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The skin acts as a stalwart barrier that protects the inner organs. It locks in the necessary moisture and electrolytes in the body and blocks harmful bacteria and allergens from entering the body. This is the key barrier function of the skin. The most important role of this barrier function is carried out by the stratum corneum.
The dermal-epidermal junction was once thought to be the most important for the barrier function of the skin. However, according to the latest reports, the stratum corneum, the layer of dead cells that we try so hard to scrub off in the shower, has been found to play the most important role in the skin barrier function. Chemical barriers of antimicrobial peptides against bacteria, such as cathelicidin and human beta defensin, play a crucial role, but the corneocytes and intercellular lipids provide actual physical barrier against water loss.
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Looking at the physical barrier function of stratum corneum, keratinocytes differentiate at the base of epidermis into corneocytes rich in keratin. These cells form a brick wall and the intercellular lipids fill the gap between the bricks like cement, providing strong support. Corneodesmosome that exist between corneocytes act as a strong rivet that hold together adjacent corneocytes.
Recently, the tight junction in the granular layer has also been found to play an important role in skin barrier. The intercellular lipids mainly consist of ceramide, cholesterol, and free fatty acid. When these components are insufficient, the skin barrier is weakened and the skin becomes drier. The dry and weaker skin is susceptible to various allergens which cause allergic reactions such as atopic dermatitis. Moisturizers can help prevent or improve dryness and supplement the weakened barrier of the atopic skin.
The use of moisturizers drastically increased along with the rising incidences of atopic dermatitis, psoriasis and ichthyosis, etc. that severely dry the skin. Also, an increasing number of elderly patients with dry eczema require sufficient moisturization. The moisturizer market is ever growing.
Occlusive moisturizer and humectant
Moisturizers can be largely classified as an occlusive that strengthens the skin barrier by forming a new layer and humectant that increases the water content of the skin. Currently available types of occlusive moisturizer include petrolatum, lanolin alcohols, jojoba oil, cocoa butter, paraffin, olive oil, and lipid mixture, etc. Petrolatum is the oldest occlusive moisturizer used in widely available product of vaseline which may be said to be the ancestor of modern cosmetics. Petroleum prevents up to 98% of water loss through skin.
Humectant is a substance with high affinity to water. It can replace natural moisturizing factor (NMF). However, it does not prevent water loss in cold and dry environment as effectively as a occlusive moisturizer. Types of humectant include glycerine (glycerol), propylene glycol, pyrrolidone carboxylic acid, sodium lactate, natural lipid mixture, and urea, etc. Glycerine is a traditional humectant that is most widely used. It infiltrates into the stratum corneum and forms strong bonds with protein and lipids to seal in water. It effectively inhibits transepidermal water loss.
-To be continued-
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