<Li> TNFα increases vascular permeability and activates vascular endothelia </Li> <Li> CXCL8 (IL - 8) forms a chemotactic gradient that directs leukocytes towards site of tissue injury / infection (CCL2 has a similar function to CXCL8, inducing monocyte extravasation and development into macrophages); also activates leukocyte integrins </Li> <P> In 1976, SEM images showed that there were homing receptors on microvilli - like tips on leukocytes that would allow white blood cells to get out of the blood vessel and get into tissue . Since the 1990s the identity of ligands involved in leukocyte extravasation have been studied heavily . This topic was finally able to be studied thoroughly under physiological shear stress conditions using a typical flow chamber . Since the first experiments, a strange phenomenon was observed . Binding interactions between the white blood cells and the vessel walls were observed to become stronger under higher force . Selectins (E-selection, L - selection, and P - selectin) were found to be involved in this phenomenon . The shear threshold requirement seems counterintuitive because increasing shear elevates the force applied to adhesive bonds and it would seem that this should increase the dislodging ability . Nevertheless, cells roll more slowly and more regularly until an optimal shear is reached where rolling velocity is minimal . This paradoxical phenomenon has not been satisfactorily explained despite the widespread interest . </P> <P> One initially dismissed hypothesis that has been gaining interest is the catch bond hypothesis, where the increased force on the cell slows off - rates and lengthen the bond lifetimes and stabilizing the rolling step of leukocyte extravasation . Flow - enhanced cell adhesion is still an unexplained phenomenon that could result from a transport - dependent increase in on - rates or a force - dependent decrease in off - rates of adhesive bonds . L - selectin requires a particular minimum of shear to sustain leukocyte rolling on P - selectin glycoprotein ligand - 1 (PSGL - 1) and other vascular ligands . It has been hypothesized that low forces decrease L - selectin--PSGL - 1 off - rates (catch bonds), whereas higher forces increase off - rates (slip bonds). Experiments have found that a force - dependent decrease in off - rates dictated flow - enhanced rolling of L - selectin--bearing microspheres or neutrophils on PSGL - 1 . Catch bonds enable increasing force to convert short bond lifetimes into long bond lifetimes, which decrease rolling velocities and increase the regularity of rolling steps as shear rose from the threshold to an optimal value . As shear increases, transitions to slip bonds shorten their bond lifetimes and increase rolling velocities and decrease rolling regularity . It is hypothesized that force - dependent alterations of bond lifetimes govern L - selectin--dependent cell adhesion below and above the shear optimum . These findings establish a biological function for catch bonds as a mechanism for flow - enhanced cell adhesion . While leukocytes seem to undergo a catch bond behavior with increasing flow leading to the tethering and rolling steps in leukocyte extravasation, firm adhesion is achieved through another mechanism, integrin activation . </P>

Which of the following cells can undergo a process called diapedesis