The Wound Healing Process

The skin has various regeneration and repair mechanisms. These are employed to eliminate any damage caused by external influences and to restore lost function

Reactions of the “horny layer”

The action of external mechanical, physical or chemical irritants causes the horny layer to thicken. Typical examples are the thickening found after intensive UV radiation and the formation of calluses on areas subject to mechanical stress (palms of the hands and soles of the feet).

Regeneration following UV-related damage

Intense UV-exposure causes primary damage to the genetic material. Secondary damage is inflicted on the cell proteins and membranes by UV-induced free radicals.

The skin is known to have many mechanisms for the repair of damaged DNA. In humans the most important are the excision repair and post-replication repair mechanisms: The excision repair mechanism is based on recognition, removal and replacement of the damaged DNA segment.
This way mutations are prevented as long as the repair mechanism is not overburdened or defective.

The post-replication repair mechanism, on the other hand, works around the damaged DNA segment, meaning that it is ignored when the genetic code is read. Only later is the damage repaired. This mechanism is so faulty however, that often more mutations are caused by the repair than by the original radiation damage.

Regeneration following ablative skin injury:

The epidermis does not scar

The layer of epidermal mother cells – the basal layer – ensures a steady renewal of the epidermis, through continual cell division (proliferation). If an injury is confined to the uppermost skin layer, this damage, known as erosion, can heal without scarring. It can however pigment.

The dermis does scar

If the damage reaches the dermis (e.g. an ulcer) and thus involves the basal membrane (dermal epidermal junction), then healing is usually accompanied by scar formation. When this happens, destroyed epidermal cells are replaced by connective tissue.

Ablative wound-healing in several consecutive stages:

1. Inflammatory Response (1 to 5 days)

On initial damage the Mast Cells in the tissue release Histamine.

Which triggers the inflammatory response, the local capillaries & arterioles dilate to improve blood circulation to the area.

The capillary walls open up slightly to allow fluid, Inflammatory Exudate (IE), to pass into the tissue. The IE contains antibodies to neutralise any foreign material and precursors to fibrin which gets laid down in a mesh to prevent the spread of any foreign material. IE will also contain leucocytes, which move through the capillary walls, to remove foreign material and dead tissue by phagocytosis.

Scab formation:

If bleeding has taken place, Haemostasis occurs soon after the injury. Platelets in the area of the wound build up a plug over the damage and release Serotonin. Serotonin cause the blood vessels to contract and reduce the blood flow. If this is insufficient to stop the bleeding then a series of chemical messengers starts the Clotting Mechanism. This results in large quantities of fibrin being laid down in the area to bind everything together. Hence a scab contains, dried blood cells, dried inflammatory exudate (and its white blood cells), platelets and any foreign matter not washed away by the flow of blood and exudate.

After the initial rush of the Inflammatory Response. It is the period when the leucocytes and the later arriving macrophages remove the dead tissue and foreign material and the fibrin net (blood clot) laid down in the tissues is dissolved.

The mighty macrophage is the key player, these white cells have both scavenger and non-scavenger functions. With no slight intended to the other inflammatory cells, without macrophages there is no healing or what healing occurs is poor.
Macrophages release tissue destroying enzymes to rid the wound of debris, leaving room for healthy cells to fill the void.

The macrophage is critical to the inflammatory stage of wound healing and also essential to new tissue development through macrophage derived growth factors (MDGF).

It will take 3-5days for the macrophages to stimulate the differentiation of fibroblasts, this next phase of wound healing is called the fibroplastic stage. The fibroplastic phase is usually established within 5 days of wounding and last for up to 4 weeks.

2. Fibroplastic stage (5 days to 4 weeks)
Fibroblasts that are normally found in low numbers in the dermis, proliferate in the wound and migrate with the help of the growth factors (MDGF) and a very important glycoprotein called fibronectin. Fibronectin acts as a conduit for fibroblasts. It binds to both the wound matrix and the fibroblast, allowing the fibroblast to march down it (the fibronectin), move along the chemotactic gradient of growth factors, and take up residence in the wound.

Once in the wound, fibroblasts begin to synthesize wound collagen (type 3) fibres and produce proteoglycans, structural proteins, and adhesive proteins — connective tissue ground substances. Other fibrous protein in the wound include elastin and reticulin.

Vitamin C, iron, and copper are essential to the synthesis of collagen, which constitutes 50% of the protein found in scar tissue.

New blood vessels start to infiltrate the wound (Granulation Tissue)
Collagen, however, cannot be synthesized in the absence of adequate oxygen supply, and the wound has derailed blood flow. Angiogenesis is required. Through their enzymatic actions, growth factors break down the vascular membrane of the venules. Endothelial cells that migrate through the defect aggregate to form vascular buds. Buds connect with contiguous buds to form loops. Loops develop a basal membrane from extracellular matrix components and then develop their own vascular buds.

This process continues until contact is made with an intact blood vessel and a capillary loop forms with directed blood flow. The wound is now adequately re-perfused and this has all been accomplished within days of the trauma. New highly vascularized tissue has a granular appearance (which may be why it’s called granulated tissue), and is recognizable by its visible pinhead-size rounded nodules. Nodules that are dark red in color and appear moist and shiny indicate good healing. Poor healing is indicated by a bluish color and a smeary fibrin appearance.

Dermal remodelling

The type 3 (wound) collagen initially laid down by the fibroblasts, has been temporarily holding the wound together while healing. The wound collagen is haphazard in its layout and not very strong.

The type 3 collagen will now begin to change to type1 collagen, giving strength and structural integrity back to the wounded area by being reorganized into regularly aligned bundles oriented along the lines of stress in the healing wound. This is done by the enzyme collagenase.

Having formed abundant collagen fibres, the fibroblasts transform either into fibrocytes or myo-fibroblasts.Myo-fibroblasts in the wound area are responsible for wound contraction; the normal process where the edges of the wound migrate toward the wound centre.
Epithelial cells divide and migrate over the basal layers to regenerate the epithelium.

Basal cells continue to divide until the epithelial stratification is restored. When the coverage of the wound surface beneath the scab is complete the scab sloughs off and the epidermis begins to keratinise. Remodelling of the collagen matrix may continue for years with the extent varying among individuals and with age. The scar is rarely as strong as the tissue it replaced.