Water is essential for the normal functioning of skin, in particular, for the normal functioning of the skin’s outer layer, i.e., the stratum corneum. Stratum corneum hydration is necessary for the proper maturation and desquamation of skin cells and a lack of water in the skin impairs the enzymatic activity necessary for the normal functioning of these processes. As a result skin appears dull, dry and flaky.
Given the importance of hydration for the normal functioning of the skin, water loss from the skin must be carefully regulated. This regulation is dependent on the complex nature of the stratum corneum, specifically on two major components:
The presence of humectants, or naturally hygroscopic ingredients within corneocytes. This is known as the Natural Moisturising Factor (NMF) and is discussed in greater detail in subsequent sections.
The lipid matrix, i.e., intercellular lipid arrangement in the stratum corneum that forms a barrier to trans-epidermal water loss (TEWL).
Dehydration often tends to go hand in hand with dry skin as dry skin lacks sufficient skin lipids which are essential for forming a barrier against excess water loss. However, that said, people with oily skin can also experience skin dehydration.
Natural Moisturising Factor (NMF) And The Importance of Skin Hydration
The NMF consists of the natural humectant substances in your skin that help it attract and hold onto water. As is evident from the preceding statement, NMF is essential for helping skin maintain adequate hydration, which is essential for its normal functioning and for its optimal functioning as a barrier.
The NMF is composed principally of filaggrin proteolysis (degradation) products including free amino acids, and amino acid derivatives like PCA, urocanic acid, inorganic salts (chlorides, phosphates, and citrates of sodium, potassium, calcium, and magnesium), sugars, lactic acid and urea. It is found within corneocytes (terminally differentiated keratinocytes that compose most of the stratum corneum, the topmost layer of the skin) and makes up approximately 10% of the mass of a corneocyte and 20-30% of the dry weight of the stratum corneum.
The ingredients that make up the NMF serve as highly efficient humectants. When hydrated, they form ionic interactions with keratin fibres. This results in reduced intermolecular forces between the fibers and increased elasticity of the stratum corneum. As a result, skin appears supple and healthy and the probability of skin damage due to mechanical stress, reduces. NMF also keeps epidermal solute concentrations balanced to prevent both over, and under-hydration of the skin, where the former causes corneocytes to shrink.
While the stratum corneum is biologically dead, it is biochemically highly active. Water in the stratum corneum is also integral for this biochemical activity, which includes the activity of enzymes. A lot of these enzymes are involved in the desquamation process, and their normal functioning is essential for several reasons, only one of which is the prevention of acne and clogged pores. Research shows a dependency of desquamatory enzyme activity on water levels within the skin.
Pathologies Associated With Filaggrin Gene Mutations, NMF Reduction And Absence
Reduction in, or depletion of, NMF in the skin is associated with dry skin, and also with several more serious skin pathologies that manifest clinically as areas of dry skin with abnormal desquamation resulting in scaling and flaking, and in extreme cases, the skin can even crack. These pathologies include ichthyosis vulgaris, psoriasis, and atopic dermatitis, where in the former two, the NMF is essentially absent.
Reduction or depletions may be the result of mutations in the filaggrin gene, or even the result of environmental factors. Low humidity for example, impairs the ability of hydrolytic enzymes to break filaggrin down into NMF, resulting in dry skin. UV radiation has also been shown to impair the breakdown of filaggrin and routine soap washing has been shown to deplete it from the superficial layers of the stratum corneum. NMF levels in the skin also fall with age as the production of filaggrin precursors decline. Research, for example, has shown diminished levels of urea in the skin of the elderly, and also in the skin of patients with atopic dermatitis. The level of PCA, another component of NMF, has also been shown to fall in the top layers of the skin as a consequence of soap-washing, and of age.
The good news amidst all of this however, is that topical application of moisturisers containing NMF appears to be beneficial in the treatment of dry skin conditions. The topical application of urea for instance, corrects urea deficits; PCA, when topically applied, has been shown to alleviate symptoms of dry skin, while lactic acid appears to work by stimulating the synthesis of ceramides in the stratum corneum.