5 Tissue Repair

• Tissue repair, also known as healing, refers to the restoration of tissue architecture and function after an injury
• Regeneration is when damaged components essentially return to a normal state
• Scarring is the deposition of connective tissue
• In Regeneration, some tissues can replace damaged components and essentially return to a normal state.
• In Connective tissue deposition, repair occurs by the laying down of connective tissue, a process that may result in scar formation.
• Fibrous scar is not normal, it usually provides enough structural stability that the injured tissue is able to function
• Cell proliferation is involved in regeneration of injured cells and tissues, driven by growth factors.
• Labile tissues are continuously dividing tissues; cells of these tissues are continuously being lost and replaced by maturation from tissue stem cells and by proliferation of mature cells.
• Stable tissues have only minimal proliferative activity in their normal state, but are capable of dividing in response to injury or loss of tissue mass.
• Permanent tissues are considered to be terminally differentiated and non-proliferative in postnatal life
• Injury to the brain or heart is irreversible and results in a scar because neurons and cardiac myocytes cannot regenerate
• Restoration of normal tissue structure can occur only if the residual tissue is structurally intact, as after partial surgical resection.
• If repair cannot be accomplished by regeneration alone, it occurs by replacement of the injured cells with connective tissue, it leads to scar formation
• Angiogenesis is the formation of new blood vessels, which supply nutrients and oxygen needed to support the repair process.
• In Formation of granulation tissue, there will be migration and proliferation of fibroblasts = type of tissue will become firmer compared to the normal tissue
• The firmer the tissue, the more stable it is
• As healing progresses, the number of proliferating fibroblasts and new vessels decreases
• Collagen synthesis is critical to development of strength in a healing wound site → makes wound or remodeled area stronger
• Infection is clinically one of the most important causes of delayed healing; prolongs inflammation and potentially increases local tissue injury
• Diabetes is a metabolic disease that compromises tissue repair for many reasons; one of the most important systemic causes of abnormal wound healing.
• Nutritional status has profound effects on repair; protein and vitamin C deficiency inhibit collagen synthesis and retard healing
• Glucocorticoids have well-documented anti-inflammatory effects, and administration may result in weakness of the scar
• Mechanical factors such as increased local pressure or torsion may cause wounds to pull apart, or dehisce
• Poor perfusion due to peripheral vascular disease, arteriosclerosis, and diabetes or due to obstructed venous drainage; also impairs healing
• Foreign bodies such as fragments of steel, glass, or even bone impede healing by perpetuating chronic inflammation.
• Type and extent of tissue injury such as in complete restoration can occur only in tissues composed of stable and labile cells, injury to tissues composed of permanent cells inevitably results in scarring and some loss of function.
• Location of injury is also important, for example, inflammation arising in tissue spaces develops extensive exudates.
• In healing by first intention, injury involves only the epithelial layer (superficial).

  principal mechanism: epithelial regeneration
• In healing by second intention, also called secondary union, differs from primary healing in several respects the repair process involves a combination of regeneration and scarring
• Carefully sutured wounds have approximately 70% of the strength of normal skin, largely because of the placement of sutures.
• Recovery of tensile strength results from excess of collagen synthesis over collagen degradation during the first 2 months of healing by cross-linking of collagen fibers and increased fiber size.
• Wound strength reaches approximately 70% to 80% of normal by 3 months but usually does not substantially improve beyond that point.
• Wound dehiscence is the separation of the tissues which were connected together
• Hypertrophic scars & keloids will go beyond the boundaries of the wound.
• Venous leg ulcers develops most often in elderly people as a result of chronic venous hypertension, which may be caused by severe varicose veins or congestive heart failure.
• Arterial ulcers develops in individuals with atherosclerosis of peripheral arteries, especially associated with diabetes.
• Diabetic ulcers affects the lower extremities, particularly the feet; there is tissue necrosis and failure to heal
• Pressure sores are areas of skin ulceration and necrosis of underlying tissues caused by prolonged compression of tissues against a bone
• Keloid is a scar tissue grows beyond the boundaries of the original wound and does not regress.
• Hypertrophic scars is a raised scar; generally develops after thermal or traumatic injury that involves the deep layers of the dermis.