Fig. 6. Tek-dependent regulation of MT1-MMP expression during ex vivo angiogenesis. (A) MT1-MMP transcription in MT1-MMP^+/lacZ explants (as monitored by lacZ activity) was assessed in the absence or presence of sTek. Wheras lacZ activity is confined to endothelial tip cells in the absence of sTek (white arrow, left panel), addition of sTek induced widespread MT1-MMP expression in growing neovessels (white arrows, right panel). Mural cells and fibroblasts stain positive for smooth muscle actin (SMA). Bar, 0.25 mm. (B) Total number neovessels (blue bars) and of neovessels displaying endothelial tip-cell-specific MT1-MMP expression (red bars) was determined in explant cultures in the absence or presence of sTek after a 7-day culture period. Whereas the total number of neovessels was unaffected by sTek, the number of capillary structures exhibiting tip-specific MT1-MMP induction was significantly reduced in the eight experiments performed (P=0.0045, F-test). (C) In murine explants, treatment with sTek (right panel) induced widespread expression of MT1-MMP (green, indicated by yellow arrows) and the concomitant decrease in type IV collagen staining (red) compared with control explants (left panel). Bar, 0.25 mm.