Elastin and collagen fibers play important roles in the mechanical properties of aortic media. Because knowledge of local fiber structures is required for detailed analysis of blood vessel wall mechanics, we investigated 3D microstructures of elastin and collagen fibers in thoracic aortas and monitored changes during pressurization. Using multiphoton microscopy, autofluorescence images from elastin and second harmonic generation signals from collagen were acquired in media from rabbit thoracic aortas that were stretched biaxially to restore physiological dimensions. Both elastin and collagen fibers were observed in all longitudinal-circumferential plane images, whereas alternate bright and dark layers were observed along the radial direction and were recognized as elastic laminas (ELs) and smooth muscle-rich layers (SMLs), respectively. Elastin and collagen fibers are mainly oriented in the circumferential direction, and waviness of collagen fibers was significantly higher than that of elastin fibers. Collagen fibers were more undulated in longitudinal than in radial direction, whereas undulation of elastin fibers was equibiaxial. Changes in waviness of collagen fibers during pressurization were then evaluated using 2-dimensional fast Fourier transform in mouse aortas, and indices of waviness of collagen fibers decreased with increases in intraluminal pressure. These indices also showed that collagen fibers in SMLs became straight at lower intraluminal pressures than those in EL, indicating that SMLs stretched more than ELs. These results indicate that deformation of the aorta due to pressurization is complicated because of the heterogeneity of tissue layers and differences in elastic properties of ELs, SMLs, and surrounding collagen and elastin.

No abstract

Observations of intracellular tension dynamics of MC3T3-E1 cells during substrate adhesion using a FRET-based actinin tension sensor

The aorta is composed of various constituents with different mechanical properties. This heterogeneous structure implies non-uniform deformation in the aorta, which could affect local cell functions. The present study investigates 3D strains of the aorta at a cell scale induced by intraluminal pressurization. After resected mouse, thoracic aortas were stretched to their in vivo length, and the aortas were pressurized at 15, 40, 80, 120, and 160 mmHg. Images of autofluorescent light of elastin were captured under a two-photon microscope. From the movement of markers in elastic laminas (ELs) created by photo-bleaching, 3D strains (ε θθ , ε zz , ε rr , ε rθ , ε rz , ε θz) between two neighboring ELs in the circumferential (θ), longitudinal (z), and radial (r) directions with reference to the dimensions at 15 mmHg were calculated. The results demonstrated that the average of shear strain ε rθ was almost 0 in a physiological pressure range (from 80 to 120 mmHg) with an absolute value |ε rθ | changing approximately by 5%. This indicates that ELs experience radial-circumferential shear at the cell scale, but not at the whole tissue scale. The normal strains in the circumferential ε θθ and longitudinal direction ε zz were positive but that in the radial direction ε rr was almost 0, which demonstrates that aortic tissue is not an incompressible material. The first principal direction in the radial-circumferential plane was 29° ± 13° from the circumferential direction. We show that the aorta is not simply stretched in the circumferential direction during pressurization and that cells in the aorta undergo complex deformations by nature.

Rupture properties of thoracic aortic aneurysms (TAAs) were measured in vitro in a pressure-imposed test to predict the ultimate stress of TAAs from their mechanical behavior in a physiological pressure range. Each quadrilateral (ca. 20 9 20 mm 2 ) specimen of TAAs or porcine thoracic aortas (PTAs) was pressurized from the inner wall until rupture or up to 4500 mmHg, while its deformation was being monitored. In-plane stress r and strain e of the specimen were calculated using Laplace's law and deformations of the markers drawn on the specimen surface, respectively. Ultimate stress r max and tangent elastic modulus H were determined from the r-e curve as its maximum stress and slope, respectively. The tangent elastic modulus H of PTA specimens tended to increase with the increase in r, while that of TAA specimens tended to reach a plateau in a low-r region. This tendency was confirmed by fitting a function H = C r (1 À exp(Àr/s r )) to the HÀr relation of specimens: The yielding parameter s r was significantly lower in TAAs than PTAs. Furthermore, the logarithm of the parameter s r correlated significantly with r max , for all specimens. These results may indicate that s r is one of the candidate indices for rupture risk estimation.