On the last day of experiment (day 13 post wounding), all the rats were sacrificed, and the middle tissue of the wound area of each animal was excised and collected for histological observation. All sample tissues were fixed in 10% neutral buffered formalin solution, embedded in paraffin wax, cut into 5 μm-thick sections, and stained with hematoxylin–eosin. The slides were photographed with an Olympus U-TU1X-2 camera connected to an Olympus CX41 microscope (Tokyo, Japan).