When the skin is cut, blood vessels are also damaged and blood seeps into the wound cavity where it clots. The fibrin strands in the clot shrink, pulling the edges closer together. Later the cells in the cut edges start to multiply and migrate into the clot. At the same time, macrophages enter the wounded area and begin to remove dead cells, the blood clot, bacteria and any debris that has entered the cut. Capillaries enter the clot, to supply blood to the multiplying epidermal and dermal cells. All this occurs beneath the scab formed by the dried blood clot. When the cut has been repaired, the fibrin and collagen that have been deposited in the wound shrink to bind the two surfaces together. A pale fibrous scar is formed if the wound is large and the repairing tissue did not receive a blood supply. In cases of severe injury, surgical stitches may be needed to pull the cut surfaces together.