The stratum corneum: the rampart of the mammalian body

Background – The stratum corneum (SC) is the outermost region of the epidermis and plays key roles in cutaneous barrier function in mammals. The SC is composed of ‘bricks’, represented by flattened, protein‐enriched corneocytes, and ‘mortar’, represented by intercellular lipid‐enriched layers. As a result of this ‘bricks and mortar’ structure, the SC can be considered as a ‘rampart’ that encloses water and solutes essential for physiological homeostasis and that protects mammals from physical, chemical and biological assaults. Structures and functions – The corneocyte cytoskeleton contains tight bundles of keratin intermediate filaments aggregated with filaggrin monomers, which are subsequently degraded into natural moisturizing compounds by various proteases, including caspase 14. A cornified cell envelope is formed on the inner surface of the corneocyte plasma membrane by transglutaminase‐catalysed cross‐linking of involucrin and loricrin. Ceramides form a lipid envelope by covalently binding to the cornified cell envelope, and extracellular lamellar lipids play an important role in permeability barrier function. Corneodesmosomes are the main adhesive structures in the SC and are degraded by certain serine proteases, such as kallikreins, during desquamation. Clinical relevance – The roles of the different SC components, including the structural proteins in corneocytes, extracellular lipids and some proteins associated with lipid metabolism, have been investigated in genetically engineered mice and in naturally occurring hereditary skin diseases, such as ichthyosis, ichthyosis syndrome and atopic dermatitis in humans, cattle and dogs. Résumé Contexte – Le stratum corneum (SC) est la zone la plus externe de l’épiderme et joue un rôle clé dans la fonction de barrière cutanée des mammifères. Le SC est composé de « briques » représentant des cornéocytes aplatis riches en protéines, et en « mortier », représentant les couches intercellulaires riches en lipides. Cette structure en « briques et mortier » confère au SC un rôle de « rempart » qui emprisonne l’eau et les solutés essentiels pour l’homéostasie physiologique et qui protège les mammifères des assauts physiques, chimiques et biologiques. Structures et fonctions – Le cytosquelette des cornéocytes contient des faisceaux serrés de filaments intermédiaires de kératine regroupés avec des monomères de filaggrine qui sont dégradés par la suite en composés hydratants naturels par diverses protéases, comme