Additionally, skin provides an “outside–inside” barrier to protect against mechanical, chemical, and microbial assaults from the external environment ( Fig. 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. The skin's most important function is to form an effective barrier between the “inside” and the “outside” of the organism. Lipid or lipid-like creams and ointments can repair disturbed barrier function.

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

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