Epidermis of Normal Skin
The skin is comprised of two distinct layers, the dermis and the epidermis with a third layer below the dermis, often referred to as the hypodermis. The epidermis is the outermost layer and is comprised of five different strata known as: the stratum basale, the innermost layer adjoining the dermis, the stratum spinosum, the stratum granulosum, the stratum lucidum and the stratum corneum, which is the outermost layer. Each layer has a distinct appearance and function as referred to in Figure 1.
The stratum basale is located immediately above the dermis and is the layer where the skin cells, keratinocytes, develop and multiply (also known as proliferation). Melanocytes are also found in this layer. They produce melanin, which is taken up by the keratinocytes and gives the skin its colour. The amount of melanin produced determines how dark the skin is, rather than the number of melanocytes. The keratinocytes are continually produced and migrate upwards, differentiating as they proceed, changing in structure and function.
Once formed, the keratinocytes migrate upwards and form the stratum spinosum. Here, they form tight junctions between the cells, called desmosomes, which prevent the ingress of microbes and other foreign substances. This layer also contains special immune cells called Langerhans cells, which take up microbes such as bacteria and viruses to prevent infection.
The keratinocytes continue to differentiate and migrate upwards forming the stratum granulosum. At this point, the keratinocytes contain keratohyalin granules and profilligrin that aggregate into highly organised structures within the cells. Also present are lamellar bodies, which produce the precursors to the molcules that ultimately form the lipid barrier in the outermost layer of the epidermis, the stratum corneum.
By the time the keratinocytes reach the stratum lucidum, they have died and no longer contain a nucleus and other cell organelles. However, although the cells are dead, they are still biochemically active. The profilligrin is converted into filligrin, which is then broken down to form natural moisturizing factors (NMF), keeping the cells hydrated. Natural moisturizing factors are hygroscopic substances that have the ability to attract and absorb water. The main components are amino acids (40%), pyrrolidone carboxylic acid (PCA) (12%), lactates (12%) and urea (7%). When these cells are hydrated, the skin looks plump and supple. However, if these cells become dehydrated, the skin can appear saggy and wrinkled.
Once the cells reach the outermost layer of the epidermis, the keratinocytes are now called corneocytes. The cells are no longer round, rather they are like flattened discs, surrounded by lipids. These lipids comprise a complex mixture of ceramides (50%), cholesterol (25%) and fatty acids (15%). Glycerol is also generated in this layer when the phospholipids, released by the lamellar bodies, are converted into free fatty acids. Glycerol is a humectant and also helps to keep the stratum corneum hydrated. The lipids surrounding the corneocytes also maintain the acid mantle of the skin. The normal pH of the skin is between 5 and 5.5 and is important for maintaining the skin barrier function.
How Does that Affect Me?
As you can see, there are a lot of important factors that contribute to keeping our skin healthy. The main function of the skin is to protect our bodies from infection and disease so it is vital that it is maintained in a healthy condition. The most important aspect of this is maintaining the integrity of the skin barrier. Fortunately, we can help our skin by using mild cleansers and applying moisturisers after washing. We also need to keep our bodies hydrated to keep the skin cells from becoming dehydrated. Frequent washing and being exposed to water for long periods of time is detrimental to the skin barrier. This is due to the fact that the bonds between the corneocytes and the lipid layer start to breakdown and allow ingress of foreign substances. This in turn can lead to sensitization and the skin developing allergies to once benign substances. This could be the reason why a product that you have used previously without any ill effect, suddenly seems to cause you problems.
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