Activation and Deactivation of Periventricular White Matter Phagocytes During Postnatal Mouse Development

Brain microglia are related to peripheral macrophages but undergo a highly specific process of regional maturation and differentiation inside the brain. Here, we examined this deactivation and morphological differentiation in cerebral cortex and periventricular subcortical white matter, the main "fountain of microglia" site, during postnatal mouse development, 0-28 days after birth (P0-P28). Only macrophages in subcortical white matter but not cortical microglia exhibited strong expression of typical activation markers alpha5, alpha6, alphaM, alphaX, and beta2 integrin subunits and B7.2 at any postnatal time point studied. White matter phagocyte activation was maximal at P0, decreased linearly over P3 and P7 and disappeared at P10. P7 white matter phagocytes also expressed high levels of IGF1 and MCSF, but not TNFalpha mRNA; this expression disappeared at P14. This process of deactivation followed the presence of ingested phagocytic material but correlated only moderately with ramification, and not with the extent of TUNEL+ death in neighboring cells, their ingestion or microglial proliferation. Intravenous fluosphere labeling revealed postnatal recruitment and transformation of circulating leukocytes into meningeal and perivascular macrophages as well as into ramified cortical microglia, but bypassing the white matter areas. In conclusion, this study describes strong and selective activation of postnatally resident phagocytes in the P0-P7 subcortical white matter, roughly equivalent to mid 3rd trimester human fetal development. This presence of highly active and IGF1- and MCSF-expressing phagocytes in the neighborhood of vulnerable white matter could play an important role in the genesis of or protection against axonal damage in the fetus and premature neonate.