Module 4: Skin Physiology - The Barrier Function

THE BARRIER FUNCTION:

Skin-Environment Interaction:

The sequence below will be used to highlight the structure and function of the barrier to water and chemicals that is so vital to an appreciation of skin function and a good skin care management programme in the workplace.

The "brick and mortar model" best describes the barrier. During differentiation epidermal cells undergo significant changes. Nuclei are lost and keratin and other proteinaceous materials are organised to form the flattened keratinocyte bricks..

During this process of differentiation and cornefication specialized structures within the epidermal cells containing specialized lipids are excreted into the intercellular spaces and aggregate into lipid bi-layers, the inter-cellular mortar. This mortar is strongest proximally and weakens distally to allow exfoliation/desquamation of the cells of the outermost layer of the stratum corneum.

Crucial to the whole skin barrier and stratum corneum is an outer layer of glandular secretions which contribute to skin pH and the maintenance of a hydrated stratum corneum. The outer layer of secretion plaster acts as an endogenous moisturiser. The humectants glycerol, urea and lactic acid, selectively accumulate in the corneocytes trapping and holding water, thus maintaining its hydration and hence function.

The barrier allows for internal water loss to maintain adequate hydration of the stratum corneum and hence its flexibility and function.

The barrier allows easy permeation of lipid soluble (hydrophobic) substances through the functional intact skin. This occurs via the intercellular lipid bi-layer mortar.

Although there is selective permeation of water-soluble (hydrophilic) compounds across the intact barrier, it is limited and the mechanism not well understood.

The stratum corneum can act as a reservoir for both hydrophilic and hydrophobic compounds.

Concentrations of substances can accumulate here facilitating and encouraging diffusion across the intact barrier. Because of the reservoir effect this absorption can continue long after the exposure has been terminated. Fortunately, the natural process of exfoliation reduces the reservoir spontaneously given time.

The intact skin is an effective barrier to infective organisms. Damaged skin is easily colonised offering easy access to pathogenic microorganisms. Several mechanisms are responsible for this protective barrier effect.