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Wound Healing (Structure & function of skin (3 Main components
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Wound Healing
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Wound healing process
DefinitionWound
An injury to living tissue cause by a cut, blow or other impact. Typically one which the skin is brokenWound healing
The physiological process by which the structure and function of the skin are returned to normalTwo kinds of wound healing depend on the wound
- Epidermal wound healing
- Restricted to the epidermis
- Common injuries - abrasions where portion of the skin is scrapped away. Minor burns
- Deep wound healing
Epidermal wound healing
- In response to injury, basal cells of the epidermis surrounding the wound break contact with the basement membrane
- Basal cells enlarge and migrate across the wound until the advancing cells from opposite sides meet
- The stop migrating due to a process called contact inhibition
As basal cells migrate a hormone called epidermal growth factor stimulate basal stem cells to divide and replace those that have moved into the woundThe relocated basal epidermal cells divide to build a new strata, thickening the new epidermis
Deep wound healingThe injury extends to the dermis and subcutaneous layer
- More difficult to heal as basement membrane or skin appendages (hair follicles, sweat and oil glands) may be destroyed
Healing process has 4 stages:
- Inflammatory phase
- Migratory phase
- Proliferative phase
- Maturation phase
Due to formation of scar tissue (fibrosis), some normal tissue function may be lost
Primary wound
- Very little tissue loss
- Tissue surface can be closed with stitches, staples or skin glue
Secondary wound
- Wound is extensive and has considerable tissue loss
- Edges cannot be brought together e.g pressure ulcers on the heal
Stages1. Inflammatory phase
- Formation of blood clot to unite wound edges
- Inflammation - Helps eliminate microbes, foreign material and dying tissue in preparation for repair
Vasodilation:
- Redness due to vasodilation of arterioles
- Increased temperature due to increased blood flow
- Swelling due to increased extravascular fluid content of injured tissue
- Pain and discomfort
Part of non-specific response to infection
- Leucocytes (WBC) are attracted to the site of injury
- Neutrophils and monocytes (which develop into macrophages) phagocytise microbes
- Monocytes my dominate the wound by day 5: remove debris, clots, foreign bodies, bacteria and viable tissue
- Also secrete substances needed to attract cells for proliferation stage
2. Migratory phase
- Beginning of repair to the wound
- Wound becomes a scab
- Epithelial cells migrate beneath the scab to bridge the wound
- Fibroblasts migrate along fibrin threads and begin synthesising scar tissue
- Damaged blood vessels begin to regrow
- Tissue filling the wound is called granulation tissue
Granulation
- Angiogenesis (formation of new blood cells) stimulated by hypoxia (deficiency in oxygen reaching the tissue) creation of new capillaries to support mitotic activity that provides replacement cells
- Macrophage activity - a consequence of inflammation, this stimulates the production and multiplication of fibroblasts, fibroblasts do most of the work
- Fibroblasts lay down ground substance and begin the secretion of collagen - this will form the scar of the wound, usually about 7 days after the injury to 6 weeks
Wound contraction and epithelisation
- Following the deposition of connective tissue the fibroblasts have congregated at the wound margins develop contractile proteins and use their properties to pull the wound together
- Complete wound healing is only possible when the epithelial cells have bridged the surface of the wound, and then any scab will slough away and production of keratin will strengthen the new epidermis
3. Proliferative phase
- Following migration, epithelial cells grow beneath the scab
- Collagen fibre formation by fibroblasts (in random patterns)
- Continued growth of blood vessels (angiogenesis)
4. Maturation phase
Final stage
- Can take 24h to 2 weeks
- Fibroblasts decrease in number
- Collagen fibre become more organised, strengthening the tissue
- Original clot removed by action of plasmin
- Blood vessels return to normal