Your skin has many functions, the most important of which is probably that of a barrier. Your skin is not only a physical barrier between the outer environment and the inside environment of your body but a biochemical barrier. It holds moisture inside your body and keeps unwanted things out of your body such as toxins, and damaging allergens. In healthy skin, there is very little that will actually permeate the skin. The role of skin care should be to protect and enhance the skin barrier so it can continue to protect you.
What composes this skin barrier function?
The outer layer of your skin is made up of cornified keratinocytes called the stratum corneum. This is a layer of dead cells. However, even though dead, that layer of skin is highly metabolically active. Cells in this layer continually slough off, or exfoliate as tiny scales call dander.
This outer later contains extracellular lipids (fats) and proteins that help prevent water loss. Much of this layer is made up of a protein called keratin. Keratin in the cells of the stratum corneum (corneocytes) help hold water. Filaggrin is another important protein in the stratum granulosum.
The structural arrangement of the stratum comeum is analogous to bricks and mortar. The bricks are the cells (called corneocytes) that contain high amount of protein, and the mortar between the cells is the extracellular lipids. These proteins and lipids in the epidermis help the skin to act as a barrier.
The lipids that compose the mortar include ceramides, fatty acids and cholesterol. Linoleic acid is one of the important fatty acids here. It is called an essential fatty acid because it cannot be made by the body and must be ingested. Deficiencies in linoleic acid can result in dry scaly skin and a compromised barrier function. Saturated fatty acids with chain lengths of 20 or more are important too. The lipids in the stratum corneum occur in broad smooth sheets between the cells.
Compounds in the skin that help it hold moisture are referred to as the natural moisturizing factor (NMF). The NMF consists of water soluble, low molecular weight compounds that bind water. These include amino acids, pyrrolidine carboxylic acid, lactate, urea, minerals, electrolytes and sugars. Excess bathing, age, sun and stress all decrease the amount of NMF in the skin causing the permeability barrier of the skin to decrease.
Even though the stratum corneum is a layer of dead cells, it is highly active metabolically. Enzymes in the stratum corneum are promoting exfoliation (loss of dead cells), processing lipids and proteins, and forming the NMF.
This specialized ‘bricks and mortar’ structure of the stratum corneum effectively produces a covering for the skin that is both flexible and protective. Proper hydration of the skin however, is necessary for these metabolic processes to occur and to maintain the barrier function of skin.
Once the skin barrier is damaged, the skin may become dry and scaly. This is primarily because there is more water loss from the epidermis. This results in increased appearance of wrinkles. Many times damage to the barrier function presents itself as sensitive skin. Additional skin problems can occur including various types of dermatitis (eczema), psoriasis or cold sores.
Damage to the barrier function can happen when skin is over cleansed or harsh cleansers are used. Some surfactant molecules bind to proteins to change their structure.
Excessive exfoliation can damage the skin barrier function as can weather such as heat, cold, dry air, wind and oxidative damage to the skin such as from UV light. Even stress can have an effect on the skin barrier function.
The skin barrier function however, can be repaired using the proper skin care products. Skin care should contain a mix of proper fatty acids, water and humectants to support the skin barrier function. For instance, Springtide Gold. People must be getting more comfortable talking about the skin barrier; the NY Times just published an article about products that damage the skin barrier here.