is continuously renewing and altering itself in response to endogenous and exogenous stimuli. To gain an understanding of the altered physiology that leads to skin becoming vulnerable, it is necessary to first understand the physiology of healthy skin. Figure 1 - Normal skin structure Epidermal self-renewal in normal skin The process of epidermal renewal is ensured by antigen p63 and CD29 (integrin -1B), expressed in epidermal stem cells in the basal layer. These proliferative progenitor cells have the capacity for self-renewal and for generating the differentiated cells that offer a better resistance to environmental hazards (see Figure 2 ). By developing a stratified epithelium of terminally differentiating cells, the epidermis generates a self-perpetuating barrier that helps keep harmful micro-organisms out and essential body fluids in. These stem cells reside in the adult hair follicles, sebaceous glands and epidermis. Figure 2 - Normal skin structure and function Transcription factor p63 is implicated in the maintenance of epithelial stem cells in stratified epithelia. It regulates the expression of extracellular matrix adhesion molecules, including basal integrins, which are necessary to orient asymmetric cell divisions during epithelial stratification, and desmosomal proteins, which are determinants of epithelial integrity and proliferative maintenance [2] . MicroRNA 203 stops the translation and suppresses the activity of p63, which results in a transition from proliferating stem cells to differentiating keratinocytes as they exit the basal layer and move outward to the skin surface. CD29 is a type 1 glycoprotein expressed on most cells, of which the isoform, known as