The clinical relevance of maintaining the functional integrity of the stratum corneum in both healthy and disease-affected skin

Abstract

It has been recognized for approximately 50 years that the stratum corneum exhibits biological properties that contribute directly to maintaining and sustaining healthy skin. Continued basic science and clinical research coupled with keen clinical observation has led to more recent recognition and general acceptance that the stratum corneum completes many vital "barrier" tasks, including but not limited to regulating epidermal water content and the magnitude of water loss; mitigating exogenous oxidants that can damage components of skin via an innate antioxidant system; preventing or limiting cutaneous infection via multiple antimicrobial peptides; responding via innate immune mechanisms to "cutaneous invaders" of many origins, including microbes, true allergens, and other antigens; and protecting its neighboring cutaneous cells and structures that lie beneath from damaging effects of ultraviolet radiation. Additionally, specific abnormalities of the stratum corneum are associated with the clinical expression of certain disease states. This article provides a thorough "primer" for the clinician, reviewing the multiple normal homeostatic functions of the stratum corneum and the cutaneous challenges that arise when individual functions of this thin yet very active epidermal layer are compromised by exogenous and/or endogenous factors.

Figure 1

Note the histologic appearance of the stratum corneum that has classically been described as “basketweave hyperkeratosis.” This appearance of the stratum corneum portrays this outermost epidermal layer as a simple lattice of keratin fibrils. In fact, this appearance is an artifact of histologic processing and misrepresents the true in-vivo appearance and structural integrity of the stratum corneum.

Figure 2

This figure depicts the “bricks and mortar” structure of the stratum corneum. The corneocytes represent the bricks and the intercellular lamellar lipid membrane represents the mortar. Corneocytes comprise primarily keratin macrofibrils, are protected externally by a cornified cell envelope, and are held together by corneodesmosomes. The intercellular lamellar lipid membrane is primarily composed of ceramides, cholesterol, and fatty acids. A mixture of multiple small hygroscopic compounds present within corneocytes, referred to collectively as natural moisturizing factor (NMF), plays a vital role in the physiological maintenance of stratum corneum hydration. (Reprinted with permission from Harding CR. Dermatol Ther. 2004;17:6–15.)

Figure 3

Major stratum corneum lipids and relative content by weight (%). Stratum corneum lipids represent 20% of stratum corneum volume.

Figure 4

The “Epidermal Factory”: Progressive layers and corresponding production steps

Figure 5

Physiological adaptions of the stratum corneum in response to factors promoting desiccation (e.g., increase in transepidermal water loss, decrease in natural moisturizing factor, damage or reduction in stratum corneum lipids, damage to stratum corneum proteins)

Figure 6

Stepwise progression from damage to permeability barrier ➙ xerosis ➙ eczematous dermatitis