Chapter 2: The Wound Healing Process

Introduction

Wound healing consists of a symphony of events that, when well orchestrated, results in reepithelialization of the skin and restoration of its barrier function. The first event after wounding is the establishment of hemostasis by plugging the defect with a fibrin clot. This is followed by overlapping stages of inflammation, repair (proliferation), and remodeling (Fig. 2.1). Cellular and humoral mediators conduct the symphony and consist of inflammatory cells (neutrophils, lymphocytes, mast cells, and monocytes/macrophages); repair cells (fibroblasts, myofibroblasts, and fibrocytes); epithelial cells (keratinocytes); and soluble proteins (cytokines, chemokines, enzymes, and growth factors). The extracellular matrix (ECM) provides the stage on which these mediators play, serving as a physical scaffold for repair and presenting a milieu of protein intermediates.

Stage Recognition by Gross Inspection

The stages of healing may be identified by gross inspection of the wound. (Fig. 2.2) Hemostasis is identified by clot formation and cessation of active bleeding. Inflammation is recognized by the cardinal signs of dolor (pain), rubor (erythema), calor (increased temperature), tumor (swelling—drainage or pus in this case), and loss of function. During the early part of the repair stage, the wound bed is covered by granulation tissue, which is marked by robust proliferation of fibroblasts and neovascularization, giving the wound a red and granular appearance. Granulation tissue serves as a welcoming surface for keratinocyte migration (epithelialization) during the latter part of repair. Keratinocytes may migrate from the periphery of the wound edges, or in the case of partial-thickness wounds in which the hair follicles are spared, epithelialization may arise and spread from the hair follicles. The repair stage is also identified by wound margin contraction, in which the wound edges approach the center of the wound. The repair stage culminates in the complete covering of the wound with epithelium, recognized by the cessation of drainage and a thin, pink layer of skin. Following closure, the scar continues to reorganize and gain strength through the remodeling or maturation stage. During this phase, the scar will increase in type I collagen content and vascularization will be reduced to normal, resulting in strengthening of the scar and reduction in erythema over time.