The Anatomy of the Stratum Corneum

The stratum corneum is the outermost layer of the epidermis. Sometimes referred to as the horny layer of the skin, the stratum corneum is composed mainly of keratin—the protein comprising human hair and nails, as well as structures such as horns, hooves, and claws of animals—and lipids (fats).

As such, the stratum corneum primarily functions as a barrier between the deeper layers of skin and the outside environment, preventing toxins and bacteria from entering the body. It also helps to keep moisture from evaporating into the atmosphere and so is important for keeping the skin hydrated.

The stratum corneum was thought to be basically inert. Since then, scientists have discovered that in fact, the stratum corneum has a complex structure and is in a constant state of change.

Corneocytes

The stratum corneum often is described as having a "brick and mortar" type of structure. In this analogy, the "bricks" are corneocytes, which originate in the deepest layer of the epidermis, the stratum spinosum, as cells called keratinocytes.

As the name suggests, keratocytes primarily consist of keratin, which again, is the protein that also makes up hair and nails. As these cells move up through the layers of the epidermis to the stratum corneum, they lose their nucleus and flatten out. It's at this point they're considered corneocytes.

Each corneocyte is about a micrometer thick, although the thickness of corneocytes also depends on factors such as a person's age, exposure to ultraviolet UV radiation, and location on the body. For example, they tend to be thicker on the hands and feet and thinner in more delcate areas such as around the eyes.

Lamellar Bodies

Lamellar bodies are organelles that form within keratinocytes. As a keratinocyte matures and moves toward the stratum corneum, enzymes degrade the envelope surrounding the lamellar bodies within it, which triggers the release of three types of lipids—free fatty acids, cholesterol, and ceramides.

Intercellular Lipids

The lipids released as the lamellar bodies degrade form the "mortar" that holds together the corneocytes that are the building blocks of the stratum corneum. This triple layer of lipids consisting of free fatty acids, cholesterol, and ceramides plays an essential role in helping to maintain the barrier properties of the stratum corneum.

Cell Envelope

Each corneocyte is surrounded by a shell called a cornified cell envelope. The cell envelope is composed of proteins that are tightly packed together, making the cell envelope the most insoluble structure of the corneocyte. Of these proteins, loricrin makes up more than 70% of the cell envelope. Other proteins in the cornified cell envelope are involucrin, small proline-rich proteins, elafin, keratin filaments, filaggrin, cystatin-A, and desmosomal proteins

Cell Envelope Lipids

Attached to the cell envelope is a layer of ceramide lipids that repel water. Because the lamellar lipid layers also repel water, water molecules are held between the cell envelope lipids and the lipid layer. This cellular structure helps maintain the water balance in the skin, allowing trapped water molecules to stay closer to the surface, thereby giving skin a healthy and hydrated glow.

Corneodesmosomes

Holding the corneocytes together are specialized protein structures called corneodesmosomes. These structures also are part of the "mortar" in the "brick and mortar" analogy. Corneodesmosomes are the structures that must be degraded for the skin to be shed.

Natural Moisturizing Factor (NMF)

Natural moisturizing factor (NMF) is composed of water-soluble compounds that are found only in the stratum corneum. These compounds comprise approximately 20% to 30% of the weight of the corneocyte. NMF components absorb moisture 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 upon contact 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.

Desquamation Process

Desquamation is, again, the clinical term for the shedding of dead corneocytes from the surface of the stratum corneum. For this process to take place, certain enzymes cause the destruction of the corneodesmosomes. How these enzymes are activated isn't fully understood. It is known that the cell cycle in the stratum corneum—from the time a cell is formed until it is shed—takes approximately 14 to 28 days.

Having a basic understanding of how the stratum corneum is formed and how it functions can be helpful when it comes to skincare. Overcleansing, using harsh exfoliants, and rubbing skin can strip the outer layer of skin of natural lipids and protective factors. Sun exposure can also cause damage to the stratum corneum. Given that everyone's skin is different, it can be helpful to consult a dermatologist to develop a healthy skin care regimen.

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