Figure 2.
A bilayered analysis of biaxial wall stress (panels A-B, E-F) and material stiffness (panels C-D, G-H) confirmed that the media carries most of the load in the normal aorta under physiological conditions, which allows medial elastin to store elastic energy. Interestingly, the same is true in TSP2-null aortas. For simulated acute increases in pressure (e.g., 15, 30, or 45% elevations in systolic pressure Psys), the adventitia carries increasingly more load in the normal aorta, thus serving as a protective sheath (panels I,K). In the absence of TSP2, however, the media must bear most of the load since the adventitia fails to provide stress shielding (panels J,L). Consistent with numerical predictions, PSR staining reveals differences in adventitial collagen between WT (panels M-N, 400ms & 100ms exposure) and Thbs2−/− (panels O-P, 400ms & 100ms exposure) mice; there is a higher content of red collagen fibers and lower content of green, yellow, and orange fibers in the absence of TSP2 at 40 weeks of age. Note that the dotted lines in the circumferential stress plots (panels A, E) represent the Laplace stress (i.e. the mean value of circumferential stress that comes from the universal solution of a pressurized tube, independent of the specific material response), which serves as a convenient check on the predictions of the bilayered stress analysis. The dotted lines in the linearized stiffness plots (panels C-D, G-H) indicate the integral mean, which can be compared with the average values obtained from the 2-D analysis.