Colorized SEM showing a venule lumen. Surface features of a monocyte (Mo) are clear. The cell attaches to the endothelium (En) by pseudopodia (arrows) as it begins migrating from the lumen on its way to becoming a macrophage. This migratory process—diapedesis—enables leukocytes to leave the circulation for functions in surrounding tissues. 3440×. (Courtesy of Dr. M. E. Todd)