pattern as reported for fetal microglia. However, astrocytes appeared to differentiate at a later stage within these regions than described for microglia. This will require further investigations for confirmation. Differentiated astrocytes were also found to associate with cortical blood vessels in a region-specific manner during the second trimester. Conclusions: These observations indicate that the differentiation and spatial distribution of astrocytes parallels those of microglia during the second trimester. It remains to be seen whether factors released by microglia can directly influence the differentiation of astrocytes in situ during development