cells, such as those in the kidney, liver, pancreas, and bone, are capable of
regeneration in cases of injury though their cells are not normally dividing.
cells are incapable of regeneration. If injury to permanent cells occurs the tissue undergoes replacement. The damaged cells are broken down and cells of another type fill in the area. Examples of permanent cells include cardiac muscle and central nervous system neurons. In cases where injury to a tissue is severe, replacement will occur no matter what types of cells are involved. In such cases the damage to an area affects every cell in that area and leaves no viable cells to undergo mitosis to regenerate the tissue. For example, a shallow cut in the skin will heal through regeneration while a deep cut will form a connective tissue scar when it heals.
Replacement
Most cases of severe injuries will heal through replacement. This process may begin as early as twenty-four hours after injury. There are five mzin components to the replacement process that occur over three phases. 1. Inflammatory Phase The inflammatory phase begins at the time of injury and lasts for three to five days after injury. The inflammatory phase is characterized by the vascular & cellular responses of inflammation. The injured area fills with blood and clotting begins. Neutrophils arrive at the area within twenty- four hours of injury and begin the process of . The fibrin framework of the blood clot is preserved as it will provide a scaffolding for further healing. 2. Proliferative Phase The proliferative phase is also known as the phase of healing. It is characterized by the cellular response of inflammation. It begins two to three days after injury and lasts five to thirty days depending on the size of the injured area. The proliferative phase overlaps the inflammatory phase for up to forty-eight hours. Fibroblasts at the site of injury secrete collagen onto the fibrin framework as well as components of the extracellular matrix and growth factors. The extracellular matrix is made up of proteoglycans (heparin sulfate and chondroitin sulfate), polysaccharides (hyaluronic acid) and proteins (collagen, fibrin, elastin) as well as water. Growth factors trigger as well as endothelial cells and fibroblasts to proliferate in the newly deposited extracellular matrix. This forms a bridge of pink tissue across the damaged area. Pink granulation tissue is a pink, vascular, friable tissue that is both anaesthetic and antiseptic.
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