Chronic inflammation is a product of the Type 4 hypersensitivity mechanisms (Refer Chapter 14 for Hypersensitivity).
Tissue Repair and Wound Healing:
While removing the agent responsible for inflammation, the defense mechanism of body causes some damage to the tissue in and around the region of inflammation. To get back the normal status, healing and repair of the damaged tissue is necessary.
Hence healing and repair of damaged tissue is the end phase of inflammation. Restoring cellular structures and tissue layers is a complex and dynamic process. It takes place in two steps one is regeneration of tissue and the other is fibroplasia and fibrosis or replacement.
Depending upon the injury, tissue involved in injury, the wound may be healed by regeneration alone or by a combination of both regeneration and replacement. Balance between these two processes determines the out come of the final response of inflammation.
If basement membranes and the underlying extra cellular matrix of damaged tissue are intact, regeneration occurs through active division of the remaining cells. Mitogenic factors released from the cells of inflammation or availability of increased nutrients indirectly produced by the reduced tissue mass may trigger proliferation of healthy cells that remain at the site of injury. If the damage is repaired by regeneration the regenerated tissue is indistinguishable from the original tissue.
Since non-dividing or quiescent cells such as nerve cells, skeletal muscle cells and cardiac muscle cells etc. are not capable of undergoing divisions, granulation tissue take up the job of filling the damaged region. Granulation tissue is combination of abundant new blood vessels and immature collagen admixed with plump fibroblasts. Eventually, many of the blood vessels regress and all that is left is collagen. The excess collagen is recognized as a “scar” or “scar tissue”. The human body has a limited capability for regeneration and the amount of excess collagen depends on the extent of tissue damage, the regenerative capacity of the injured tissue, as well as an individual’s tendency, to produce excess collagen.