Sketch on physiological relevance of adhesive microvilli.

<p>Geometrical considerations illustrate the impact of microvilli on binding probability of leukocytes. General assumptions are: ICAM-1 clusters (<i>adhesive platforms</i>, <i>EAP</i>) cover 10% of the endothelial surface and are equally distributed at mean distance of 2b = 2.5 μm, and leukocytes have smooth, spherical shape, radius a = 5 μm. (<b>A</b>) In a simple model of a flat endothelial surface, the chance of a leukocyte to meet an EAP upon first contact just equals the ICAM-1 surface coverage (10%). (<b>B</b>) With an arbitrary convolvement of membrane surface as in real cells (cytoskeletal roughness), many EAPs become inaccessible for leukocytes. The binding probability decreases close to zero, depending on the degree of surface roughness. (<b>C</b>) This sterical shielding effect is avoided, when EAPs are elevated above ground level. With the geometrical assumptions made above, EAPs reach a binding probability of 95% when lifted upwards by 160 nm.