Differences in fibrillar collagen composition and packing appear to determine biaxial mechanical properties of the graft and remodeled adjacent aorta. (A) Representative images of Picrosirius Red (PSR) staining (20X magnification) for the adjacent aorta and TEVG at 3, 6, and 12 months. L indicates the lumen. (B) The relative area fraction of collagen fiber birefringence colors, as quantified by PSR colorimetric analysis, revealed a significantly greater proportion of thicker, more densely packed collagen fibers (red and orange) in the adjacent aorta at 6 months. These fibers appeared to turn over by 12 months, with the adjacent aorta achieving a fiber distribution and density similar to that at 3 months. In contrast, no such variation in fiber thickness and packing was observed in the luminal and adventitial neotissue deposited within the graft. These data are consistent with the observed biaxial mechanical behaviors of both the graft and the adjacent IAA presented in Figures 6A and 6B. Values are mean ± SEM. *p < 0.05, **p < 0.005, ns indicates not significant.