INTRODUCTION

Understanding of the structure and functioning of the stratum corneum has advanced greatly in the 4 years since the first edition of this book. Advances in physical and biomolecular techniques are unraveling the dynamic behavior of the bricks and mortar model that skin scientists have long used as their mantra. For the cosmetic scientist hoping to use the new insights to propel cosmeceuticals more effectively through the skin barrier there is disappointment. The recent science does more to explain why the stratum corneum is such an excellent barrier than it does to reveal new strategies to enhance penetration. Delivery of actives across the skin barrier remains a major challenge and continues to limit the physiologic usefulness of topically applied products.

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.

Fig. 2.1 The stratum corneum sitting above the living epidermal layers: (A) a typical bricks and mortar representation that does not reflect the relative proportions of skin cells, and (B) a truer to life representation of the stratum corneum structure with closely opposed and heavily cross-linked corneocytes and intercellular lipids. Corneocyte length is 50–100 times greater than thickness

Fig. 2.2 The four key processes for the formation and functioning of the stratum corneum

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.

Fig. 2.3 (A) Stratum corneum fully hydrated at the skin surface with normal desquamation and release of effete corneocytes. (B) Stratum corneum with reduced hydration in the superficial layers leading to incomplete desquamation and a build up of attached cells at the skin surface (skin dryness)

Water is also important for maintaining the elasticity of the stratum corneum. Without water the skin feels tight and may crack in regions subject to stretching forces (e.g. knuckles). Ultraviolet (UV) radiation, which we mostly associate with sunburn in the short term and photoaging over time, may also damage the stratum corneum by disrupting the conversion of filaggrin to NMF (natural moisturizing factor).