[Microglia: origin and development]

As suggested by Del Rio Ortega a long time ago, it is now widely accepted that microglia are the resident macrophages of the central nervous system. Microglia represent about 10% of the adult brain cell population. We have previously shown that the late embryonic and adult mouse brain contain potential microglial progenitors. We report here that microglial progenitors can be detected in neural folds from embryonic day 8. They originate from the yolk sac in which macrophage progenitors are found from embryonic day 7. We also report that the bulk of microglial cells (about 95%) appear during post-natal development. A major finding is that microglia arise by an intense in situ proliferation comparable to that of neural cells. Taken together, these results show that adult mouse microglia originate from cells migrating from the yolk sac and whose progeny actively proliferates in the brain during development.