Phases of wound healing
- The four main stages of wound healing are:
- Haemostasis
- Inflammation
- Regeneration
- Repair
- These stages are not discrete but overlap
Haemostasis
- Damaged endothelial lining exposes platelets to sub-endothelial
collagen
- Damaged endothelium releases von-Willebrand factor and tissue
thromboplastin
- von-Willebrand factor facilitates platelet adhesion to
sub-endothelial collagen
- Adhered platelets release ADP and thromboxane A2
- This leads to further platelet aggregation
- Tissue thromboplastin activates the coagulation pathways
- Fibrin is the end product of the coagulation pathways
- Fibrin forms a plug into which platelets and red blood cells are
trapped
- This results in clot formation
Inflammation
- Platelets release platelet derived growth factor and transformation
growth factor β
- These are chemotactic to neutrophils and monocytes
- Neutrophils and macrophages phagocytose foreign material and
bacteria
Regeneration and repair
- Platelet derived growth factor and transformation growth factors are
mitogenic to epithelium and fibroblasts
- This lead to proliferation of epithelial cells and fibroblasts
- Fibroblasts produce collagen
- Vascular endothelial growth factor is mitogenic to endothelial cells
- It is released by monocytes in response to hypoxia
- It promotes angiogenesis
- Wound strength by the end of 1st week is 10% of the original
- By the end of 3rd month it reaches 70% and thereafter plateaus
Time Line of events
- Within first 24 hours
- Neutrophils are the predominant cell type
- This is the phase of acute inflammation
- Epithelial cells start proliferating and migrating into the wound
cavity
- By the 2nd and 3rd day
- Macrophage and fibroblasts are the dominant cell types
- Epithelial cell proliferation and migration continues
- Angiogenesis begins
- Granulation tissue appears
- Collagen fibres are present but these are vertical
- They do not bridge the wound gap
- Granulation tissue comprises newly formed new capillary loops
- By the end of 5th day
- Fibroblasts are the predominant cell type
- They synthesise collagen
- Collagen now bridges the wound edges - bridging collagen
- Epidermal cells continue division and epidermis is now
multi-layered
- Abundant granulation tissue is present
- During 2nd week
- Acute inflammation begins to reduce
- Collagen continues to accumulate
Aberrations in wound healing
Keloid scar
- Overgrowth of scar tissue
- Scar tissue extends beyond wound margin
- More common in black races
- Uncommon in children
- Typical sites are sternum, back, ear lobes
- Triamcinolone injections directly into the tissue is the initial
treatment
- Other option is excision of the overgrowth
- They tend to recur
Hypertrophic scar
- Overgrowth of scar tissue but it does not extend beyond wound edge
- Can occur in any race
- More common in children
- Usually growth subsides with time
- May even regress
- More common over flexor surfaces and after burns
Bibliography
Teller P, White T K. The physiology of wound healing:
Injury through maturation. Surg Clin North Am 2009; 89:
599-610. |