Introduction

As the outermost layer of the epidermis, the stratum corneum is the first line of defense for the body, serving an essential role as a protective skin barrier against the external environment. The stratum corneum aids in hydration and water retention, which prevents cracking of the skin, and is made up of corneocytes, which are anucleated keratinocytes that have reached the final stage of keratinocyte differentiation. Corneocytes retain keratin filaments within a filaggrin matrix, and the cornified lipid envelope replaces the keratinocyte plasma membrane. These flat cells organize in a brick and mortar formation within a lipid-rich extracellular matrix. Pathophysiology of the stratum corneum is typically secondary to either protein or lipid defects. Other clinically significant signs include parakeratosis, which is the incomplete maturation of keratinocytes, and the morphological retention of nuclei in the stratum corneum. Abnormal parakeratosis of the stratum corneum can appear in patients with psoriasis, chronic eczema, and squamous cell carcinoma.[1] Scaling, or visible peeling and flaking of the skin, furthermore is a salient manifestation of diseases of the stratum corneum.[2][3][4]

Structure

The stratum corneum is the outermost layer of the epidermis and marks the final stage of keratinocyte maturation and development. Keratinocytes at the basal layer of the epidermis are proliferative, and as the cells mature up the epidermis, they slowly lose proliferative potential and undergo programmed destruction. These finally differentiated, enucleated keratinocytes are termed corneocytes and retain only keratin filaments embedded in filaggrin matrix. Cornified lipid envelopes replace the plasma membranes of the previous keratinocytes, and the cells flatten, connecting to one another with corneodesmosomes and stacking as layers to form the stratum corneum. This outermost barrier level is made up of a network of corneocytes and an extracellular lipid matrix. The stratum corneum functions as a two-compartment system, with the hydrophobic, protein-rich corneocytes sequestered in a lipid-enriched matrix. This network is organized in a “bricks and mortar” formation, with the extracellular matrix organizing into lamellar membranes.[5][6]

The human stratum corneum comprises 15 or so layers of flattened corneocytes and is divided into two layers: the stratum compactum and the stratum disjunctum. The stratum compactum is the deep, dense, cohesive layer, while the stratum disjunctum is looser and lies superficially to the stratum compactum. As the stratum disjunctum continues to lose adhesiveness secondary to decreased inter-corneocyte adhesion, the cells desquamate.