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AT-A-GLANCE
One of the most important functions of the skin is to form a barrier between the organism and the external environment.
The skin protects our bodies from physical damage caused by desiccation, physical stress, infection, overheating or heat loss, and ultraviolet (UV) irradiation.
The skin is covered by the epidermis, a cornified, stratified epithelial cellular sheet that is equipped with a barrier formed by the stratum corneum and tight junctions.
The stratum corneum is an air–liquid interface barrier on the body surface that prevents excessive water loss (inside–outside barrier) and the entry of harmful substances from the environment (outside–inside barrier).
The stratum corneum barrier is composed of corneocytes and intercorneocyte water-impermeable lipid lamellae. Corneocytes are wrapped with cornified cell envelope and corneocyte lipid envelope and contain keratin filaments associated with filaggrin, which is degraded to natural moisturizing factors.
The tight junctions seal the intercellular space between neighboring cells at the second layer in the stratum granulosum and form a liquid–liquid interface barrier that limits molecular movement through the paracellular pathway.
Langerhans cells are located in the epidermis under the tight junction barrier in steady state but extend their dendrites to the outside tight junction barrier upon activation to capture external antigens at the tips of the dendrites.
Antimicrobial peptides, lipids, the acidic pH of the stratum corneum, and continuous daily desquamation (daily detachment of dead skin cells) control the skin microbiota and protect us from infection by bacteria, yeasts, fungi, and viruses
UV light is reflected from the stratum corneum and absorbed by urocanic acid and melanin molecules, which protect genomic DNA from UV irradiation damage.
Sweating, blood flow control, and heat storage in subcutaneous adipose tissue protect us from cold and overheating.
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The skin is the integument of vertebrates (Table 14-1). One of the key functions of the skin is to form a protective physical barrier between the body and the external environment. The limitation of molecular movement across the skin is largely dependent on the epidermis and especially the stratum corneum in mammals. The epidermis prevents the inward and outward passage of water, electrolytes, lipids, and proteins, as well as insults from chemicals, bacteria, fungi, virus, toxins, and allergens. Defects in the formation of the epidermal barrier cause various congenital diseases (Table 14-2).
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