Research reportMicroglia derive from progenitors, originating from the yolk sac, and which proliferate in the brain
Introduction
del Rio-Hortega first suggested that microglia originate from meningeal macrophages penetrating the brain during late embryonic development 36, 37. Data accumulated during the last 15 years convincingly show that microglial cells belong to the hemopoietic system and display macrophage markers 13, 20, 34; however, their time of appearance, site of origin and mode of penetration into the brain rudiment are unclear. A major question is whether embryonic and adult microglia have distinct origins or whether adult microglia are the progeny of embryonic microglia. Indeed, the morphology of embryonic and adult microglia are quite different, the first one being qualified as ameboid, while the adult microglia had been shown to be essentially ramified. It had been reported that, in rodents, post-natal and adult microglia are derived from monocytes which are brought in by the blood circulation from an haemopoietic organ, i.e., foetal liver after mid-gestation or bone marrow during adulthood 24, 35. In contrast, recent reports have shown that, in avians, circulating blood cells do not cross vessel walls and thus do not penetrate into the brain rudiment 5, 19.
We have previously determined that the late embryonic mouse brain contains potential microglial progenitors with a high in vitro proliferative capacity which give rise to cells expressing “classical” macrophage-microglial markers, such as Mac-1, F4/80 and Fc-R [1]. We thus investigated the time of appearance of microglial progenitors in the developing brain and their presence in the potential sites of origin, i.e., the early sites of haemopoietic cell generation, the yolk sac (YS) [28]and the P-Sp/AGM 7, 12, 27, 31. The existence of microglial progenitors with a high proliferative capacity during embryonic development led us to consider that adult microglia might be derived from these microglial progenitors. We therefore also investigated whether microglial cells proliferate in the brain during late embryonic and post-natal development.
We report here that cells with properties of microglial progenitors can be first detected at embryonic day (ED) 8 in the early neural folds. Their number rapidly increases to reach a plateau during late gestation. These progenitors most probably originate from the yolk sac, as they were only found in this haemopoietic site before their appearance in the brain rudiment. We also determined that microglial cells other than progenitors appear during mid-gestation and that the bulk of microglia is born during the first two post-natal weeks. A major finding was the high percentage of mitotic microglial cells during this developmental period, and the presence of these cells throughout the brain parenchyma which thus may account for the adult microglial population. These results are consistent with the hypothesis according to which microglia progenitors derive from yolk sac, colonize the brain rudiment during early developmental stages, and give rise to microglial cells which proliferate in situ and differentiate into adult microglia.
Section snippets
Mice
Mature BALB/c or C57Bl/6 females were caged with breeding males of the same strain and examined for vaginal plug; the day of plug observation was considered as day 0.5 of gestation. Pregnant females were sacrificed by cervical dislocation. Individual embryos were staged according to the number of somite pairs and general morphology 16, 42. The dissection of the YS and intra-embryonic haemogenic sites were performed as described by Godin et al. [12]and Cumano et al.[7]; the brain rudiment was
Microglial progenitors originate from the yolk sac
We previously characterized the microglial progenitors in late embryonic and adult brain as cells that adhere to astroglial monolayers, express macrophage-microglial markers (Mac-1, F4/80 and FC-R) and proliferate vigorously to give colonies containing up to 2×105 cells after 4 to 6 weeks. To determine the nature and origin of these microglial progenitors, it was necessary to determine the first stage at which they can be detected in the brain rudiment. Single cell suspensions of brain rudiment
Discussion
Although most authors agree that microglia belong to the haemopoietic system and thus is of mesodermal origin, at least two questions remain to be answered: when do the first microglial cells appear in the brain rudiment and what is their origin; are terminally differentiated microglia related to embryonic microglia 6, 11, 24? We have previously shown that the late embryonic and adult brain contain cells which have the characteristics of potential microglial progenitors [1], a finding confirmed
Acknowledgements
This work was supported by institutional funds from the Centre National de la Recherche Scientifique (CNRS), and grants from Agence Nationale de Recherche sur le SIDA (ANRS), Fondation de France, Fondation pour la Recherche Médicale and CANAM. We wish to gratefully acknowledge Michel Louette for photographical work and Suzanne Barrey for secretarial assistance.
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