Fig. 2. Time-lapse photography of plasma kallikrein-induced MMP-1 activation and capillary tube regression. ECs were suspended in collagen matrices and cultured in the absence or presence of 1 µg/ml kallikrein. Sequential time-lapse photographs were taken of individual EC fields at the time in hours denoted in each panel. (A) In the absence of kallikrein, ECs form sprouts, vacuoles and tubular networks, a process known as morphogenesis. (B) In the presence of kallikrein, morphogenesis is initiated (arrows); however, capillary tubes and vacuoles collapse (arrowheads), eventually resulting in individual EC rounding, aggregation and death. (C) ECs were allowed to undergo morphogenesis and establish EC networks for 48 hours prior to the addition of kallikrein. An EC-lined tube (arrows) is visible undergoing tube regression (arrowheads) prior to the onset of gel contraction. Bar, 100 µm.