The exact developmental origin of microglia is still under debate. In the present study we investigated which heamatopoietic tissues and which features of the organotypic brain slice culture promoted microglia ramification. The potential of cells derived from embryonic yolk sac, embryonic aorta-gonad-mesonephros and adult blood monocytes was examined. These tissues were co-cultured with brain slices after the brain slices had first been maintained in vitro for 1 day, 5 days and 9 days. When brain slices had been maintained in culture for 1 day before the donor cells were added, the donor cells took several days to ramify. However, when donor tissues were added to brain slices that had been 5 or 9 days maintained in culture, the donor cells exhibited a ramified morphology within a day. Therefore changes in organotypic brain slices had an effect on the transformation of cells to the microglial morphology. When adult blood monocytes were added to brain slice cultures there was no evidence of any tendency to ramify over 6 days of co-culture. This study did not support the suggestion that microglia cells derive from bone-marrow (BM) cells or from circulating monocytes.

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