Notch signaling is required for arterial maturation in vivo. Representative image of P6 retinas, following (A) vehicle (DMSO) or (B) γ-secretase inhibitor (DAPT) treatment to block Notch signaling from P2-P5, immunostained with Abs specific to CD31 (red) and αSMA (green). DAPT treatment results in an immature vascular plexus, represented by enhanced vascular branching (arrows) and reduced αSMA⁺ perivascular cell coverage of arteries (arrowheads). (C) Vehicle-treated pups show continuous VSMC coverage of arteries, marked by αSMA, unlike (D) DAPT-treated pups where individual VSMCs are clearly visible (arrowhead) and arterial coverage is interrupted (arrows). (E) DAPT treatment prevents arterial maturation, measured as a 40% decrease in arterial VSMC (αSMA⁺) coverage compared with controls. Values represent means ± SEM. DMSO (n = 17), DAPT (n = 25), where n is the number of arteries analyzed. P = .004. Scale bars: (A-B) 600 μm; (C-D) 25 μm.