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Skin Barrier at a Glance
  • The most important function of the skin is to form a barrier between the organism and the environment.
  • The skin barrier prevents excessive water loss (inside–outside barrier) and the entry of harmful substances from the environment (outside–inside barrier).
  • The physical barrier is predominantly located in the stratum corneum.
  • The stratum corneum barrier is composed of corneocytes and intercellular lipids, cholesterol, free fatty acids, and ceramides.
  • Keratins and cornified envelope proteins are important for the mechanical stability of the corneocytes.
  • The cornified envelope protein involucrin binds ceramides covalently, forming a backbone for the subsequent attachment of free ceramides.
  • The nucleated epidermis through tight junctions and desmosomes also contributes to the barrier.
  • Experimental barrier disruption increases epidermal lipids and changes in epidermal differentiation.
  • The signals for barrier recovery are cytokines and the calcium ion gradient.
  • Several diseases are characterized by a probably genetically disturbed barrier function. The disturbed barrier function contributes to disease pathology, in particular in contact dermatitis, atopic dermatitis, forms of ichthyosis, and psoriasis.
  • Lipid or lipid-like creams and ointments can repair disturbed barrier function.

The skin's most important function is to form an effective barrier between the “inside” and the “outside” of the organism. Life on dry land requires the presence of a barrier to regulate water loss and prevent desiccation, commonly referred to as the “inside–outside” barrier. Additionally, skin provides an “outside–inside” barrier to protect against mechanical, chemical, and microbial assaults from the external environment (Fig. 47-1).1 To perform these functions the epidermis undergoes keratinization, a process in which epidermal cells progressively mature from basal cells with proliferative potential to the lifeless, flattened squames of the stratum corneum (SC) (Fig. 47-2). Both the SC and the deeper skin layers protect the skin from mechanical forces, ultraviolet (UV) radiation, cold and hot temperatures, and invasion of chemicals and microbes. To effectively perform this multiplicity of functions, the skin contains different types of barriers. The physical barrier consists mainly of the SC, but the nucleated epidermis, in particular the tight junctions, provides another important barrier component. The chemical/biochemical (antimicrobial) barrier consists of lipids, acids, lysozymes, and antimicrobial peptides (discussed in Chapter 10). The humoral and cellular immune system provides a barrier to infectious disease, but immune hyperactivity may lead to allergy (Table 47-1).

Figure 47-1

Functions of the epidermal “inside–outside” and “outside–inside” barrier.

Figure 47-2

The nucleus in the stratum granulosum (SG) should be flattened. In the basal cell layer please add a hemidesmosome towards the basal membrane (please review the former graphic). The lamellar bodies should be hatched (including the membrane fused bag shaped lamellar bodies).

Table 47-1 Different Skin Barriers

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