Curious to learn more about the layers of skin?
Want to know what the epidermis function – the epidermis is the outermost layer of the skin – is all about? Scientist, David Layton, takes a look at one of the integumentary system’s most important components: the stratum corneum.
The outer or top layer of skin, known as the stratum corneum, provides an essential physical barrier between 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. Key components of the SC are corneocytes, which are flat, non-viable cells with protein and lipid envelopes. The extracellular matrix between the corneocytes contains 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.
Normal Functioning for Healthy Skin
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.
Dry Skin Formation (Xerosis)
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. Diligence in monitoring the occurrence 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.
- 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.
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.