The Layered Structure of Coronary Adventitia under Mechanical Load

Abstract

The mechanical loading-deformation relation of elastin and collagen fibril bundles is fundamental to understanding the microstructural properties of tissue. Here, we use multiphoton microscopy to obtain quantitative data of elastin and collagen fiber bundles under in situ loading of coronary adventitia. Simultaneous loading-imaging experiments on unstained fresh coronary adventitia allowed morphometric measurements of collagen and elastin fibril bundles and their individual deformation. Fiber data were analyzed at five different distension loading points (circumferential stretch ratio λ(θ) = 1.0, 1.2, 1.4, 1.6, and 1.8) at a physiological axial stretch ratio of λ(axial) = 1.3. Four fiber geometrical parameters were used to quantify the fibers: orientation angle, waviness, width, and area fraction. The results show that elastin and collagen fibers in inner adventitia form concentric densely packed fiber sheets, and the fiber orientation angle, width, and area fraction vary transmurally. The extent of fiber deformation depends on the initial orientation angle at no-distension state (λ(θ) = 1.0 and λ(axial) = 1.3). At higher distension loading, the orientation angle and waviness of fibers decrease linearly, but the width of collagen fiber is relatively constant at λ(θ) = 1.0-1.4 and then decrease linearly for λ(θ) ≥ 1.4. A decrease of the relative dispersion (SD/mean) of collagen fiber waviness suggests a heterogeneous mechanical response to loads. This study provides fundamental microstructural data for coronary artery biomechanics and we consider it seminal for structural models.

A validated 3D microstructure-based constitutive model of coronary artery adventitia

“…Our experimental studies showed that coronary adventitia is divided into outer and inner adventitia, as shown in Fig. 1 (3,6). The outer adventitia, consisting of thicker and wavier collagen bundles and few elastin fibers, supports the vessel and connects with the surrounding tissue rather than significantly resisting the transmural pressure ( Fig.…”

“…The cross sections of the frozen segment were sectioned by a cryostat microtome, mounted on microscope slides, and viewed by an FV1000-MPE multiphoton microscope (MPM; Olympus America, Center Valley, PA), which is equipped with a Mai Tai DeepSee tunable laser (Spectra-Physics, Santa Clara, CA). A combination of second harmonic generation and two-photon excitation fluorescence was set up to detect signals of collagen and elastin fibers simultaneously (3,6,49). The excitation wavelength of the laser was 830 nm, and emission wavelengths for collagen (second harmonic generation) and elastin (second harmonic generation) were 415 and 520 nm, respectively.…”

“…Smooth muscle cells in media predominate active responses of blood vessels, whereas elastin and collagen fibers only contribute to passive mechanical behaviors (5,21). The mechanical properties of individual fibers have been broadly studied in literature (1,13,15,20,44,46), and microstructural parameters and deformation in coronary adventitia have been quantitatively measured in our recent studies (3,6). The mechanical function of ground substance, an amorphous, gel-like material containing glycosaminoglycans (GAGs), proteoglycans, glycoproteins, and fibroblasts, has yet to be tested, although some related studies have been performed for other types of tissues (22,27,31,42,44).…”

“…At physiological pressures, the adventitia is less stiff than the media, whereas the latter serves as the most important mechanical layer to resist luminal pressure. At higher pressures (e.g., hypertension), collagen fibers reach their straightened lengths, and the adventitia becomes a stiff tube to prevent the media from overstretch and rupture (3,6,44,48). Smooth muscle cells in media predominate active responses of blood vessels, whereas elastin and collagen fibers only contribute to passive mechanical behaviors (5, 21).…”

On the Presence of Affine Fibril and Fiber Kinematics in the Mitral Valve Anterior Leaflet

“…However, it has been observed that valvular tissues are largely strain-rate insensitive (63), so that the responses measured here are likely to represent the in vivo response. The SHG imaging systems has been successfully utilized previously in several studies (7,64,65), although it does have resolution limitations and the results presented herein are necessarily limited by the technique's ability to distinguish fiber structures. Overall, our fiber-level results are a good approximation of the ensemble fiber responses, but we cannot rule out local (i.e., submicron scale) nonaffine deformations of individual fibers.…”