The Anatomy of the Stratum Corneum

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Stratum Corneum Overview

The epidermis is the outermost section of the skin and it's made up of five layers. The stratum corneum is the outermost of those five layers and largely acts as a barrier.

Before the mid-1970's, the stratum corneum was thought to be biologically inert, like a thin plastic sheet protecting the more active, lower layers of the skin. Over the past few decades, scientists have discovered that the biological and chemical activity of the stratum corneum is actually very intricate and complex.

Understanding the structure and function of the stratum corneum is vital because it is the key to having skin that is healthy and attractive. These illustrations will take you through the important components of the stratum corneum.

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The Corneocyte

The stratum corneum has a "brick and mortar" type of structure, and the "bricks" in this analogy are protein complexes called corneocytes (see illustration). A corneocyte is made of tiny threads of keratin in an organized matrix. The keratin can hold large amounts of water between the fibers/threads. The stratum corneum contains about 12 to 16 layers of corneocytes, and each corneocyte has a mean thickness of 1 micrometer, depending on the following factors: age, anatomical location, and exposure to UV radiation.

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Lamellar Bodies

Lamellar bodies are formed in the keratinocytes of the stratum spinosum and stratum granulosum. When the keratinocyte matures to the stratum corneum, enzymes degrade the outer envelope of the lamellar bodies, releasing types of lipids called free fatty acids and ceramides.

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Intercellular Lipids

Free fatty acids and ceramides that are released from the lamellar bodies fuse together in the stratum corneum to form a continuous layer of lipids. Because there are two types of lipids, this layer is referred to as a lamellar lipid bilayer. This lipid bilayer plays a major role in maintaining the barrier properties of the skin and is analogous to the "mortar" in the brick and mortar analogy.

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Cornified Envelope

Each corneocyte is surrounded by a protein shell called a cell envelope. The cell envelope is composed primarily of two proteins, loricrin and involucrin. These proteins contain extensive links between each other, making the cell envelope the most insoluble structure of the corneocyte. The two sub-types of cell envelopes are described as "rigid" and "fragile," based on the interaction of the lamellar lipid bilayer with the cell envelope.

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Cornified Envelope Lipids

Attached to the cell envelope is a layer of ceramide lipids that repel water. Because the lamellar lipid bilayer also repels water, water molecules are held between the cell envelope lipids and the lipid bilayer. This helps maintain the water balance in the stratum corneum by trapping water molecules, instead of letting them get absorbed into the lower layers of the epidermis.

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Corneodesmosomes

The "rivets" that hold the corneocytes together are specialized protein structures called corneodesmosomes. These structures are also a part of the "mortar" in the "brick and mortar" analogy. Corneodesmosomes are the major structures that must be degraded for the skin to shed in a process called desquamation.

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Natural Moisturizing Factor (NMF)

Natural moisturizing factor (NMF) is a collection of water-soluble compounds that are found only in the stratum corneum. These compounds comprise approximately 20 to 30 percent of the dry weight of the corneocyte. NMF components absorb water from the atmosphere and combine it with their own water content, allowing the outermost layers of the stratum corneum to stay hydrated despite exposure to the elements. Because NMF components are water-soluble, they are easily leached from the cells with water contact—which is why repeated contact with water actually makes the skin drier. The lipid layer surrounding the corneocyte helps seal the corneocyte to prevent the loss of NMF.

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Desquamation Process

The desquamation or exfoliation process of the stratum corneum is actually very complex and only parts of this process are fully understood. It is known that several enzymes degrade the corneodesmosomes in a specific pattern, but the exact nature of these enzymes or how they become activated in order to start the exfoliation process is not known. Water and pH play a significant role in the activity of these enzymes.

Sources:

van Smeden J, Hoppel L, van der Heijden R, Hankemeier T, Vreeken RJ, Bouwstra JA. LC/MS analysis of stratum corneum lipids: ceramide profiling and discovery . J Lipid Res. 2011 Jun;52(6):1211-1221.

Walters RM, Mao G, Gunn ET, Hornby S. Cleaning formulations that respect skin barrier integrityDermatol Res Pract. 2012;2012:495917.

Johnson, AW. (2015). Cosmeceuticals: Function and the Skin Barrier. Procedures in Cosmetic Dermatology - Cosmeceuticals. Ed. Zoe Diana Draelos. Elsevier, 11-17.

Verdier-Sévrain S, Bonté F. Skin hydration: a review on its molecular mechanisms. J Cosmet Dermatol. 2007 Jun;6(2):75-82.

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