Microglia: origin and development

As suggested by Del Rio Ortega a long time ago, it is now widely accepted t hat microglia are the resident macrophages of the central nervous system. M icroglia represent about 10 % of the adult brain cell population. We have p reviously shown that the lair embryonic and adult mouse brain contain poten tial microglial progenitors. We report here that microglial progenitors can be detected in neural folds from embryonic day 8 8. They originate from th e 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 inte nse in situ proliferation comparable to that of neural cells. Taken togethe r these results show: that adult mouse microglia originate from cells migra ting from the yolk sac and whose progeny actively proliferates in the brain during development.