The Stratum Corneum: Protector of Your Internal Organs

The stratum corneum is the outermost layer of the skin and it serves as the primary barrier between the inner layers of skin and the exterior environment.  It protects against external environmental agents such as harmful chemicals and microbes and our internal organs. It also controls the loss of body water through the epidermis to ambient air. The barrier function of the stratum corneum (SC for short) depends on a complex combination of skin structure plus biomechanical and biochemical processes.

Structure of the Stratum Corneum

The two key components of the SC are corneocytes, which are flat, non-viable cells with protein and lipid envelopes, and (2) an extracellular matrix of  lipids (fats) arranged in lamellar, sheet-like structures or bilayers. In simple terms, the corneocyte and lipid matrix is often described as a “brick and mortar” construct, with the corneocytes representing the bricks and the lipids the mortar. Each strata, or layer, works together to form the stratum corneum. Cohesion of the corneocytes is facilitated by protein structures called corneodesmosomes that connect adjacent corneocytes.

Function of the Stratum Corneum

New skin cells are constantly being produced, while at the same time corneocytes are simultaneously released from the skin surface to create a healthy, homeostatic balance. An important aspect of the normal shedding of corneocytes is the gradual degradation of near-surface corneodesmosomes via enzymatic processes. Weakened corneodesmosomes, which hold corneocytes together, facilitate their release to the exterior environment. Because corneocytes are very small, their discharge from the skin surface is not even noticeable. Nevertheless, it may come as surprise to know that shed skin cells are an important component of the organic matter content of indoor dust!

Proper functioning of the stratum corneum is maintained by the hydration properties of the SC, which primarily involve (1) the transport of water from within the deeper layers of the skin to the surface and (2) retention of water. The corneocytes contain a mixture of hydroscopic (water loving) compounds collectively referred to as the Natural Moisturizing Factor (or NMF) that absorb free or unbound water to maintain skin hydration. The lipid matrix in turn serves as a semipermeable membrane that helps control the transport of water from the hydrated inner skin layers to the exterior environment. The major lipids include ceramides, fatty acids, and cholesterol.

Hydration of the Stratum Corneum

The proximate cause of “cosmetic” dry skin that is manifested as visible flakes on the skin surface is the reduced action of hydrolytic enzymes in degrading the corneodesmosomes. As a result, corneocytes are released in visible clumps rather than single cells due to the continuing adherence to corneodesmosomes. A trigger event for the dry-skin process is typically low environmental humidity and temperature. Drying stresses in turn can disrupt the micro organization of the lipid matrix, which contributes to its dehydration. Changes in the level of the NMF in corneocytes can also exacerbate dry skin formation.

Dry Skin Management

Moisturizing lotions and creams on the market today contain a large variety of ingredients that can serve to inhibit the movement of epidermal water to air by forming an occlusive barrier (e.g., petrolatum) and/or absorb water (e.g., a humectant substance such as glycerin) to increase skin hydration. Use of a room humidifier that elevated indoor humidity will help alleviate drying stresses on skin. Diligence in monitoring indoor drying stresses along with the appearance of dry skin and then proactively applying moisturizing lotions to both prevent and control skin dehydration can go a long way to restoring the healthy functioning of the stratum corneum.

Summary

• The stratum corneum is the exterior layer of skin that provides barrier protection against environmental agents.
• A dehydrated stratum corneum can lead to dry skin formation and reduced barrier function, which can subsequently increase the risk of harmful exposures to environmental agents and negatively affect your overall skin anatomy.
• Proactive management in detecting dry skin conditions and then actively treating the impacted areas with moisturizing lotions and creams is the key to restoring a healthy stratum corneum.

References

Harding, C. R., A. Watkinson, A. V. Rawlings, and I. R. Scott. “Dry Skin, Moisturization and Corneodesmolysis.” International Journal of Cosmetic Science 22, no. 1 (February 2000): 21–52. doi:10.1046/j.1467-2494.2000.00001.x.

Proksch, Ehrhardt, Johanna M. Brandner, and Jens-Michael Jensen. “The Skin: An Indispensable Barrier.” Experimental Dermatology 17, no. 12 (December 1, 2008): 1063–72. doi:10.1111/j.1600-0625.2008.00786.x.