Author: 绿水青山
Reviewers: 食野; 未名

  Throughout our lives, we are inevitably affected by things in the surrounding environment. The body naturally responds by first trying to adapt. If a stimulus is too strong, however, the body may be injured. Today we will look at how the body repairs that damage.

  An injury is the destruction or loss of some of the body’s cells and tissues; repair is the process of mending and restoring what is missing. Repair generally takes one of two forms: regeneration or fibrous repair. In regeneration, cells of the same type divide at the injured site and restore the tissue’s original function. This form of repair is usually more complete and can restore all function. Fibrous repair uses fibrous connective tissue. It produces a scar and does not completely restore function.

  The body’s regenerative capacity has several general characteristics. Tissues that are frequently injured or continually renewed regenerate more readily than other tissues, and less differentiated cells regenerate more readily than highly differentiated ones. Regenerative capacity also varies with an organism’s evolutionary path and species. Salamanders, geckos, many plants, and other organisms have especially strong regenerative abilities.

  Tissue regeneration can be broadly divided into complete and incomplete regeneration. In complete regeneration, the repaired tissue functions much as it did before. In incomplete regeneration, fibrous connective tissue replaces the original tissue. The cells used for repair differ from those in the original tissue, some function is lost, and a scar forms.

  Different tissues and different parts of the body generally regenerate in different ways after injury. Let us consider several examples.

Epithelial tissue

  The cells in epithelial tissue are labile cells. Because they divide frequently, they have a strong capacity for regeneration. Their ability to regenerate also depends on surrounding connective tissue and other factors. If the connective tissue remains intact, complete regeneration is likely and the original function can be restored relatively well. If that connective tissue is destroyed and can no longer provide its supporting framework, the body instead undertakes fibrous repair. Fibrous repair is often incomplete and cannot restore the original function.

Fibroblasts

  Connective tissue repair relies mainly on fibroblasts. When tissue is injured, these cells are activated and increase in number. They begin producing large amounts of collagen and other “supporting materials,” gradually filling the damaged area. The newly formed structures then stabilize, cellular activity declines, and relatively dense fibrous tissue develops to complete the repair.

Blood vessels

  Blood-vessel regeneration can be divided into the regeneration of small and large vessels. Small vessels such as capillaries generally regenerate by sprouting and joining surrounding vessels to form a network, and the repair is usually complete. Large vessels have more difficulty regenerating completely. Severe damage may require surgical suturing before their function can be restored.

  Now that we have a general picture of how several tissues regenerate, let us examine what happens after an ordinary injury, using a common wound as an example.

  Immediately after tissue is damaged, severed blood vessels bleed. The blood mixes with exudate and detached epithelial cells around the wound and coagulates into a scab. The scab adheres to the wound surface, so forcibly tearing it off may enlarge the wound, expose it to the air again, and increase the risk of infection.

  Granulation tissue then begins to fill and surround the wound. This tissue is rich in newly formed capillaries and immune cells, which help remove pathogens and suppress infection. Because granulation tissue contains so many capillaries, the wound bleeds very easily at this stage.

  In the next stage, fibroblasts inside the wound begin forming scar tissue, which contracts and reduces the wound. Finally, epithelial cells cover the tissue again, completing the repair process.