Perivascular microglial cells. This image illustrates the proximity of microglia to cerebral capillaries in the adult rat hindbrain. A 30-µm rat brain section was stained for the microglial marker IBA1 (red) and major histocompatibility complex (MHC) II (green), which is upregulated in activated microglia but also stains endothelial cells. Nuclei were stained with 4ʹ,6-diamindino-2-phenylindole (DAPI; blue). The white arrow highlights a surveillance microglia and the arrowhead highlights an activated microglia that has increased expression of MHCII. Note the capillary between the two microglia. This image is a maximum intensity projection of a 10-µm segment of the brain slice and was processed for brightness, contrast, and RGB levels. Adapted from Herndon JM, Tome ME and Davis TP. Development and maintenance of the blood-brain barrier. In: LR Caplan, J Biller, MC Leary, et al. (eds) Primer on cerebrovascular diseases, 2nd ed. San Diego: Elsevier Academic Press, 2017.