The very thin stratum corneum (SC), 10–30 µm or 1/10 the thickness of a piece of paper, is the skin’s outer and most exposed layer, and the permeability barrier between the internal milieu and hostile environmental assault. Because it is exposed externally, the skin provides protection and acts as a barrier to the outside environment in order to maintain internal homeostasis. Although the SC has many functions, its main ability is to serve as a protective barrier that prevents excess fluid and electrolyte loss, allowing life to exist in a fragile terrestrial environment.
A wall of protection
The SC accounts for up to 75% of the epidermis, making it a virtual wall of protein protection for the cellular and inner-cellular inhabitants. “Stratum Corneum Defense Functions: An Integrated View,” written by Peter M. Elias, MD, states, “The permeability barrier is mediated by the organization of the extra cellular lipids of the SC into a series of parallel membrane structures, and its distinctive composition.”1
This fascinating upper layer of the epidermis is composed of fibrous protein-enriched corneocytes and an essential lipid-enriched intercellular matrix. During corneocyte formation, the plasma membrane of viable cells is replaced by a ceramide monolayer, covalently bound to the underlying cornified envelope. This lipid envelope functions as a template for accumulating extra cellular lipid layers.
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According to the article “Adhesion and Debonding of Biological Soft Tissues—Stratum Corneum,” by Kenneth Wu of the Reinhold H. Dauskardt Research Group at Stanford University in Stanford, California, “The layered construction of the skin represents a composite material in which the components possess specialized functionalities to accommodate a variety of conditions from mechanical stresses to variable ambient moisture, and to resist the presence of toxic chemicals, pathogens and radiation.” E Proksch, et al, in the study “Barrier Function Regulates Epidermal Lipid and DNA Synthesis,” explain, “The lipids are a mixture of sphingolipids, cholesterol and free fatty acids, which form intercellular membrane bi-layers.”2
These bi-layers house essential intercellular residents that are affected by exposure to the environment, and other elements, including professional spa treatments and daily cleansing that remove these inner-cellular lipid substances. “The hygroscopic water-soluble substances within the stratum corneum are responsible for much of the water-binding ability of the horny layer,” states JD Middleton in the article “The Mechanism of Water Binding in Stratum Corneum.”3 The ability of corneum to bind water is reduced when it is extracted with solvents, which remove lipids, and then with water, which removes the water-soluble substances. This fact is important to note because it affects the outcome of both professional skin care services and home care treatments, and should be front and center when organizing skin care remedies.
Lipids in the intercellular spaces of the SC provide the permeability barrier of the skin. Corneocytes are held together by attachments called desmosomes. The lipids between them help to conserve moisture because it is difficult for water to pass through them.4
For the skin to sustain balance, it is important to understand what occurs in this tiny SC ecosystem. According to Proksch’s study, “Lipid synthesis occurs in the keratinocytes in all nucleated layers of the epidermis.”2 The study also states that these newly synthesized lipids are then delivered by lamellar bodies to the interstices of the SC during critical epidermal differentiation.2
The disruption of barrier function by topical fatty acid removal, such as through the use of cleansers, pretreatment of peels, masks and other constituents during professional spa treatments—particularly peeling, microdermabrasion, ultrasound and other therapies or medical modalities that impact the SC environment—results in an increase in the synthesis of free fatty acids, sphingolipids and cholesterol in the living layers of the epidermis, leading to barrier repair. This is the natural course of reproduction to replace the lipid loss. Because ceramides are a lipid family available in high concentrations in the cell membrane, it subscribes to this logic. Ceramides can act as signaling molecules that help regulate differentiation proliferation, the apoptosis of cells and programmed cell death.5 Given this physiological fact, ceramide replacement is important for the balance of the SC ecosystem to overcome the impact of lipid loss.
Studies clearly demonstrate that three key lipids—cholesterol, free fatty acids and ceramides—are required for the permeability barrier to maintain skin health, and these lipids must be supplied together in a proper proportion for normal barrier recovery. All three lipids have different chemical compositions and different functions throughout the body. Nine different types of ceramides exist in the SC and they account for up to 50% of the lipids in the SC.6 The lipid layer provides a firm, smooth structure that coheases cells together and maintains the moisture-retention ability of the skin. Application of a ceramide-rich serum used as the final step of a skin care treatment or as an integral part of a client’s home care regimen can be a major point of difference in a results-oriented skin recovery outcome.
The crucial role of ceramides in the skin’s barrier function is still being researched, as well as how these natural interstitial substances are important in managing conditions of the skin that are unbalanced.5 The bottom line is ceramides are the major lipid constituent of lamellar sheets present in the intercellular spaces of the SC, according to “Penetration and Growth of DPPC/DHPC Bicelles Inside the Stratum Corneum of the Skin” by L. Barbosa-Barros, et al.7 “These lamellar sheets are thought to provide the barrier property of the epidermis,” states the study “Ceramides and skin function,” by L Coderch, et al.8 The dead cells form this compound system, and the lamellae are the skin’s actual barrier against the hostile environment of UV rays, bacteria, chemicals and other substances.
Ceramides in skin care
A drop in the level of ceramides results in the skin becoming dry and hard, leading to fine lines and wrinkles. In skin care, ceramides are used for replenishing natural moisture levels due to their water-binding capacity. Formulations containing lipids identical to those in skin and, in particular, ceramide supplementation used twice daily, improves acne, rosacea, and photoaged, dehydrated, irritated skin. In addition to other imbalanced skin conditions, it is important to note that many facial cleansers contain a comedogenic ingredient named sodium lauryl sulfate that can lead to severely irritated skin. Serums that contain ceramide 1, ceramide 3, or both, can replace the transepidermal water loss and reduce the irritation brought on by this ingredient.
“Scientists have also learned that people who suffer from eczema have significantly fewer ceramides in their SC. On the other hand, individuals who suffer from psoriasis have the same number of ceramides compared to people with normal skin. However, the psoriasis-sufferers have less ceramide 1, 3, 4 and a subset of 5 and 6; and more ceramide 2 and another subset of 5,” according to Heather Brannon, MD, in the article “Ceramides: Skin Lipids That Keep Skin Moisturized.”9 Ceramide serums containing human-identical ceramide 1 and ceramide 3 could offer potential relief as part of a skin care protocol for psoriasis-sufferers.
An essential lipid
Ceramides are a principal indigenous material of the skin needed to maintain the health and proper balance of the SC. Science has proven ceramides are an essential part of daily skin lipid supplementation because they originate in the SC’s natural ecosystem and when lost through aging, trauma, cleansing or other causes, they require continued replenishment to maintain the necessary balance, energy, health and hydration of the skin.
1. PM Elias, Stratum Corneum Defense Functions: An Integrated View, J of Inv Derm 125 183–200 (2005)
2. E Proksch, et al, Barrier function regulates epidermal lipid and DNA synthesis, Br J Dermatol 128 (5) 473–82 (May 1993)
3. JD Middleton, The Mechanism of Water Binding in Stratum Corneum, Br J Dermatol 80 (7) 437–50 (July 1968)
4. www.pgbeautygroomingscience.com/the-epidermis.html (Accessed Sept 9, 2010)
5. http://en.wikipedia.org/wiki/Ceramide (Accessed Sept 9, 2010)
6. dermatology.about.com/od/skincareproducts/a/ceramide.htm (Accessed Sept 9, 2010)
7. L. Barbosa-Barros, et al, Penetration and Growth of DPPC/DHPC Bicelles Inside the Stratum Corneum of the Skin, Langmuir 24 (11) 5700–06 (2008)
8. L Coderch, et al, Ceramides and skin function, Am J Clin Dermatol 4 (2) 107–29 (2003)
9. dermatology.about.com/od/skincareproducts/a/ceramide.htm (Accessed Sept 9, 2010)