Monocytes induce a single cell cycle in ECs. (A) Monocytes added to subconfluent human umbilical vein endothelial cells (HUVECs) at time 0 induce a single cycle of ³H-thymidine incorporation in HUVECs. A second addition of monocytes at time 51 hours induces an additional cycle of EC proliferation. (B) At 27 hours after monocyte addition, EC number nearly doubles as measured by image analysis from phase-contrast microscopy (n ≥ 3).