The epidermis is a dynamic system whose metabolic activity is regulated in large part by the integrity of the permeability barrier. This barrier resides in the stratum corneum and comprises a unique 2-compartment system of structural
protein-enriched corneocytes embedded in a
lipid-enriched intercellular matrix.
Lipid extraction or metabolic imbalances, such as essential
fatty acid deficiency, produce barrier abnormalities that in turn result in epidermal hyperproliferation, scaling, and
inflammation. When the barrier remains intact,
lipid imbalances, such as an abnormal
cholesterol sulfate:
cholesterol ratio in recessive
X-linked ichthyosis, can lead to abnormal corneocyte adhesion (visible scale). Both cellular and intercellular
proteins also participate in normal desquamation, and
protein abnormalities may provoke abnormal scaling (such as
filaggrin in
ichthyosis vulgaris). Thus, perturbations of the stratum corneum may be the catalyst for a number of
skin diseases, rather than the end result of processes that are initiated in subjacent skin layers.