TABLE 1.
The Stratum Corneum and Simultaneous Multitasking: Individual Epidermal Barrier Functions
| BARRIER FUNCTION | MAJOR COMPONENTS |
|---|---|
| Permeability Barrier | Formation of stratum corneum lipids in specific ratio from precursor lipids |
| Production of lamellar bodies packaging precursor lipids and some antimicrobial peptides | |
| Formation of natural moisturizing factor from filaggrin (converted from profillagrin) | |
| Formation of cornified envelope and the corneocyte-lipid envelope | |
| Maintenance of water gradient, calcium gradient, acid mantle (acidic pH) | |
| Response of primary pro-inflammatory cytokines to impairment of permeability barrier | |
| Antimicrobial Barrier | Maintenance of an acidic skin pH decreases skin colonization by pathogenic bacteria |
| Antibacterial activity of stratum corneum lipids (eg, free fatty acids, sphingosine, other) | |
| Genetically-encoded primary antimicrobial peptides (defensins, cathelicidins, dermcidins) synthesized in SC, present in sebum, and in sweat (dermicidin-derived) | |
| Multiple agents with antimicrobial activity as alternative function (some chemokines, some neuropeptides, other) | |
| Antioxidant Barrier | Network of enzymatic and non-enzymatic antioxidant systems to counter oxidative stress |
| Antioxidants present in epidermis (stratum corneum, skin surface lipids) and dermis | |
| Hydrophilic non-enzymatic antioxidants include ascorbic acid (vitamin C) and uric acid | |
| Major lipid-soluble non-enzymatic antioxidant is alpha-tocopherol (vitamin E) | |
| Co-antioxidants (ascorbic acid, ubiquinol [Coenzyme Q10]) allow tocopherol regeneration | |
| Gradients in stratum corneum for ascorbic acid and tocopherol (lowest near surface) | |
| Interceptive antioxidant enzymes (catalase, superoxide dismutases, glutathione peroxidases) | |
| Antioxidant repair enzymes (eg, methionine sulfoxide reductase) | |
| High concentration of alpha-tocopherol in sebum accounts for high levels in facial sebaceous gland stratum corneum (sebum serves as a physiologic delivery pathway) | |
| Immune Response Barrier | Dendritic cells involved in immune surveillance and antigen recognition (eg, plasmacytoid dendritic cells, myeloid dendritic cells, Langerhans cells) |
| Toll-like receptors involved in recognition of microbial pathogens and other agonists | |
| Antimicrobial peptides and some of their enzymatic conversion products (eg, LL-37) | |
| Innate and acquired immune response pathways and balance with T regulatory cell system | |
| Photoprotection Barrier | Epidermal melanin barrier (degree of protection related to Fitzpatrick Skin Type) |
| Stratum corneum protein barrier | |
| Antioxidants within stratum corneum (protection against photo-oxidative stress) | |
| Optical reflective properties of the stratum corneum (stratum corneum thickness more important than epidermal thickness for protection against ultraviolet/solar radiation) |