Actin polymerization results in the sequential formation of filopodia, lamellipodia, and stress fibers in platelets, but the molecular mechanisms regulating these processes were unknown. In 15 June
Antibody treatment froze the platelets at the rounded, early stage of activation, before the formation of filopodia or lamellipodia. Analysis indicated that the proportion of platelets at the rounded stage rose from 2.85% before treatment to 63% afterwards. Addition of a recombinant Arp2 protein reversed this effect.