Although often depicted as planar, the lymphocyte cell surface is covered with finger-like projections called microvilli. These structures have been proposed to spatially segregate adhesion receptors used in extravasation (passage from the blood stream to peripheral tissues), resulting in a temporal segregation of receptor function during this multi-step process. I postulate that microvilli play roles in many other, if not all, cell surface processes carried out by lymphocytes. My laboratory investigates the mechanisms of lymphocyte microvillar assembly, which has yet to be determined for any type of microvillus. Microvilli are actin-dependent structures, and my laboratory has shown that they are made up of parallel actin bundles that run the length of the microvillus, similar to epithelial microvilli. However, we find that these microvilli are >100-fold more dynamic than epithelial microvilli, protruding and retracting on a timescale of seconds to minutes. Our goals are: 1) to identify molecules required for lymphocyte microvillar assembly; 2) to develop reagents that specifically disrupt these molecules, thereby disrupting microvilli; 3) to use these reagents to test the role of microvilli in extravasation and other lymphocyte cell surface processes. Our studies will lead to a clearer understanding of cell surface structure in general, and of the lymphocyte lymphocyte cell surface in particular.