SHEAR STRESS-INDUCED REORGANIZATION OF THE SURFACE-TOPOGRAPHY OF LIVING ENDOTHELIAL-CELLS IMAGED BY ATOMIC-FORCE MICROSCOPY

We report the first topographical data of the surface of living endoth elial cells at sub-light-microscopic resolution, measurements essentia l for a detailed understanding of force distribution in the endotheliu m subjected to flow. Atomic force microscopy was used to observe the s urface topography of living endothelial cells in confluent monolayers maintained in static culture or subjected to unidirectional shear stre ss in laminar flow (12 dyne/cm(2) for 24 hours). The surface of polygo nal unsheared cells was smooth, with mean excursion of surface undulat ion between peak height (over the nucleus) and minima (at intercellula r junctions) of 3.4 +/- 0.7 I-Lm (mean +/- SD); the mean height to len gth ratio was 0.11 +/- 0.02. In cells that were aligned in the directi on of flow after a 24-hour exposure to laminar shear stress, height di fferentials were significantly reduced (mean, 1.8 +/- 0.5 mu m), and t he mean height to length ratio was 0.045 +/- 0.009. Calculation of max imum shear stress and maximum gradient of shear stress (partial deriva tive tau/partial derivative x, where tau is shear stress at the cell s urface) at constant flow velocity revealed substantial streamlining of aligned cells that reduced partial derivative tau/partial derivative x by more than 50% at a nominal shear stress of 10 dyne/cm(2). Aligned cells exhibited ridges extending in the direction of flow that repres ented imprints of submembranous F-actin stress-fiber bundles mechanica lly coupled to the cell membrane. The surface ridges, approximate to 5 0 nm in height and 200 to 1000 nm in width, were particularly prominen t in the periphery of the aligned cells. These observations (1) repres ent the first measurement of endothelial surface topography in living cells, (2) demonstrate significant changes in surface topography as a result of exposure to hemodynamic forces, probably as a result of subm embranous cytoskeletal reorganization, and (3) facilitate computation of detailed cell-surface force distribution.