Detection of CFSE-labeled monocytes in brains of day 10 BDR mice. CFSE-labeled monocytes were observed within the parenchyma, most prominently in the periventricular and perivascular regions, in brains of day 10 recipient BDR mice. A, CFSE-labeled monocytes present in the lumen of the choroid plexus and also within the cerebral parenchymal area surrounding the choroid plexus. B, CFSE-labeled monocytes present in the cerebral parenchyma in the subfornical organ region of the brain (both the choroid plexus and the subfornical organ are potential routes used for leukocyte transmigration into brain parenchyma). C, Immunohistochemical staining for vWF (labels cerebral vascular endothelium) demonstrated that CFSE-labeled monocytes were present within the cerebral parenchyma (closed arrow) in areas clearly distinct from cerebral blood vessels, and were also observed in close proximity to cerebral blood vessels (open arrow). A, B, C, Brain sections from n = 3 day 10 BDR mice. C, A blood vessel in the periventricular region in a day 10 BDR mouse. Scale bars: A, C, 5 μm; B, 20 μm. BV, Blood vessel.