vva. ter -v-a.p ~r

The stratum corneum or 'cornified layer' is a multicellular, metabolically inactive surface layer of the skin comprised of flattened, stacked cells which are the intact remains of what was once living epidermis tissue. The stratum corneum exhibi~ regional differences in thickness over the body, being as thick as several hundred micrometres on the palms of the hand and the soles of the feet in adults, the average thickness over the body being about 10 1J.lIl. This tissue is also very dense, about 1.5 g cm-3 in the dry state1 . Its cellular origin is in the basal layer of the epidermis where cell division begins a complex process in which cells migrate outward and towards the body surface. In this process the cells flatten and dehydrate and undergo intracellular changes such that, when they become the stratum corneum, they are dense, keratin-filled and metabolically inactive disc-like concentrates of their original forms. In spite of being such a thin membrane, the stratum corneum forms the main barrier to microbes, radiation and chemicals brought into contact with the skin. It is also important as a thermal barrier and as part of the temperature-regulating mechanism. The general rationale for the use of topical dosage forms involves the manipulation of or assistance of this barrier function of the stratum corneum. The required degree of elasticity of this tissue is dependent upon the proper formation of the stratum corneum (psoriatic plaques tend to split and crack) and on the presence of adequate natural lipids, hygroscopic substances and moisture2 . Both lipids and water plasticize this tissue, that is, they tend by their presence to make it less brittle and excessive removal of either lipid or moisture leads to 'chapping'. The importance of water in the stratum corneum was first reported by Blank3 in 1952. He observed that applied oils did not soften the dehydrated stratum corneum, but that water was readily absorbed by the tissue and softened it. Most determinations on dry skin have been performed using abnormal tissue, such as callus3 , or diseased skin, such as that present in ichthyosis, psoriasis and essential fatty acid defic­ iency4-6 These experiments suggest that such dry skin is thickened, hyperproliferative, depleted in 'natural moisturizing factors', de­ ficient in water-binding capacity, has an abnormal lipid composition and an abnormal permeability to water7 .