Skin Repair

• Healing of a wound or surgical incision may be by:
  • First intention
  • Second intention.

Healing by First Intention

• Healing by first intention occurs when the incised ends remain in close apposition to each other and bacterial contamination is minimal.
  • This may be induced by suturing.
  • For example, a surgical incision.
• Results in minimal scarring.

Process

• Initially, the incision ruptures the dermal blood vessels.
  • The exuded blood forms a fibrinous clot between and above the incision.
    • The clot functions to:
      1. Stem the flow of blood from the injured site.
      2. Adhere the two ends together.
      3. Prevent infection from entering the injured area.
• The basal layer of epithelium wthin 1mm of the wound edge begins to lose its connections with adjacent basal and overlying epithelium.
  • Undergoes mitosis.
  • Begins to migrate down both sides of the wound under the clot using pseudopodia.
    • As they are migrating, cells differentiate to some degree to form more superficial layers of the epithelium.
      • Gives a rather thick, if not very strong, barrier of epidermis.
    • In the area of migration, the skin is usually hypopigmented and lacks hair follicles.
• Within 2-4 days, the migrating basal layer of epithelium from either side meet together under the clot.
  • It is thought that there is passage of substances, from one side to another that prevents further migration and mitosis.
  • This gives reconstituion of an intact barrier to micro-organisms.
• While the epithelial changes are occuring, there is a sudden proliferation of local fibroblasts and accompanying endothelial cells in the dermis surrounding the incision.
  • These grow across the narrow divide from each side and link up in the middle.
    • Takes about 12 hours to accomplish.
  • In the early stages (days 4-7), their alignment may be vertical, but in later stages (7-21 days) both fibroblasts and capillaries line up horizontally across the incision.
  • This dermal repair forms the major portion of strength between the two sides at this time.
    • Gains in strength over a long period of time as the collagen contracts and remodels according to the stresses imposed upon it.

Factors Inhibiting Healing

• Factors inhibiting proper wound healing include:
  1. Protein deficiency
     • May be absolute, as in starvation, or resolute, as in some of the endocrine deficiencies.
       • E.g. hypothyroidism
  2. Vitamin C deficiency
     • Vitamin C is essential for fibroplasia and to maintain the integrity of endothelial and epithelial cells.
  3. Cold
  4. Ageing
  5. Contamination
     • Infection tends not to be a complication as bacteria are generally excluded.
       • Retained foreign material such as hair portions or suture material inadvertently left in the wound will cause infection and/or a foreign body reaction.
  6. Movement
     • Gives persistent trauma.

Healing by Second Intention

• Healing by second intention occurs when the gap between the ends of the incision is too wide to allow close approximation of the ends.
• This process of granulation tissue repair in a large wound is also the underlying process in the repair of:
  • Infarcts and thrombi in vessels.
  • Surface ulcers and diphtheresis.
  • Pyogenic membrane in abscesses.
  • Diffuse fibrosis ( cirrhosis ) in the liver.

Process

• In comparison to healing by first intention, there is a more massive fibroblastic and endothelial proliferation in the wound which fills and repairs the defect.
• There is also considerable surface exudation.
  • The exudate is composed of fibrinous fluid and numerous inflammatory cells, mainly neutrophils and macrophages.
    • The cells are scavengers, and engulf necrotic debris and any bacteria present
    • Macrophages and their secretions are also important for the promotion of fibroplasia.
• The fibroblasts tend to align themselves roughly horizontal to the surface, but the endothelium is perpendicular to the surface.
  • The upper vessels form loops near to the surface.
    • Gives the gross appearance of red granules - this is granulation tissue.
• Well-formed granulation tissue tends to be fairly resistant to surface infection; however, it is rather delicated and so susceptible to trauma and subsequent introduction of infection.
  • Infection is therefore a common complication in the early stages of healing.
• Once the gap has been filled with granulation tissue and is free of infection, the epithelium migrates across.
  • As it migrates, the epithelium secretes collagenolytic substances.
• The epidermis is usually hypopigmented and lacks hair follicles unless they have survived in the granulation tissue.

Factors Inhibiting Healing

• Several factors inhibit healing by second intention.
  1. Movement
     • Movement before sufficient strength has been attained in the bond between the edges can inhibit healing.
  2. Infections
  3. Corticosteroids
     • Prevent proper collagen matrix formation.
     • Inhibit leukocyte emigration and phagocytosis.
       

       Scar tissue (Courtesy of BioMed Archive)
     • Diminish any acute inflammatory response by generally stabilising cellular membranes.

Scarring

• As the fibroblasts mature into fibrocytes, the collagen also matures and contracts and there may be extensive scar formation.
  • There may be considerable depression of the surface in such a scar.
  • The scar may interfere with movement in the area.
• The scar tends to diminish in size over a long period of time, as the underlying collagen remodels according to the stresses imposed upon the area.