a,b, Cerebral infarct volume (A, white area in each slice) was measured 3 days after permanent MCA occlusion. c, Pial collaterals (black arrows) after dilation, fixation and filling with contrast material. d, Native collateral diameter is smaller in aged mice (non-occluded). Aging impairs collateral remodeling (% increase is relative to 3-months-old, p=0.03); inset, change in diameter. White bar, 3-months-old; black bar, 24-months-old.