When one suffers a severe cut to the skin, the tissue is repaired by two methods: regeneration and fibrosis. Regeneration replaces dead or damaged cells with the same type of cell, thus restoring normal function. Fibrosis replaces damaged tissue with scar tissue, which holds the tissue together but does not restore normal function. Upon damage to the tissue, severed blood vessels bleed, and the blood pools in the cut. Mast cells release histamine, which promotes vasodilatation. Vasodilatation causes more blood to seep into the wound; however, it brings with it antibodies to help prevent infection, clotting proteins for clot formation, and blood cells. A clot forms in the tissue. The clot gently binds together the edges of the cut and prevents pathogens from entering into the healthy tissue. The surface of the clot forms a scab, while macrophages beneath the scab remove the damaged tissue. New blood capillaries grow into the wound. Macrophages, and fibroblasts migrate into the wound. The macrophages remove the clot, and the fibroblasts produce collagen fibers, which will form the scar tissue. This is fibrosis. Surface epithelial cells begin to migrate into the wounded area beneath the scab. This is regeneration. Eventually the scab loosens and falls off. As fibrosis in the connective tissue continues, capillaries withdraw from the area.

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