The developmental origin of microglia remains a controversial subject. While it is generally accepted that primitive fetal macrophages that migrate from the yolk sac to the brain become microglia, it also has been argued that there is a second source of microglia that are of neuroectodermal lineage. To determine whether progenitors in the dorsolateral subventricular zone (SVZDL) are capable of producing microglia as well as macroglia, we infected perinatal rat SVZDL cells with a mixture of two replication-deficient retroviruses, placed these progenitors in vitro and then varied the media formulations to promote microglial differentiation. Mixed macroglial clones were obtained, but no heterogeneous clones containing microglia were observed, regardless of the media components. Among the macroglial clones, we observed every possible combination of type 1 astrocyte and O-2A lineage cells. Some clones were homogeneous and contained cells belonging to a single macroglial lineage. Other clonal clusters were heterogeneous and were comprised of type 1 astrocytes and oligodendrocytes, type 1 and type 2 astrocytes, or type 2 astrocytes and oligodendrocytes. Of 130 clones examined, where we used triple immunofluorescence with antibodies that recognize microglia, 2 clonal clusters contained OX-42+ microglia that were retrovirally labeled, but all of the cells in those clones expressed the microglial marker and none expressed either GFAP or O4. In addition, we isolated neural stem cells from the perinatal SVZDL and assessed their capacity to generate macroglia and microglia. Confirming and extending our previous analyses, neural stem cells generated homogeneous and heterogeneous macroglial clones, but they did not generate microglia. We conclude that brain macroglia and microglia do not share a common precursor, even though the neural stem cells in the SVZDL cells can produce neurons, astrocytes and oligodendrocytes. Therefore, the microglia that reside in the SVZDL are immigrants from nonneural precursors.

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