A classic definition of wound healing is that it is a dynamic process, involving multiple types of cells, cytokines, and chemical mediators that work together in a complex interaction. Although knowledge of wound healing is important in all medical fields, it is particularly significant to dermatologists as they create and care for more wounds than all other specialties.1 The basic science of wound healing can be divided into five phases: hemostasis, inflammation, proliferation, tissue remodeling, and finally resolution (Fig. 10-1).2 However, these phases do not always occur sequentially but are rather highly integrated and often overlap. We will describe these phases of wound healing and also discuss several important factors that impair wound healing.
Phases of wound healing. Upon injury, the initial hemostatic response involves platelets as the primary responders. This is followed by inflammation which increases vascular permeability, allowing access for neutrophil and macrophage-derived factors. Proliferation of endothelial cells and keratinocytes promotes re-epithelialization. Ultimately progression to remodeling and resolution of the wound occurs.
Wounds can be classified as either acute or chronic on the basis of the length of the time it takes to complete epithelialization, one of the elements of the proliferative phase of the healing process. Although the exact time distinction used in classification can be arbitrary and may be affected by a number of factors such as the shape, location, and cause of the wound, chronic wounds are generally associated with physiological impairments that slow or prevent healing, such as advanced age or underlying disorders.
The depth of a wound has a significant impact on how wounds heal, the healing process and its clinical management. For example, if the injury is limited to the epidermis, the epidermis can relatively easily regenerate itself, without scarring. This, similar to limited injury in the liver, is one of the few instances in which regeneration, as opposed to repair, occurs after injury. However, with deeper wounds, involving part or all of the dermis and deeper layers, regeneration does not occur, but rather a repair process is initiated that usually results in scarring. Interestingly, regeneration can sometimes occur in specific situations even when the dermis is involved. One example of this is in early (first two trimesters) fetal skin wounds. Although not entirely understood, this is likely due to a reduced inflammatory response and the presence of higher concentrations of glycosaminoglycans and type III collagen and decreased levels of transforming growth factor b1 (TGF-b1), seen in fetal dermis. Overall, these factors are likely important in mediating regeneration rather than repair.3
Wound classification is based on the depth of involvement. Superficial wounds involve only the epidermis and upper dermis. If only the epidermis ...