Stratum corneum Lipids and Water

Lipids in the intercellular spaces of the stratum corneum provide the permeability barrier of the skin. The primary function of the barrier is to prevent water loss to the environment. Secondarily, the barrier limits or prevents the penetration of potentially toxic substances that may contact the skin surface. The main lipids comprising the barrier are ceramides, cholesterol and long-chain saturated fatty acids. The ceramides are structurally heterogenous. In the human, there are 9 series of ceramides. Three of these are acylceramides consisting of long ω-hydroxy acids amide-linked to a long-chain base and bearing linoleate ester-linked to the ω-hydroxyl group. The base component can be sphingosine, phytosphingosine or 6-hydroxysphingosine. The other ceramides contain normal or α-hydroxy acids amide-linked to one of these bases. Linoleate is an essential fatty acid, without which the barrier of the skin cannot be maintained. This is thought to be reflected in the roles of the acylceramides in barrier formation and function. The intercellular lipids of the stratum corneum are organized into elaborate multilamellar structures. Water molecules hydrogen bond to polar head groups of the lamellae; however, there is no free water between the lamellae. Most of the water in the stratum corneum is inside the corneocytes. In a variety of pathological conditions, the lipid composition and organization are altered, leading to a reduced capacity to hold water and increased transepidermal water loss.