Wound Healing Example
Wound healing occurs in three major stages: hemostasis phase, inflammatory phase, proliferative phase, and maturation/remodeling phase. We use these phases to help us learn what is happening during wound healing, but in reality they overlap quite a bit. Here is a brief description of each of these stages.
Hemostasis Phase: This phase occurs very quickly and directly after injury, it involves blood leaking from damaged vessels in the area of the would and the coagulation of that blood. Coagulated blood has the effect of preventing further bleeding, releasing chemical signals that help initiate inflammation and healing, and creating an initial framework that can help hold the damaged tissue together. A blood clot at the skin surface also forms a scab when it dries, the scab is natures band-aid, it creates a barrier that keeps the healing tissue from drying out while helping to prevent aliens from getting into the healing tissue.
Inflammatory Phase: The inflammatory phase starts right after injury and may last a few days, it is mediated by chemicals released during blood clotting and also released by damaged cells in the tissue. Inflammation involves the dilation of blood vessels in the tissue (brings more nutrients and heat) as well as an opening up of spaces between the cells of blood vessel walls that allows plasma proteins and white blood cells (WBCs) to enter the tissue from the blood stream. This leads to an increase of fluid in the tissue, the presence of active clotting proteins and antibodies in the tissue, and activated phagocytic WBCs in the tissue. The clotting proteins prevent or at least slow any aliens from leaving the area while antibodies bind known aliens and tag them for phagocytosis, the increase in fluid in the tissue causes a shift of more fluid entering the lymphatic system and gives the WBCs room to phagocytize debris and any aliens. Chemicals released during this phase stimulate cells in the damaged tissue to divide, leading to the next phase.
Proliferative Phase: The proliferative phase begins as the inflammation subsides and may last a few weeks, it involves cells in the tissue dividing in order to replace dead cells, produce more fibroblasts (cells that produce collagen and other ECM components), and produce new capillary blood vessels to replace damaged ones and supply nutrients to the healing tissue. Macrophages phagocytize the initial blood clot components. Granulation tissue may form and grow to fill space if there is a large gap between parts of the tissue. Collagen fibers are laid down during this phase, and wound contraction also happens during this phase.
Maturation/Remodeling Phase: The maturation/remodeling phase begins a few to several weeks after injury and is the longest phase, sometimes lasting years. This phase involves first the development of and then the long term replacement of fibrotic tissue scarring. The amount of scarring that occurs depends on the size of the initial wound and any disruption of the wound during healing (this and the risk of infection is why you should not pick at a scab); other factors can contribute to scarring such as genetics (some people are just more prone to develop keloid scarring for example). The continuous remodeling that occurs for years explains why scars tend to fade over time.