Origin of brain macrophages and the nature of the so-called microglia.

Two aspects of the so-called microglia were studied by silver impregnation and 3H-TdR ARG in light and electron microscopy. (1) So-called microglioblasts are glioblasts differentiated from matrix cells. They are progenitors of the so-called resting microglia as well as of astrocytes and oligodendroglia. (2) Brain macrophages in stab wounds, experimental Japanese encephalitis and retrograde degeneration of the facial nucleus are all found to be of hematogenous origin. Infiltrating hematogenous cells cannot stay permanently in the brain parenchyma unless pathological alterations persist indefinitely.