Are Resting and/or Reactive Microglia Macrophages?

Abstract

According to recent submicroscopic, cytokinetic, and functional (particularly cytoimmunologic) investigations, no relationship exists between «resting» microglia (the small argyrophilic cells appearing in undamaged brain tissue, first described by RIO HORTEGA) and «reactive» microglia (the argyrophilic cells appearing under pathologic conditions). While «resting» microglia are apparently cells of neuro-ectodermal origin, all observations tend to indicate that «reactive» microglia are derived from extravasated blood monocytes and should be called brain macrophages. In the intact brain parenchyma, no macrophages are demonstrable. Free subarachnoidal cells in the cerebrospinal fluid (CSF), perivascular cells, and epiplexus and/or supraependymal cells in the CSF-containing spaces of the normal central nervous system are cells of the mononuclear phagocyte system and must be considered as CSF macrophages. According to rough estimates, the normal adult central nervous system contains a maximum of 280,000 CSF macrophages.