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INTRODUCTION

KEY POINTS

  • Epidermal differences include stratum corneum structure, lipid ­content, and melanin dispersion.

  • Dermal differences include varied structural organization and ­concentration of dermal components.

  • Although few definitive conclusions can be made with sparse research, biological skin differences do exist.

  • These biological differences in skin structure and function account for lower rates of skin cancers and less pronounced photoaging, but also increased incidence of keloids and a variety of pigmentary disorders.

STRATUM CORNEUM

Structure and Function

The stratum corneum forms the interface between the external environment and the body and influences barrier function and subsequently the potential for irritant reactions. The stratum corneum contains approximately 12 to 16 layers of corneocytes, each with a mean thickness of 1 μm. The primary function of the stratum corneum is to prevent evaporative water loss from the aqueous interior cell layers. The stratum corneum also protects against mechanical insults, foreign chemicals, microorganisms, and ultraviolet (UV) light. This layer was initially thought to be biologically inert, offering only a layer of protection for the more active layers underneath. However, in the past 30 years, the stratum corneum has been also found to have important biological properties.

The stratum corneum consists of a two-compartment system, termed bricks and mortar, that is composed of polyhedral corneocytes surrounded by a matrix of lipid-enriched membranes. The corneocytes are filled with keratin filaments and osmotically active small molecules, including filaggrin, loricrin, and involucrin, which also play an important role in natural moisturizing and the elastic properties of the skin. Additionally, the mechanical strength and chemical resistance of the skin barrier are due to these extensively cross-linked proteins into the corneocyte cornified envelope. Lipids in the intercellular spaces of the stratum corneum are organized into elaborate multilammelar ­structures composed of ceramides, cholesterol, and long-chain saturated fatty acids. These lipids maintain an optimal ratio to mediate the permeability barrier against excessive water and electrolyte loss. Corneodesmosomes connect adjacent corneocytes in the stratum corneum and comprise various proteins such as desmosomal cadherins, desmogleins, and desmocollins. The site of corneodesmosome hydrolysis, where proteolytic enzymes are involved in the desquamation and shedding ­process, is termed the aqueous pore pathway for water, drug, and xenobiotic movement in the epidermis.1 In a variety of pathologic conditions, the structure, composition, and organization of the stratum corneum may be altered, leading to a reduced capacity to hold water and increased transepidermal water loss (TEWL).

Structural Differences

Attention has been focused on the thickness, density, and compactness of the stratum corneum when comparing skin of color with white skin. The thickness of the stratum corneum in white and black skin is generally thought to be similar.2 A comparative study investigating the number of tape strips required to completely remove the stratum corneum (a measure of the number of layers of the stratum corneum) demonstrated a greater variability in ...

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