Elsevier

Developmental Brain Research

Volume 117, Issue 2, 18 November 1999, Pages 145-152
Developmental Brain Research

Research report
Microglia derive from progenitors, originating from the yolk sac, and which proliferate in the brain

https://doi.org/10.1016/S0165-3806(99)00113-3Get rights and content

Abstract

Microglia, the resident CNS macrophages, represent about 10% of the adult brain cell population. Although described a long time ago, their origin and developmental lineage is still debated. While del Rio-Hortega suggested that microglia originate from meningeal macrophages penetrating the brain during embryonic development, many authors claim that brain parenchymal microglia derive from circulating blood monocytes originating from bone marrow. We have previously reported that the late embryonic and adult mouse brain parenchyma contains potential microglial progenitors [F. Alliot, E. Lecain, B. Grima, B. Pessac, Microglial progenitors with a high proliferative capacity in the embryonic and the adult mouse brain, Proc. Natl. Acad. Sci. U.S.A. 88 (1991) 1541–1545]. We now report that they can be detected in the brain rudiment from embryonic day 8, after their appearance in the yolk sac and that their number increases until late gestation. We also show that microglia appear during embryonic development and that their number increases steadily during the first two postnatal weeks, when about 95% of microglia are born. Finally, the main finding of this study is that microglia is the result of in situ proliferation, as shown by the high proportion of parenchymal microglial cells that express PCNA, a marker of cell multiplication, in embryonic and postnatal brain. Taken together, our data support the hypothesis that terminally differentiated brain parenchymal microglia are derived from cells originating from the yolk sac whose progeny actively proliferates in situ during development.

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.

References (46)

  • A.L Medvinsky et al.

    Definitive hematopoiesis is autonomously initiated by the AGM region

    Cell

    (1996)
  • S.C Moore et al.

    Phenotypes and alloantigen-presenting activity of individual clones of microglia derived from the mouse brain

    J. Neuroimmunol.

    (1992)
  • E.B Pedersen et al.

    Enriched immune-environment of blood–brain barrier deficient areas of normal adult rats

    J. Neuroimmunol.

    (1997)
  • V.H Perry et al.

    Immunohistochemical localization of macrophages and microglia in the adult and developing mouse brain

    Neuroscience

    (1985)
  • E.R Stanley et al.

    Factors regulating macrophage production and growth

    J. Biol. Chem.

    (1977)
  • K Takahashi et al.

    Differentiation, maturation and proliferation of macrophages in the mouse yolk sac: a light microscopic, enzyme-cytochemical, immunohistochemical, and ultrastructural study

    J. Leukoc. Biol.

    (1989)
  • R.J Tushinski et al.

    Survival of mononuclear phagocytes depends on a lineage specific growth factor that the differentiated cells selectively destroy

    Cell

    (1982)
  • W.S Walker

    Separate precursor cells for macrophages and microglia in mouse brain: immunophenotypic and immunoregulatory properties of the progeny

    J. Neuroimmunol.

    (1999)
  • F Alliot et al.

    Microglial progenitors with a high proliferative capacity in the embryonic and the adult mouse brain

    Proc. Natl. Acad. Sci. U.S.A.

    (1991)
  • P.H Cossmann et al.

    Mesoderm-derived cells proliferate in the embryonic central nervous system: confocal microscopy and three-dimensional visualization

    Histochem. Cell. Biol.

    (1997)
  • M.A Cuadros et al.

    First appearance, distribution and origin of macrophages in the early development of the avian central nervous system

    J. Comp. Neurol.

    (1993)
  • C.J.A De Groot et al.

    Determination of the origin and nature of brain macrophages and microglial cells in mouse central nervous system, using non-radioactive in situ hybridization and immunoperoxidase techniques

    Glia

    (1992)
  • L Faff et al.

    Intracellular pH regulation in cultured microglial cells from mouse brain

    J. Neurosci. Res.

    (1996)
  • Cited by (0)

    View full text