As recent studies have clearly demonstrated, the stratum corneum (SC) forms a primary barrier capable of opposing the penetration of any chemical substance from the outside. It is characterized by protein-enriched flat cells, called corneocytes, embedded in a lipid-enriched intercellular matrix formed by bymolecular lipidic layers (lamellae) (ceramides, triglycerides, free fatty acids, cholesterol, etc.) whose structures retain a small amount of water linked to the NMFs (sodium PCA). These extracellular lipids of the SC appear to arise in the form of ellipsoidal organelles (lamellar bodies) in the first suprabasal cell layer (stratum spinosum), and they continue to accumulate in the stratum granulosum until they account for up to 25% of the volume of the cytosol. The lipids in the lamellar bodies serve to provide for barrier function and are essential to trap water and prevent excessive water loss at level of stratum corneum. Infact, it seems that these lipids are specifically compartmentalized into the intercellular spaces to exert their water-holding properties. The degradative enzymes which are found within the intercellular spaces, may mediate the changes in lipid composition that occur during transit through the SC: from basically polar or hydrophilic lipids (glycosphingolipids, phospholipids and free sterols) into basically nonpolar hydrophobic lipids (ceramides, free sterols and free fatty acids). Similar phenomena occur in the corneocytes. The amount of fibrous proteins (keratins) and basic proteins rich in histidine and glycine increases. Finally, special glycoprotein molecules provide communication between intra and extra cellular environment. These molecules, called "adhesion molecules", preserve the functional and structural integrity of the tissue.