Chapter 2 Cosmeceuticals: Function and the Skin Barrier
THE STRATUM CORNEUM STRUCTURE AND FUNCTION
The stratum corneum is an essentially impermeable structure with layers of pancake-like cells (corneocytes) embedded in a matrix of lamella lipids (Fig. 2.1). This complex structure is continuously renewed by transformation of cells (keratinocytes) migrating up from the living epidermis. There are four key processes of stratum corneum function (Fig. 2.2). The main features are summarized in Table 2.1. When all four processes are functioning optimally the stratum corneum is an excellent moisture barrier (without which all mammals would quickly dehydrate and perish) and a very effective barrier to micro-organisms and chemicals.
THE STRATUM CORNEUM BARRIER AND THE ENVIRONMENT
Although the stratum corneum is an excellent and resilient barrier, the superficial layers are readily disturbed by low humidity, wind, sun, detergents, solvents, and other chemicals. The result is dry skin. What we experience as dry skin is not simply skin that lacks water but dysfunctional skin where aberrant desquamation leads to an accumulation of corneocytes at the skin surface (Fig. 2.3). The skin feels rough; looks dull because light is scattered by the uneven surface; looks pale because the pinky glow from the microcirculation is obscured; may show visible scaling, and is susceptible to irritation. All this is a consequence of dehydration at the skin surface.
• The stratum corneum and moisturization
Simple moisturizers are not normally classed as cosmeceuticals but in practice they have much greater benefit for skin than many cosmetic ingredients described as cosmeceutical actives. Moisturization is not the most dramatic or exciting effect of skincare products but it is a benefit delivered with great certainty and to a maximum extent. An effective moisturizer contains a good humectant, such as glycerol, to hold water in the stratum corneum, and lipid emollients that seal in moisture and prevent the washout of humectant when the skin is next in contact with water. Moisturizers reverse the negative effects of skin dryness leaving skin soft and smooth with a natural looking healthy glow. Moisturizers also restore the elasticity of the stratum corneum, making skin feel firmer and more vibrant. Skin looks better, looks healthier, and looks rejuvenated. All this from using a good moisturizer!
• The stratum corneum and cosmeceuticals
Moisturizers exert their effects in the upper regions of the stratum corneum whereas cosmeceuticals are supposed to penetrate the stratum corneum barrier to exert their effects more deeply although still at a level where they are properly regarded as cosmetic rather than drugs. To penetrate the barrier and reach a skin target in sufficient concentration to be effective is not easy. Strategies to enhance penetration are discussed later but there is an increasing recognition that there are benefits for overall skin condition by maintaining barrier integrity. Moisturizers help skin restore and maintain barrier function by activating skin processes that are switched off in dry skin, but is it possible to directly enhance barrier integrity?
