Wound Healing

Introduction

           Wound healing is a complex process that is not fully understood.  Involved in natural wound healing are many cell types, growth factors, and other proteins, among other influential factors.  All tissues undergo some degree of tissue repair/regeneration.  The process varies for different tissues, but generally follows a consistent development.
           The outcome of wound healing, or tissue repair, is the substitution of viable cells for dead cells.  This can occur via regeneration or replacement.  In regeneration, the cells are replaced with the same types that were lost and tissue function is generally restored.  In replacement, the tissue is eventually replaced with a different type of tissue, namely scar tissue.  Most wounds heal through a combination of the two processes; the specific combination depending on the type of tissue and the nature and extent of the wound. [1] Ideally, the wound would heal completely by regeneration.  Healing in soft epithelial tissue occurs with the concurrent actions of three basic phenomena: wound contraction, formation of granulation tissue, and reepethialization. [2]

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Dermal Wound Healing

            Skin repair is similar to wound repair in other tissues, especially epithelialized tissues.  One important difference, however is that skin is exposed to air and the external environment.  Evolution has likely caused wound repair in the skin to adapt and emphasize survival of the organism via the prevention of infection over functionality of replaced tissue.  Scar tissue effectively seals an open wound, but provides poor functionality compared to the original tissue.  Cosmetic related concerns provide a desire for improved cutaneos wound healing with less scarring, but perhaps at the expense of prolonged healing time.

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Biology of Wound Healing

            After wounding of the skin, a blood clot forms, which may or may not be able to completely close the gap of the wound, depending on its size.  An inflammatory response induces vasodilatation and brings increased numbers of blood cells to the area.  Neutrophils invade the area and ingest bacteria and tissue debris to help clear the area for repair.  Fibroblasts migrate into the clot and produce collagen and other extracellular matrix components.  The clot is eventually broken down and replaced by granulation tissue, which consists of fibroblast, collagen, and capillaries.  Scar is formed when a large amount of this granulation tissue persists after the healing of the wound.  In large wounds, a large amount of granulation tissue forms and wound contraction occurs as a result of the contraction of fibroblasts in the granulation tissue.  Wound contraction can lead to large and disfiguring/debilitating scars. [1]
           Below are schematics of the healing process for a dermal wound at day 3 and at day 5.  Click images for larger views.

      
Images source: Reference [14]

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Ways to Improve Healing

            Using tissue engineering strategies, it may be possible to reduce scar tissue formation and instead promote tissue regeneration over replacement in epidermal and dermal wounds.  In conjunction with topical antibiotics to prevent infection, down regulation of fibroblast activity and granulation tissue formation could lead to reduced scar formation.  A topical salve that would reduce scar tissue formation and promote regeneration in dermal applications is would have great clinical and surgical applications, as well as use as a self-treatment option for superficial wounds.

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