The inflammatory response is stimulated by the release of chemical mediators from the cells damaged at the injury. There is a vascular response and a cellular response.
- Vascular: vasodilation and vasopermeability causing hyperaemia and the production of oedema within the tissues (via increased hydrostatic pressure and decreased osmotic pressure). WBCs marginate, platelets adhere to the vessels walls and endothelial cells swell. The exudate dilutes irritant substances, forms a fibrin clot around the intact tissues, and the fibrin also develops into a meshwork to trap foreign particles and debris.
- Cellular: chemotaxis (by chemical mediators), phagocytosis and site clearance. Lactic acid is the end product of phagocytosis and this is a stimulant of the proliferative phase of healing.
Macrophages and lactate stimulate the proliferative stage. Fibroblasts and endothelial cells are attracted to the damaged area from neighbouring tissues. Fibroblasts are responsible for secreting collagen and an amorphous ground substance. The ground substance provides a linking mechanism for the collagen fibres (primarily type III at this stage). The collagen fibres are deposited in a haphazard manner and are relatively weak and not specialised to the parent tissue at this time. Angiogenesis also occurs which is the growth of new blood vessels within the damaged tissue to provide more oxygen and remove debris/co2. The revascularisation is what makes the tissues look red around the scar caused by an injury. Myofibroblasts are responsible for drawing the edges of the wound together (from contraction) reducing the size of the final scar. When the damage is healed these cells die from a process called apoptosis. As the granulation tissue matures there is a process of devascularisation with obliteration of the lumen of the vessels.
The remodelling stage can begin as early as the first 1/2 weeks post injury and last upto 2 years. This is the stage in which the collagen fibres align in the direction of stress (from movement). It is also made stronger by the deposition of stronger type I collagen fibres and the reabsorption of the original type III collagen. It never becomes as strong as the parent tissue.
Healing of Muscle Strains:
Muscle can regenerate. Satellite cells are ‘stem cells’ that have the ability to differentiate into myoblasts and to form new muscle fibres. However if a patient suffers a third degree strain and the muscle is completely severed, the two segments heal by dense scar tissue formation. Muscle does not regenerate across the scar and functional continuity is not restored.
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