Phase-contrast intravital microscopy showing a platelet rolling on a mesenteric venule after stimulation by the calcium ionophore A23187. “o” indicates the location of the platelet at time 0. Arrowheads point toward the rolling platelet at 0 seconds (top), 1.58 seconds (middle), and 2.96 seconds (bottom). A single much larger and more slowly rolling leukocyte is also seen. Bar represents 30 μm. Reproduced with permission from PNAS.⁷