Hsiao-Ying Shadow Huang Engineered Tissue Mechanics Laboratory, Departments of Bioengineering and Mechanical Engineering, University of Pittsburgh, Pittsburgh, PA 15219 Jun Liao Michael S. Sacks 1 Ph.D. e-mail: msacks@pitt.edu Engineered Tissue Mechanics Laboratory, Department of Bioengineering, University of Pittsburgh, Pittsburgh, PA 15219 In-Situ Deformation of the Aortic Valve Interstitial Cell Nucleus Under Diastolic Loading Within the aortic valve (AV) leaflet resides a population of interstitial cells (AVICs), which serve to maintain tissue structural integrity via protein synthesis and enzymatic degradation. AVICs are typically characterized as myofibroblasts, exhibit phenotypic plasticity, and may play an important role in valve pathophysiology. While it is known that AVICs can respond to mechanical stimuli in vitro, the level of in vivo AVIC defor- mation and its relation to local collagen fiber reorientation during the cardiac cycle remain unknown. In the present study, the deformation of AVICs was investigated using porcine AV glutaraldehyde fixed under 0 – 90 mm Hg transvalvular pressures. The result- ing change in nuclear aspect ratio (NAR) was used as an index of overall cellular strain, and dependencies on spatial location and pressure loading levels quantified. Local col- lagen fiber alignment in the same valves was also quantified using small angle light scattering. A tissue-level finite element (FE) model of an AVIC embedded in the AV extracellular matrix was also used explore the relation between AV tissue- and cellular- level deformations. Results indicated large, consistent increases in AVIC NAR with trans- valvular pressure (e.g., from mean of 1.8 at 0 mm Hg to a mean of 4.8 at 90 mm Hg), as well as pronounced layer specific dependencies. Associated changes in collagen fiber alignment indicated that little AVIC deformation occurs with the large amount of fiber straightening for pressures below 1 mm Hg, followed by substantial increases in AVIC NAR from 4 mm Hg to 90 mm Hg. While the tissue-level FE model was able to capture the qualitative response, it also underpredicted the extent of AVIC deformation. This result suggested that additional micromechanical and fiber-compaction effects occur at high pressure levels. The results of this study form the basis of understanding transval- vular pressure-mediated mechanotransduction within the native AV and first time quan- titative data correlating AVIC nuclei deformation with AV tissue microstructure and deformation. DOI: 10.1115/1.2801670 Keywords: extracellular matrix, fiber architecture, nucleus aspect ratio, finite element method, statistics Introduction Aortic valve AV leaflet interstitial cells AVICs are a hetero- geneous group with characteristics of smooth muscle and fibro- blasts i.e., myofibroblasts. As the most numerous AV cell type, the AVIC population constitutes 30% volumetric density in mice leaflets 1, and in AVIC isolated from human leaflets, 78% expressed smooth muscle -actin 2. Their unique profiles of cell-cell and cell-ECM adhesion molecule expression 3,4, as well as the observation that age-related decreases in AVIC number accompany collagen fiber degeneration 5, suggest that AVICs are responsible for maintaining the valvular extracellular matrix ECM. Recently, Merryman et al. 6 measured the stiffness of interstitial cells isolated from the leaflets of all four heart valves using micropipette aspiration and correlated it with smooth muscle -actin and heat shock protein-47 HSP47, a measure of collagen biosynthetic activity. Results suggested that interstitial cells respond to local tissue stress by altering cellular stiffness and biosynthetic levels. At the tissue level, Chester et al. 7 exposed strips of AV leaf- lets to high KCl 90 mM and endothelin levels under uniaxial tension, with both treatments generating modest force levels cir- cumferential direction: 0.31–0.66 mN, radial direction: 0.11–0.23 mN. From these results, it was speculated that these AVIC generated forces may subtly modulate leaflet motion during the opening/closing phases. Based on these novel findings, our laboratory utilized bidirectional flexure to reveal insights into how the AVIC population can alter native leaflet stiffness at the low strain levels experienced in bending 6. These results indicated that changes in AVIC stiffness depended on bending direction and thus indicated layer specific behaviors. Further, a significant basal tone was observed and quantified for the first time. The results of both studies indicate that while the AVIC mechanical contribution to the leaflet tissue biomechanical behavior is negligible, AVICs are clearly tightly bonded to the surrounding ECM. As observed in vascular smooth muscle cells 8, we speculated that the con- tractile properties of AVICs are related to their role in managing ECM formation and strongly influenced by the local stress envi- ronments of the valve leaflet. We have recently demonstrated that AV layers have substan- tially different mechanical properties, partly due to their different structures 9 and the presence of residual strains 10,11. These results suggest that AVICs located in different layers may be ex- posed to different strains during the cardiac cycle. This led us to speculate that tissue-level structural changes observed under in- creasing transvalvular pressure e.g., collagen fiber rotation, straightening, and compaction 9 transduce concomitant strains to the embedded AVICs. Quantitative knowledge of how AVICs respond to tissue-level stress is thus a necessary step in under- standing and modeling how organ level stresses are translated to cellular-level events. Yet, how the valvular tissue ECM strains are 1 Corresponding author. Contributed by the Bioengineering Division of ASME for publication in the JOUR- NAL OF BIOMECHANICAL ENGINEERING. Manuscript received September 3, 2006; final manuscript received April 19, 2007. Review conducted by Cheng Dong. 880 / Vol. 129, DECEMBER 2007 Copyright © 2007 by ASME Transactions of the ASME Downloaded 11 Dec 2007 to 18.82.1.32. Redistribution subject to ASME license or copyright; see http://www.asme.org/terms/Terms_Use.cfm