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OBJECTIVE:: The aim of this study was to characterize the hydrodynamic interactions between rolling and free-stream leukocytes in an in vivo model of selectin-mediated rolling, and to identify those physical mechanisms that influence the dynamics of transient adhesion with the walls of postcapillary venules. METHODS:: Postcapillary venules of diameter 22-37 µm in the cheek pouch of anesthetized hamsters were visualized using intravital microscopy, with selectin-mediated rolling occurring in response to surgical preparation. RESULTS:: Rolling velocity was found to be a strong function of the center-to-center separation with the nearest cell, and also was found to correlate strongly with the number of nearby cells. These effects are shown to be beyond that attributable to variations along the length of the vessel. Adherent leukocytes were observed to provide a nucleation site, precipitating further adhesion events of free-stream cells. CONCLUSIONS:: The dynamics of the transient adhesion of leukocytes to the vessel wall in postcapillary venules is strongly dependent on the local concentration of adherent leukocytes, due to the complex hydrodynamics induced by their presence. The results are shown to agree well with theoretical considerations of the flow field induced by multiple nearby cells, suggesting a need for the future exploration of multicellular effects in the microcirculation.Microcirculation (2003) 10, 401-409. doi:10.1038/sj.mn.7800203 [ABSTRACT FROM AUTHOR]