Tissue Repair

Tissue Repair

The steps of tissue repair are based on division and migration of cells. These activities are controlled by growth factors also known as wound hormones, which are released by injured cells. Destroyed tissue is replaced with the same type of tissue in a process called regeneration. When fibrosis occurs, fibrous connective tissue is produced in greater quantities, forming scar tissue. Regeneration or fibrosis occurs as a result of two factors: the type of tissue that was damaged and the severity of the injury. When the skin is injured, both regeneration and fibrosis occur for tissue repair.

Inflammation begins the process of tissue repair. The capillaries dilate and become highly permeable. White blood cells and plasma fluid (containing strong clotting proteins, antibodies, and other helpful sub-stances) reach the injured area to construct a clot. This holds the wound edges together to isolate the injured area. Bacteria, toxins, and other harmful agents are prevented from spreading to the surrounding tissues. The exposed part of the clot dries and hardens, form-ing a scab. Lymphatic vessels or macrophages eventu-ally remove debris, parts of destroyed cells, and excess fluid that may be left behind.

A phase called organization begins wherein the blood clot is replaced by granulation tissue . This del-icate pink tissue contains capillaries in the formation of a new capillary bed. It is granular in appearance due to the presence of the capillaries and bleeds easily if disturbed. Fibroblasts proliferate to produce growth factors and new collagen fibers. Certain fibroblasts contract to pull the wound margins together or to pull existing blood vessels into the wound. Macrophages digest the original blood clot. Collagen fibers continue to be deposited. Granulation tissue is very resistant to infection (because it produces substances that inhibit bacteria) and eventually becomes scar tissue. When sufficient matrix has accumulated, the fibroblasts either return to resting or die.

The surface epithelium regenerates and grows under the scab, which soon detaches. Underneath, the fibrous tissue matures and contracts. The epithelium eventually looks like the surrounding skin and is fully regenerated, with an underlying area of scar tissue. Based on the wound’s severity, the scar tissue may be invisible, appear as a thin white line, or may appear more substantially. This entire process describes how a skin cut, scrape, or puncture heals. Simple infections heal only by regeneration (such as a sore throat or a pimple). Severe and destructive infections cause clot formation or scarring.