Wound Healing and Skin Structure

Study Notes

The skin acts as a protective barrier against physical damage, pathogens, and fluid loss, also contributing to immune-neuroendocrine functions.
It consists of two main layers: the epidermis and the dermis.
The epidermis contains various cells including keratinocytes, melanocytes, and immune cells, along with a basement membrane.
The dermis is home to skin appendages, mast cells, fibroblasts, and a complex extracellular matrix crucial for support and communication.
Sensory axons are densely innervated, transmitting sensations like pain, temperature, and pressure to the brain.
Skin circulation involves parallel arterial-venous networks regulated by sympathetic nerves, ensuring oxygen supply and waste removal.
Lymphatic vessels aid in draining fluid and immune cells from the skin, connecting to local lymph nodes before converging to the thoracic duct.

Restoration of skin integrity post-injury involves a collaboration of mononuclear cells, resident cells, cytokines, and growth factors.
Wound healing occurs in three overlapping phases: inflammatory, proliferative, and remodeling.
The inflammatory phase is characterized by neurogenic inflammation and hemostasis, beginning seconds after injury and lasting about one hour.
Neutrophils rapidly infiltrate the wound within the first 24 hours, followed by inflammatory monocytes/macrophages that dominate during the proliferative phase.
Lymphocytes reach a peak by day four post-injury, continuing for weeks.
The remodeling phase starts two weeks post-injury, focusing on tissue reorganization and scar formation, which may last for months.

A wound is defined as a disruption in the body’s structural continuity, which can be superficial or deep.
Healing quality depends on local factors, systemic mediators, existing health issues, and injury type.
Chronic wounds result from inadequate repair processes unable to restore integrity in a timely manner.

Inflammatory Phase: Mediated by vasoactive substances, involves leaky capillaries allowing neutrophil migration through diapedesis.
- Neutrophils perform phagocytosis and release IL-1 and TNF-α, activating fibroblasts.
- Macrophages clear debris and secrete growth factors like PDGF and TGF-β, essential for later healing stages.
Proliferative Phase: Focuses on fibroblast proliferation, collagen synthesis, re-epithelialization, angiogenesis, and nerve repair.
- Fibroblasts and growth factors are central to creating a stable extracellular matrix.

Laser resurfacing can cause tissue ablation, collagen shrinkage, and long-term remodeling.
Neocollagenesis observed for up to three months post treatment as indicated by heat shock protein expression.

Ideal dressings maintain a moist wound environment for epithelial migration and healing.
Different types of dressings cater to specific wound conditions (hydration, absorption, infection control).
Gauze:
- Composed of woven or non-woven fibers, inexpensive, and absorbent.
- Wet-to-dry dressings can cause pain and harm healing due to mechanical debridement.
- Petroleum impregnated gauze prevents adhesion and provides hydration benefits.
Hydrocolloids:
- Contains hydrophilic colloids that form a gel to absorb moisture and promote an optimal healing environment.
- Remain effective for up to seven days; disadvantages include potential skin maceration and malodor.
Alginates:
- Derived from brown seaweed, ideal for highly exudative wounds due to their absorbency and hemostatic properties.
- Forms a gel that maintains moisture but may lead to confusion with infection if the gel appears yellow and foul-smelling.