36) Tissue Repair

Tissue repair requires cells to divide and migrate activities that are initiated by growth factors (wound hormones) released by injured cells. Tissue repair occurs in two major ways: by regeneration and by fibrosis. Which of these occurs depends on the type of tissue damaged and the severity of the injury. Regeneration is replacement of destroyed tissue with the same kind of tissue, whereas fibrosis involves proliferation of fibrous connective tissue called scar tissue.

The Steps of Tissue Repair:
1) Inflammation- the tissue trauma causes injured tissue cells, macrophages, mast cells and others to release inflammatory chemicals, which cause the capillaries to dilate and become very permeable. This allows white blood cells and plans fluid rich in clotting proteins, antibodies and other substances to seep into the injured area. The leaked clotting proteins construct a clot, which stops the loss of blood, holds the edges of the wound together and effectively isolates the injured area. This prevents bacteria, toxins and other harmful substances from spreading to surrounding tissues. The part of the clot exposed to air quickly dries and hardens, forming a scab. The inflammatory events leave excess fluid, bits of destroyed cells and other debris in the areas which are eventually removed via lymphatic vessels or phagocytized by macrophages.
2)Organization and restored blood supply- organization is the first phase of tissue repair and can occur during the inflammation process. During organization, the blood clot is replaced by granulation tissue (delicate pink tissue composed of several elements). The granulation tissue contains capillaries that grow in and lay down a new capillary bed. These capillaries are fragile and bleed freely. Also present in granulation tissue are proliferating fibroblasts that produce growth factors as well as new collagen fibers to bridge the gap. Some have contractile properties that pull the margins of the wound together. As organization proceeds, macrophages digest the original blood clot and collagen fiber deposit continues. The granulation tissue is destined to become scar tissue and is highly resistant to infection because it produces bacteria-inhibiting substances.
3) Regeneration and fibrosis take effect- during organization, the surface epithelium begins to regenerate, growing under the scab (which soon detaches). As the fibrous tissue beneath matures and contracts, the regenerating epithelium thickens until it finally resembles the adjacent skin, The end result is a fully regenerated epithelium and an underlying areas of scar tissue. The visibility of the scar depends on the severity of the wound.This repair is permanent.

-The repair process described above follows the healing of a wound that breaches the epithelial barrier. In pure infections (a pimple or sore throat) healing is solely by regeneration. Only severe, destructive infections lead to scarring.