$336 Journal o f Biomechanics 2006, Vol. 39 (Suppl 1) Oral Presentations

Supported by NIH grant R01 HL076499.

4176 Tu, 15:15-15:30 (P22) Permeabi l i ty o f col lo idal part ic les in a f iber matr ix

X.'~ Chen 1,2, '~ Liu 1 , J.M. Yang 2. 1Department efMechanical Engineering, The Hong Kong Polytechnic University, Hung Hem, Kowloon, Hong Kong, 2. School of Engineering, University of Science and Technology of China, Hefei, China

Surface glycocalyx, as a barrier to material exchange between circulating blood and body tissues, was always treated as a periodic square array of cylindrical fibers. Previous study treated the glycocalyx as porous media and simulated by continuum theory. However, it has recently been found that a relatively hexagonal fibre-matrix structure may be responsible for the ultrafiltration properties of microvascular walls Squire et al. 2001). The fibre- matrix is an underlaying three-dimensional meshwork with a fibre diameter of 10-12 nm and characteristic spacing of about 20 nm. To study the permeable characteristics of nanosize particle in such fibre-matrix structure, the porous medium assumption may not be appropriate. Molecular dynamics (MD) simulation is a powerful method to simulate the fluid flow at the molecular level, it has successfully been applied in many fields including hydrodynamics and demonstrated surprising results at the nanoscales different from their macroscopic counterparts. Here we use MD to investigate the permeable characteristics of nano-particle in the new quasi- periodic ultra-structure of the endothelial glycocalyx. As a first attempt, fibre- matrix is simplified as a two dimensional periodic system in which the colloidal particles, fluid solvent, fibers are all treated as atomic systems, and the study is focused on the effect of particle size on transport properties. Acknowledgement: Support given by the Research Grants Council of the Government of the HKSAR under Grant Nos. PolyU 5273/04E and PolyU 5221/05E is gratefully acknowledged.

References [1] Squire JM, Chew M, Nneji G, Neal C, Barry J, and Michel C. J. Struc. Biology,

2001; 136, 239-255.

4117 Tu, 15:30-15:45 (P22) Components o f the endothel ia l cell g lycoca lyx mediate mechanot ransduct ion M.'~ Pahakis, J.R. Kosky, J.M. Tarbell. Department efBiemedical Engineering, The City College of New York, New York, NY, USA

The mechanisms by which endothelial cells (ECs) sense the external me- chanical forces imposed by blood flow, and sequentially transduce these signals into intracellular biochemical responses with vasoregulating properties, are of great interest in cardiovascular physiology and pathophysiology. This study was designed to test the hypothesis that the glycocalyx serves as a fluid shear stress sensor/transducer on the surface of ECs, and that certain components of it are crucial to this function. More specifically, the separate glycocalyx components examined in this study were: a) heparan sulfate, b) chondroitin sulfate, c) hyaluronic acid and d) sialic acid. In order to test their mechanotransducing properties, each of them was enzymatically digested from the surface of bovine aortic ECs, and the partially depleted cells were exposed to shear stress (20dyne/cm2). Nitric oxide (NO) and prostacyclin (PGI2) were measured as endpoint indicators of mechanotransduction, given their characteristic roles as vasotonic EC products in response to mechanical stimuli. It was observed that the significant production of NO under shear stress was inhibited by the separate pretreatment with heparanase, hyaluronidase, and neuraminidase, but not chondroitinase. Immunohistochemistry imaging and assay techniques verified that the enzymatic treatments were effective in removing a substantial fraction of their target components without significantly affecting any of the others. Shear induced PGI2 production was unaffected by any of these enzymes, regardless of the dose. These results indicate that heparan sulfate, hyaluronic acid and sialic acid components of the EC glycocalyx contribute to mechanotransduction that mediates NO production in response to shear stress. The lack of influence of these treatments on PGI2 production suggests a transduction site remote from the apical surface.

15.5. Lymphatic Biomechanics and Tissue Stresses 5701 Tu, 16:00-16:15 (P25) Interstit ial f low and au to logous chemotax is

M.A. Swartz, M.E. Fleury, J.D. Shields. Institute of Bioengineering, Ecole Polytechnique F#d6rale de Lausanne, Switzerland

Chemotaxis of cells is driven by gradients of chemokines such that cells migrate up the gradient, towards higher concentrations. These gradients are typically assumed to be generated by other downstream cells, which are created by diffusion. However, we hypothesized that pericellular gradients

of cell-secreted chemokines may be drastically influenced by slow interstitial fluid flow, which is present to some degree in all living tissues, for example the flow that results from slow-draining lymph, which drives interstitial flow velocities in from the range of 0.1-2.0 ~trn/s. In addition, many chemokines are secreted in precursor forms that contain extracellular matrix (ECM) binding motifs with affinity for ECM components such as sulfated proteoglycans. These bound chemokines can later be converted to a soluble form by cell-mediated proteolysis. Here we investigate the role of interstitial fluid convection on pericellular gradi- ents as well as explore the ECM binding characteristics of certain chemokines on such gradients. Through computational modeling, we show that these two factors - flow and ECM binding of chemokines - actually synergize to create and amplify pericellular morphogen and chemokine gradients, even when Peclet numbers are very small. We then explore an in vitro model of cell chemotaxis through a 3D ECM under very low levels of interstitial flow. We used a tumor cell model that secretes CCL21, a chemokine known to be important for lymphocyte homing to lymphatics as well as more recently tumor cell metastasis to lymph nodes. Under interstitial flow, these tumor cells migrate fivefold higher in the direction of flow; however, this flow-enhanced migration can be completely blocked by blocking CCL21 activity (or the ligation of its receptor CCR7). Therefore, as the flow-enhanced cell migration is entirely mediated through chemokines, it supports our computational model. These results demonstrate a novel mechanism for cell homing to lymphatic capillaries, which drain tissue fluid and thus create interstitial flow field always directed towards them. They also suggest a more generalizable mechanism of flow-enhanced morphogenesis with ECM-binding morphogens such as VEGF and bFGE

6330 Tu, 16:30-16:45 (P25) Estimating wall shear stress in contract ing mesenter ic micro lymphat ics

J.B. Dixon 1 , G. Cote 1 , A. Gashev 2, J. Moore 1 , D. Zaweija 2. 1Biomed. Eng., TAMU, College Station, TX, USA, 2Med. Phys., TAMUS Health Science Center, College Station, TX, USA

The lymphatic system plays important roles in maintaining tissue homeostasis, lipid transport, and immune cell trafficking. Historically the driving force of flow in this system has been divided into the intrinsic and extrinsic pump, which work to promote flow from the initial lymphatics in the interstitial spaces, through the collecting lymphatics, to the large transport lymph ducts. The primary mechanism behind the intrinsic pump is the spontaneous contraction of the lymphatic wall of collecting lymphatics in conjunction with valves located roughly every 1-2 millimeters. Lymphatic contractile activity is inhibited by flow in isolated lymphatics presumably through wall shear stress, however there are virtually no in situ measurements of lymph flow in these vessels. We determined lymph fluid velocities across multiple contraction cycles in several rats by tracking lymphocyte movement while simultaneous measuring vessel diameter. Lymph velocities ranged from -2.5 mm/sec to 9.0 mm/sec with the average velocity being 0.9 mm/sec. The average diameter of the contracting lymphatics was 90 ~tm and the RMS contraction velocity was 43 ~tm/sec. From these values, we estimated the wall shear stress experienced throughout the contraction cycle. Given the low Wormersley parameter and low Reynolds number, we calculated the shear stress at an instant in time for the given vessel diameter and particle velocity using a Poiseuille flow model. Estimates of the wall shear averaged 0.6 dynes/cm 2 with maximum shear stresses approaching 12.0 dynes/cm 2 . We also determined portions of the contraction cycle at which Poiseuille flow is not a reasonable approximation and have found that this is the case roughly 20% of the time. However, Poiseuille flow is valid during the periods of observed maximum shear stress, which is the most relevant case for investigating the mechanisms behind the flow inhibition phenomenon reported on previously. Support: Whitaker Foundation Special Opportunity Grant, NIH HL-075199 and HL-070308.

5569 Tu, 16:45-17:00 (P25) Pressure regulat ion in the cochlear per i lymphat ic low pressure system under the inf luence o f s tat ionary and dynamic vo lume processes

E.J. Haberland 1 , H.J. Neumann 2. 1Dept. e fENT and HNS, University ef Halle, Germany, 2Dept. ef ENT and HNS, City Hospital Martha-Maria, Halle, Germany

The bony labyrinth contains perilymph (PL). The PL system is a low-pressure system with a weak pressure homeostasis. Nevertheless, an exchange of PL that is modulated by stationary and dynamic volume processes takes place via connecting canals. The individual influences on the sequential pattern of pressure have been investigated in an experimental study in guinea pigs. Here, the pressure-dependence of individual regulation phenomena is described and a hydrodynamic model of the entire system is presented.