In mice. https://www.biorxiv.org/content/10.1101/2021.08.02.454769v1 TL;DR: microglia can influence cerebral vasculature. (Paragraphs mine for legibility) Microglia are brain-resident immune cells with a repertoire of functions in the developing, mature and pathological brain. Their wide-ranging roles in physiology include the clearance of cellular debris, elimination of excess synapses, regulation of neuronal activity and contributions to blood vessel development. Despite these known roles for microglia, the extent of their interactions with the vasculature and potential regulation of vascular physiology has been insufficiently explored. Here, using in vivo acute and longitudinal two-photon imaging in transgenic mice combined with electron microscopy, fixed tissue immunohistochemistry, pharmacological treatments and laser speckle imaging, we document the steady-state interactions between ramified CX3CR1+ myeloid cell somata and capillaries in the brain. We first confirm that these myeloid cells are bona fide microglia by molecular, morphological and ultrastructural approaches. Then we give a detailed spatio-temporal characterization of these capillary-associated microglia (CAMs) comparing and contrasting them with parenchymal microglia (PCMs) in their static, dynamic and chronic morphological activities including during microglial depletion and repopulation. Molecularly, we identify microglial-specific purinergic P2RY12 receptors as a receptor regulating CAM interactions under the control of released purines from pannexin 1 (PANX1) channels. Furthermore, to elucidate roles for microglia in vascular structure and function, we eliminated microglia and showed that this triggered capillary dilation, blood flow increase, and impaired vasodilative responses. We find that P2RY12-/- and PANX1-/- mice recapitulate these vascular impairments suggesting purines released through PANX1 channels play important roles in activating microglial P2RY12 receptors to regulate neurovascular structure and function.