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Research Papers

Structural and Functional Differences Between Porcine Aorta and Vena Cava

[+] Author and Article Information
Jeffrey M. Mattson

Department of Mechanical Engineering,
Boston University,
Boston, MA 02215
e-mail: jmattson@bu.edu

Yanhang Zhang

Department of Mechanical Engineering,
Department of Biomedical Engineering,
Boston University,
110 Cummington Mall,
Boston, MA 02215
e-mail: yanhang@bu.edu

1Corresponding author.

Manuscript received December 1, 2016; final manuscript received March 3, 2017; published online June 6, 2017. Assoc. Editor: Kristen Billiar.

J Biomech Eng 139(7), 071007 (Jun 06, 2017) (8 pages) Paper No: BIO-16-1490; doi: 10.1115/1.4036261 History: Received December 01, 2016; Revised March 03, 2017

Elastin and collagen fibers are the major load-bearing extracellular matrix (ECM) constituents of the vascular wall. Arteries function differently than veins in the circulatory system; however as a result from several treatment options, veins are subjected to sudden elevated arterial pressure. It is thus important to recognize the fundamental structure and function differences between a vein and an artery. Our research compared the relationship between biaxial mechanical function and ECM structure of porcine thoracic aorta and inferior vena cava. Our study suggests that aorta contains slightly more elastin than collagen due to the cyclical extensibility, but vena cava contains almost four times more collagen than elastin to maintain integrity. Furthermore, multiphoton imaging of vena cava showed longitudinally oriented elastin and circumferentially oriented collagen that is recruited at supraphysiologic stress, but low levels of strain. However in aorta, elastin is distributed uniformly, and the primarily circumferentially oriented collagen is recruited at higher levels of strain than vena cava. These structural observations support the functional finding that vena cava is highly anisotropic with the longitude being more compliant and the circumference stiffening substantially at low levels of strain. Overall, our research demonstrates that fiber distributions and recruitment should be considered in addition to relative collagen and elastin contents. Also, the importance of accounting for the structural and functional differences between arteries and veins should be taken into account when considering disease treatment options.

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References

Figures

Grahic Jump Location
Fig. 1

Movat's stain of aorta (a) and vena cava ((b) and (c)) from adventitia (left) to intima (right). Scale bars are 100 μm. The adventitia is primarily collagen (yellow) and the media consisted of collagen, elastin (black), smooth muscle (red), and GAGs (blue, appears green due to overlay with yellow). Collagen and elastin assay results were significantly (p < 0.05) different in aorta and vena cava (d). See online for color figure.

Grahic Jump Location
Fig. 2

Average stress–stretch curves (a) from equibiaxial tests of aorta and vena cava with the vena cava being more anisotropic than aorta. Average tangent modulus (b) was determined by taking the derivative of a sixth-order polynomial fit to the stress–stretch curves.

Grahic Jump Location
Fig. 3

Multiphoton images of undeformed media and adventitia for both aorta and vena cava. In vena cava, elastin (green) is generally longitudinally oriented and medial collagen (blue) is generally circumferentially oriented, but these fibers are more uniformly distributed in aorta. Adventitial collagen (blue) is wavy, but straightens with stretch. Circumferential (C) and longitudinal (L) directions are at 0 deg and ±90 deg, respectively. Images are 425 μm × 425 μm. See online for color figure.

Grahic Jump Location
Fig. 4

Multiphoton images of vena cava under mechanical loading. Biaxial grip-to-grip strain at 5% and 10% where applied in both circumferential (C) and longitudinal (L) directions at 0 deg and ±90 deg, respectively. Images are 425 μm × 425 μm. See online for color figure.

Grahic Jump Location
Fig. 5

Straightness parameter (Ps) for adventitial collagen. Vena cava adventitial collagen fibers are recruited immediately with stretch and fully straightened by 10% strain, whereas aorta fibers show a delayed recruitment beginning between 15% and 20% strain. Aorta data from Ref. [18].

Grahic Jump Location
Fig. 6

Average fiber distributions in aorta and vena cava, where the numbers represent grip-to-grip strain for circumferential (C) and longitudinal (L) directions at 0 deg and ±90 deg, respectively. The most striking difference is the longitudinally oriented elastin in vena cava. Aorta collagen fibers are more circumferentially aligned than in vena cava when unloaded, but with stretch, vena cava collagen fibers become more circumferentially aligned. Aorta data from Ref. [18].

Grahic Jump Location
Fig. 7

Average vena cava fiber distributions, where the numbers represent grip-to-grip strain for circumferential (C) and longitudinal (L) directions at 0 deg and ±90 deg, respectively. The circumference was only stretched to 10% strain because of the high degree of anisotropy. Medial elastin (a) remains longitudinally oriented with stretch. Medial collagen (b) becomes more circumferential with stretch, but the distribution widens with greater longitudinal stretch. Adventitial collagen (c) behaves similarly to medial collagen.

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