Yh. Zhao et al., Dynamic contact forces on leukocyte microvilli and their penetration of the endothelial glycocalyx, BIOPHYS J, 80(3), 2001, pp. 1124-1140
We develop a theoretical model to examine the combined effect of gravity an
d microvillus length heterogeneity on tip contact force (F-z(m)) during fre
e rolling in vitro, including the initiation of L-, P-, and E-selectin teth
ers and the threshold behavior at low shear. P-z(m) grows nonlinearly with
shear. At shear stress of 1 dyn/cm(2), F-z(m) is one to two orders of magni
tude greater than the 0.1 pN force for gravitational settling without flow.
At shear stresses >0.2 dyn/cm2 only the longest microvilli contact the sub
strate; hence at the shear threshold (0.4 dyn/cm(2) for L-selectin), only 5
% of microvilli can initiate tethering interaction. The characteristic time
for tip contact is surprisingly short, typically 0.1-1 ms. This model is t
hen applied in vivo to explore,the free-rolling interaction of leukocyte mi
crovilli with endothelial glycocalyx and the necessary conditions for glyco
calyx penetration to initiate cell rolling. The model predicts that for art
eriolar capillaries even the longest microvilli cannot initiate rolling, ex
cept in regions of low shear or flow reversal. In postcapillary venules, wh
ere shear stress is similar to2 dyn/cm(2), tethering interactions are highl
y likely, provided that there are some relatively long microvilli. Once tet
hering is initiated, rolling tends to ensue because F-m(z) and contact dura
tion will both increase substantially to facilitate glycocalyx penetration
by the shorter microvilli.