Figure 1.

Leukocyte extravasation cascade. Leukocytes initially engage with the endothelium via selectin and VCAM-1 mediating interactions during rolling (i), followed by G-protein-mediated activation (ii) and subsequent integrin-mediated firm adhesion (iii). Transmigration across the BBB may occur via paracellular or transcellular routes (iv). It remains to be determined whether systemically infused MSCs possess similar or distinct features and mechanisms enabling them to transmigrate across the BBB and home to the CNS system in vivo.