From ArticleWorld

The healing is, in its strict sense, the process of regeneration and repairing damaged or dead cells in an organ or tissue. The cells in the necrotic area are destroyed and then replaced.

The replacement

There are two ways repairing occurs: by regeneration or repair.

  • Regeneration: this involves the replacement of the dead cells by the same tissue they originated from. This requires that the damaged cells can replicate. This is the case of most cells, but some, like the cells of the cardiac muscle and the neurons, cannot replicate. It also requires a collagen framework which it will use as a base. This is quite possible to happen, as a collagenous membrane exists along most cells, and it is not easily destroyed. The common example is that of the recovery from acute tubular necrosis (ATN) in the kidney. When the epithelial cells in the kidney die because of oxygen starvation or poisoning, the cells that survive can replicate and eventually grow back.
  • Repair: healing by repair occurs when the destroyed cells cannot replicate, as in the cardiac muscle for example. It may also happen if the collagen membrane around the cells is damaged or destroyed. At first, a number of phagocyte agents are sent by the body towards the wound, to destroy any potential bacteria and release a number of chemical agents that stimulate the fibroblasts and endothelial cells to migrate and divide in the affected area. The fibroblasts produce large quantities of type III collagen which will fill the area left open by the wound, a process known as granulation. After the granulation tissue is completed, type III collagen ends up replaced by type I collagen, and eventually, a scar of collagen and a small number of fibroblasts will appear. This happens in the case of wound and bone healing.

There are some cases when the collagen scar doesn't form. This is the case of brain healing, after a cerebrovascular accident for example.