The stratum corneum is the skin’s barrier and is the top layer of the epidermis. In the epidermis, keratinocyte stem cells reside in the basal layer, which is the lowest layer of the stratified epithelia. These cells divide further and differentiate as they move upwards in the epidermis towards the stratum corneum. Found within the stratum corneum are corneocytes which are dead keratin-filled squamous cells that are tightly bound to hydroxyl molecules. Studies have shown that corneocytes are encased by cornified envelopes. These envelopes contain intercellular spaces that are filled with layers of hydrophobic and hydrophilic structures, which are formed by various intercellular lipids and act as a protective skin barrier. These spaces also include various enzymes (proteases) that control desquamation and processing of antimicrobial peptides. The antimicrobial peptides act as a microbial barrier and help to control the growth of both commensal and pathogenic bacteria. In addition, the intercorneocyte space provides a pathway which external lipophilic substances pass to reach the stratum corneum sublayers of the epidermis.
With age or unhealthy skin, the stratum corneum barrier thickens with dead keratinized skin cells and disrupts the function of the stratum corneum, making the skin unhealthy and compromised. Results of a thickened stratum corneum include lack of cell nutrients and cell protection, weakened skin immune cells, altered lipid composition and organization, and less protection from ultraviolet radiation and environmental irritants. Studies have shown that the function of the stratum corneum is crucial in barrier function and lipid composition.

NATURAL DESQUAMATION PROCESS
The desquamation process of the stratum corneum involves several enzymes that degrade the corneodesmosomes in a specific pattern. Corneodesmosomes are a class of proteins that hold the corneocytes together. It is believed that the activity of these enzymes are influenced by trans-epidermal water and the pH of the skin. The skin barrier function is maintained through this process. The lipid structure plays an important role in the natural desquamation of the skin and plays a role in the mechanism of compromised skin conditions. With skin disease, the natural desquamation of the skin can become disrupted, contributing to diseases such as eczema, dermatitis, and many other skin diseases.

IMMUNE RESPONSE, DISRUPTED SKIN BARRIER
A disruption in the skin’s barrier and function can occur as a result of an innate or adaptive immune response. Innate immunity is present at birth where certain defenses in the immune system are naturally present in the body. Innate immunity is non-specific to pathogens. Adaptive immunity is developed after an infection or vaccination, where the body can recognize to fight off a specific infectious pathogen. Cells such as tissue resident memory T-cells (TRM) are capable of producing cell-signaling cytokines and play a critical role in the development of some skin diseases. Clinical studies suggest that epidermal TRM T-cells are retained in cutaneous diseases, such as psoriasis. Immunity has a direct impact on the natural desquamation of the skin and should be considered when assessing the underlying causes of skin disease.

BARRIER INTEGRITY
Research has shown that the function of the stratum corneum is crucial with onset of disease and when managing skin conditions. Strengthening the stratum corneum will help to strengthen the skin’s barrier and function. The skin barrier can be strengthened by exfoliating dead keratinize skin cells of the stratum corneum that build up over time. This can improve skin conditions that have been compromised due to a disruption in the natural desquamation process. Exfoliating will thin the stratum corneum and stimulate new skin cells, as well as thicken the underlying layers of the epidermis. Stimulating cell renewal will encourage better product penetration and moisture retention and will stimulate a stronger and more intact protective barrier.

EXFOLIATION
Exfoliation can be either chemical or physical and usually includes the use of hydroxy acids, enzymes, or herbal peels. Chemical exfoliation is actually gentler on the skin compared to physical exfoliation. Chemical peels that contain hydroxy acids are keratolytic agents that dissolve the cellular bonds that hold dead keratinized cells together, which can help to normalize the natural desquamation process. Exfoliation with hydroxy acids can create histological changes in the epidermis and dermis and allows the skin to become more permeable, allowing optimal penetration of topical cosmeceuticals. Chemical exfoliation can help to improve the skin’s barrier and function, strengthen the epidermis, stimulate cell renewal, and increase moisture retention in the skin. Ideally, chemical exfoliation of the stratum corneum can be used to manage and improve a variety of skin conditions.

Linda Gulla is a NSPEP physician-endorsed master aesthetician and is a published writer in cosmetic dermatology, whose material has been reviewed and endorsed by dermatologist Dr. Eric Schweiger, as well as the renowned Dr. Abdala Kalil. As a published writer, Gulla’s expertise can be found in the Milady Advanced Esthetics 2nd Edition. Gulla has shared her expertise with family physicians and dermatologists as an adjunct instructor with the National Procedures Institute, where her material was reviewed by over seven medical review boards and was ACCME accredited. Gulla is founder of the Institute of Advanced Aesthetics and Health Sciences and is recognized as an approved provider with the NCEA COA. Her online self-study program can be found at iaahs.com.