The skin barrier function
YOUR SKIN BARRIER FUNCTION:
or, how a dead layer of skin cells keeps you alive
We all recognise the importance of the skin in protecting the body from bacteria, viruses, heat, cold, chemicals; it is our largest organ. It is also more complex, finely balanced and vulnerable than we usually acknowledge.
I want to look at what comprises a healthy skin barrier – what it does, what threatens it, and how specific ingredients in skin products can help to protect it.
WHAT IS THE SKIN BARRIER?
The stratum corneum or horny layer is the outermost of five layers of the epidermis, which is itself the outer layer of skin. The stratum corneum varies in thickness around the body but comprises between 10 and 30 layers of dead skin cells. These cells are primarily protein (keratin), and the layer includes ceramides, lipids, cholesterol and fatty acids.
The cells shed constantly in healthy skin as cells move out through the epidermis layers and replace them. The whole skin cycle takes on average 28 days, of which the shedding of the stratum corneum takes 14 days. (This cyclical process is relevant to your expectation of changes to the skin with targeted skincare products).
The stratum corneum, this ‘dead’ layer of cells has been described as the end product of the epidermis, and its raison d’etre (Kligman 2004).
HOW DOES THIS ‘DEAD LAYER’ PROTECT THE SKIN?
The stratum corneum regulates epidermal water content, managing trans-epidermal water loss – through a delicate lipid membrane of waterproofing ceramides, cholesterol and fatty acids between the cells.
Within the cells, a mixture of natural moisturising factor compounds maintains hydration by being hygroscopic (attracting water from the air).
- It protects against oxidants from outside which can damage the skin – antioxidants.
- It limits infection of the skin due to antimicrobial peptides.
- Immune mechanisms respond to microbes, allergens and other invaders
- It protects against damage from UV radiation
- (It can signal disease states – providing an indication of internal illness)
- Detect, protect, respond to, adapt against circumstances
WHAT DOES THIS COMPLEX SYSTEM TELL US?
When the skin barrier is damaged, this complex arrangement of protections is jeopardised and the result can be dehydrated skin, infections, irritations, and access of allergens (and as recent research has shown, even the development of atopic or allergic processes such as eczema).
Far from being inert, this layer is a finely balanced system. Advances in product development for skin care now focus minutely on the actions of each layer of the skin, and what can be done to protect, mimic or improve its actions.
This micron thin layer of ‘dead’ cells is a finely tuned system. It adapts quickly to recover if there is an increase in trans-epidermal water loss. It immediately releases lipids stored within the stratum corneum, so that within minutes this loss can be partially reversed. Within 2 to 3 hours there is a marked increase in lipid production within the skin.
What does this mean in practise? – Strip the skin with aggressive oil-removing soaps or cleansers, and it will immediately try to repair itself by producing more oil – and hence the dehydrated-yet-oily situation that acne prone skin can develop.
WHAT DAMAGES THE SKIN BARRIER?
External stressors include:
- Alteration of the pH, the normally acidic state of healthy skin, its ‘acid mantle’ – (ie through the use of alkaline soaps)
- Aggressive or inappropriate skincare – cleansers with abrasives, exfoliants or additives that are irritants;
- Use of astringents such as alcohol based toners
- Reduced environmental humidity – winter heating and air conditioning
- Photo damage – sunburn
- Psychological stress
- Exposure to irritants, allergens, microbial organisms
- Topical medications, especially if poorly formulated
Internal stressors include:
- Genetic or ethnic factors or age
- Underlying diseases, such as eczema, rosacea, psoriasis – even when the disease is apparently under control or relatively asymptomatic
- These factors cannot be changed, and so make the stratum corneum more vulnerable to external stressors
These stressors lead to
- barrier damage,
- Increased transepidermal water loss, reduced natural moisturising factors
- Inflammation – release of cytokines – a cascade of changes
SKIN CARE CONSIDERATIONS FOR GOOD BARRIER FUNCTION
When the barrier is challenged through external factors (seen most obviously in dehydration, inflammation, peeling), or where there are chronic skin disorders such as eczema or psoriasis or rosacea which affect the barrier, special care can be taken to help to repair its delicate balance.
- Well-formulated gentle cleansers – without abrasives or soaps or alkalines. Preferably non-foaming.
- Moisturisers which protect the barrier – containing humectants (to attract water to the skin), emollients (to soften the skin) and occlusives (to seal and prevent the loss of moisture)..
- Additives or actives which target specific problems or disease-related symptoms, ie lipids, humectants such as hyaluronic acid, niacinamide, antioxidants, amino acids
- Increased acidification (associated with normalising of barrier function and reduction of allergic antigens (Hogan et al)
Elias, P. M. (2008). Skin Barrier Function. Current Allergy and Asthma Reports, 8(4), 299–305. Available at https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2843412/ Accessed on 13 September 2018
Del Rosso, J. Q., & Levin, J. (2011). The Clinical Relevance of Maintaining the Functional Integrity of the Stratum Corneum in both Healthy and Disease-affected Skin. The Journal of Clinical and Aesthetic Dermatology, 4(9), 22–42.
Hogan, M, Peele, K and Wilson, N “Skin Barrier Function and Its Importance at the Start of the Atopic March,” Journal of Allergy, vol. 2012, Article ID 901940, 7 pages, 2012. https://doi.org/10.1155/2012/901940. Available at https://www.hindawi.com/journals/ja/2012/901940/ Accessed on 13 September
Kligman A.M. A brief history of how the dead stratum corneum became alive. In: Elias PM, Feingold KR, editors. Skin Barrier. New York: Taylor & Francis; 2006. pp. 15–24.
Skin image adapted from https://opentextbc.ca/anatomyandphysiology/chapter/5-1-layers-of-the-skin/