Photomicrographs of HUVEC monolayers before and at intervals after treatment with HAH buffer with or without 0.1 U/mL thrombin. (A) Nearly confluent monolayer before treatment with thrombin. (B through F) The same microscope field 2 minutes (B), 5 minutes (C), 10 minutes (D), 20 minutes (E), and 40 minutes (F) after a 5-minute exposure to thrombin. (G) Nearly confluent monolayer before treatment with HAH buffer. (H) The same microscope field as in (G) 40 minutes after a 5-minute exposure to HAH alone. The monolayer treated with thrombin (B through F) demonstrates a progressive and pronounced ICGF compared with the pretreatment monolayer (A). The monolayer exposed to HAH alone demonstrated no detectable ICGF after 40 minutes (G and H). Morphometric quantification of gap formation is tabulated in Table 2.