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.