How is the structure of the epidermis related to its functions?

The three most important functions of the epidermis are protection from environmental insult (barrier function), prevention of desiccation, and immune surveillance. The stratum corneum is an especially important cutaneous  barrier that protects the body from toxins and desiccation. Although many toxins are nonpolar compounds that can move relatively easily through the lipid-rich intracellular spaces of the cornified layer, the tortuous route among cells in this layer and the layers below effectively forms a barrier to environmental toxins. Ultraviolet light, another environmental source of damage to living cells, is effectively blocked in the stratum corneum and the melanosomes. The melanosomes are concentrated above the nucleus of the keratinocytes in an umbrella-like fashion, providing photoprotection for both epidermal nuclear DNA and the dermis.



The prevention of desiccation is another extremely important function, as extensive loss of epidermis is often fatal (e.g., toxic epidermal necrolysis). In the normal epidermis, the water content decreases as one moves from the basal layer to the surface, comprising 70% to 75% of weight at the base and decreasing to 10% to 15% at the bottom of the stratum corneum.

Immune surveillance against foreign antigens is a function of the Langerhans cells that are dispersed among the  keratinocytes. Langerhans cells internalize external antigens and process these antigens for presentation to T lymphocytes in the lymph nodes. Inflammatory cells (i.e., neutrophils, eosinophils, lymphocytes) are also capable of intercepting and destroying microorganisms in the epidermis.