Immunohistochemistry analysis of VEGF protein expression in rat dental pulp tissue after orthodontic force. VEGF protein was weakly stained in control group (a). VEGF protein became slightly stained after 1 d of orthodontic force (b). (c–e) VEGF protein expression became positively stained after 3 d, 7 d, and 2 w after orthodontic force and also reached the peak level at 2 w group. VEGF protein expression then declined in the 4 w group (f). Arrows indicate the vessel wall (images on the left: magnification ×200, bar = 100 μm; images on the right: magnification ×1000).