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.