The skin serves as a physiologic barrier to chemical penetration and microbial invasion from the environment, as well as a barrier to fluid and solute loss from within.9 The basic structure of the skin includes the stratum corneum or horny layer, which is about 10 to 20 µm thick; the viable epidermis, 50 to 100 µm thick; the dermis, 1 to 2 mm; and the hypodermis, 1 to 2 mm thick.10 The stratum corneum is a semipermeable laminated surface aggregate of corneocytes (horny cells). Corneocytes are flat polyhedral-shaped non-nucleated remnants of terminally differentiated keratinocytes derived from the viable epidermis.
Corneocytes are composed primarily of insoluble bundled keratin surrounded by a cell envelop stabilized by cross-linked proteins and covalently bound lipids. Interconnecting the corneocytes are corneodesmosomes, which contribute to stratum corneum cohesion. The intercellular lipids, which are generated during the terminal differentiation of keratinocytes, form the only continuous domain of the skin and are essential for a competent barrier function.9 It is of interest to note that hand hygiene products and procedures can decrease skin-barrier function by extracting skin lipids, chemically inducing irritation, and physically stripping the stratum corneum.1
Under the stratum corneum is the viable epidermis, which is composed primarily of keratinizing epithelial cells, i.e., keratinocytes.11 This layer also contains melanocytes involved in skin pigmentation; Langerhans cells, which are important for antigen presentation and immune responses; and Merkel cells, which contain neuroendocrine peptides and appear to have a sensory function. The viable epidermis does not contain a vascular network and the keratinocytes obtain nutrients from the dermis by passive diffusion through the interstitial fluid.
The barrier function of the skin is under homeostatic control and the rate of keratinocyte proliferation directly influences the integrity of the skin barrier. Under basal conditions, differentiated keratinocytes require about 2 weeks to exit the nucleated compartment and an additional 2 weeks to move through the stratum corneum.12 However, keratinocytes have the capacity for increased rates of proliferation and maturation to levels far greater than this when stimulated by injury or infection. In such cases, the return to normal barrier function is biphasic: 50 to 60% of barrier recovery typically occurs within 6 hours with complete normalization of barrier function in 5 to 6 days.