CNS macrophages differentially rely on an intronic Csf1r enhancer for their development

The central nervous system hosts parenchymal macrophages, known as microglia, and non-parenchymal macrophages, collectively termed border-associated macrophages (BAMs). Microglia, but not BAMs, were reported to be absent in mice lacking a conserved enhancer: the -intronic regulatory element (FIRE). However, it is unknown whether FIRE deficiency also impacts BAM arrival and/or maintenance Here, we show that macrophages in the ventricular system of the brain, including Kolmer's epiplexus macrophages, are absent in mice. Stromal choroid plexus BAMs are also considerably reduced. During normal development, we demonstrate that intracerebroventricular macrophages arrive from embryonic day 10.5, and can traverse ventricular walls in embryonic slice cultures. In embryos, the arrival of both primitive microglia and intracerebroventricular macrophages was eliminated, whereas the arrival of cephalic mesenchyme and stromal choroid plexus BAMs was only partially restricted. Our results provide new insights into the development and regulation of different CNS macrophage populations.