Cerebral amyloid angiopathy in Alzheimer disease. (a) Small leptomeningeal arteries (SLAs) on the surface of the cerebral cortex show dense deposits of amyloid-beta (Aβ) in their walls, whereas the medium-sized arteries (MLAs) show a more dispersed pattern of deposition of Aβ. A cortical artery with cerebral amyloid angiopathy (CxA) is surrounded by plaques of Aβ in the brain parenchyma (pan-Aβ immunohistochemistry). (b) Branch of a leptomeningeal artery entering the cerebral cortex. Amyloid (red) is in the basement membranes (as outlined by collagen IV staining ) surrounding smooth muscle cells  (dark spaces) in the tunica media (confocal microscopy of Congo red stained preparation). Scale bars = 100 μm (a) and 50 μm (b).