The epidermis is the layer of the skin on the outside. The layer is relatively thin. Its thickness varies with the part of the body. On the eyelids, it is only 0.05 mm while on the soles of the feet and palms of the hands it can be as thick as 1.5 mm.
The epidermis is comparatively tough and plays an important role in protecting the body from external assault and retaining the body’s water. It is exposed to the elements and therefore prone to damage due to various environmental causes. Over 90% of this layer is composed of cells called keratinocytes and some others include melanocytes producing melanin and merkel cells which are the touch receptors and immune-regulating cells. The deepest layer of the epidermis is composed of living cells while the upper layer is dead. In between layers contain cells at various stages of their life cycle. The surface layer called the stratum corneum contains 10-15% of the total water content of this skin layer while the deeper layers contain more than 70%. Although this layer appears thin, it is a composite layer comprising of several other layers within it. The layers are discussed below.
These layers are described from the deepest to the outermost. They are five in all.1- Stratum basale (AKA Basal Cell Layer)
The basal cell layer the first layer of the epidermis from the inside and it appears on top of the dermis forming what is called the Dermo-Epidermal Junction (DEJ). All the skin cells that form the epidermis originate from this layer including the two important ones called Keratinocytes (90%) and melanocytes (5-10%). With time, the oldest of these cells are pushed towards the surface of the epidermis, where they are eventually shed off. The activity of the basal layer is high with the full cycle of exchange of the epidermis in 28 days in teenage years. Later on in life this increases by 30-50%. Basal cells are connected with other basal cells and overlying spinous cells via desmosomes which are the focal point in the action of peeling agents such as glycolic and lactic acids to induce exfoliation. Melanocytes are also present which produce melanin. Melanin gives the skin its colour and associated problems of dullness, pigmentation and melanoma (discussed in next section).
Skin has two basement membrane zones lying between the dermis and the epidermis and other below the dermis surrounding the dermal blood vessels. The BM is a gate keeper controlling diffusion of bioactive molecules, storing large amounts of cytokines and growth factors serving as a reservoir for their controlled release. It is composed of glycoproteins and proteoglycans. Aesthetic practitioners try to utilise the power of the BM skin rejuvenation treatments such as micro-needling.
It is also here that melanin pigment producing cells called melanocytes are formed. These pigments give the skin its colour. All humans are born with the same number of melanocytes roughly. Melanin absorbs UV radiation from the sun which prevents the harmful rays from inducing inflammatory elements in the skin such as free radicals reactive oxygen species (ROS). Melanocytes produce melanin in response to internal stimulus by the body such as hormones or external stimulus such as free radicals produced by smoking, pollution and UV rays. UV radiation can cause general as well as localised production of melanin depending on areas exposed longer to the sun such as do the areas around the eyes and mouth resulting in uneven skin tone. Melanin is only removed when the skin cells die and are shed of via the process of exfoliation. In the teenage years due to optimal biological influences (hormones, natural anti-oxidants etc.) the skin is able to effectively regulate melanin production and distribution. Increased melanin production due to external factors is quickly recovered from in weeks (after a sunny holiday) due to the high turnover of skin cells. However, due to slowing down of the skin cycle and impaired regulatory mechanisms with ageing from the early 20s, we see signs of increased melanin deposition in the form of focal skin dullness, increased size of freckles and later on to more severe conditions like melanoma and hyper-pigmentation.
Desmos’ is Greek for chain or bond. To remain structurally stable, the basal cell layer cells are held to each other and to layers of cells above by special adhesion proteins called desmosomes. These proteins are also needed for transport between the cells. Cells formed in this layer are gradually pushed upwards until they are shed-off as dead skin cells. Certain ingredients can interfere with this bond including alpha hydroxy acids found in citrus fruits and milk.
Desmosomes maintain the integrity (strength) of the basal cell layers. Age or pathological reasons can make this layer lose this property and present with signs and symptoms of ageing.2- Stratum spinosum (AKA Spiny Layer)
The other name for this layer is squamous cell layer. As noted above, skin cells are gradually pushed towards the surface. As the cells of stratum basale do this, they become more flattened. Among the five layers of the epidermis, this layer is the thickest with up to 10 layers of keratinocytes. This layer contains special immune cells called Langerhans cells which respond to any form of skin damage and trigger an immune response that sets repair mechanisms in action. Keratinocytes in the stratum spinosum produce a protein called keratin, a water resistant protein, which protects the keratinocytes from physical stress as well as contributing to the skin barrier.3- he stratum granulosum
As the keratinocytes age, they are further pushed away towards the surface. This leads to the formation of a thin layer called stratum granulosum. These cells are too far from the dermis to receive nutrients via diffusion and start to die. Keratinocytes start to lose their nuclei and other cellular components. The cell becomes granular in nature. They contain lipids (ceramides, fatty acids, and lipids) that give the epidermis its water sealing properties forming part of the Skin Barrier Function as it progresses towards the upper most layer. This layer and the ones above it contain very little water and as mentioned earlier, majority of the water in the skin is below this layer.4- The stratum lucidum
This layer is made up of 3-5 layers of dead keratinocytes present only in the fingertips, palms, and soles of the feet. The layer appears translucent under a microscope and hence the name. Although this layer contains largely of dead cells, it is said to contain various chemicals and enzymes secreted by the stratum granulosum layer. By this time, the cells are flattened out completely.5- Stratum corneum
Stratum corneum is the outermost layer of the epidermis. This is a tough layer and offers the skin protection against chemical and physical as well as other forms of environmental assaults. It consists of 10-20 flattened layers of cells with the outermost cells ready for shedding off. This process of dead cells falling off is a continuous process and is called desquamation.
The keratin filled cells are dead but do still contribute to retaining water as the layer contains less than 10-15% of the total epidermal water. In the absence of water, the SC is intrinsically brittle and rigid. The layers must maintain some hydration to maintain its flexibility, integrity and catabolic activity. The SC does this via biophysical mechanisms.
The surface of the skin is naturally acidic due to superficial excretions such as sweat, sebum and ions. The acidity helps to regulate certain key functions in the stratum corneum including exfoliation and the natural skin bacterial flora. This acidic film is known as the Acid Mantle.
Over or under-desquamation leads to either skin water loss or permeability issues. When this cycle is out of balance, it leads to either hastened or inordinate keratinization. This leads to a markedly dry, thickened and scaly skin that in turn leads to permeability barrier dysfunction.
Studies have revealed that in young adults, the Stratum Corneum transit time was as quick as 20 days, whereas in older adults it stretched to 30 days or more. This natural slowdown from the early 20s leads to increase layers of dead skin cells as well as melanin within it contributing to uneven, dull skin tone. moisturisers, foundations and creams can further increase the amount of dead skin layers as they can impede natural exfoliation even more due to their “gluing” effect. This can lead to further dullness, enlarged pores and irregular skin texture.
As mentioned above, the stratum corneum layer plays a major role in maintaining a healthy hydration status of the skin. It does this by mainly controlling trans-epidermal water loss. The two main substances that help to maintain this process are the natural moisturising factor (NMF) and certain lipids that are found outside the cells (extra-cellular). These lipids form the brick and motor analogy of the layers that assures a waterproof skin. The NMF is made up of amino acids, lactic acid and urea. NMF gradually fall with age resulting in much dryer and scaly skin as seen in the elderly. The stratum corneum properties are therefore also named as the Skin Barrier Function. Helping the Barrier Function is also the Acid Mantle which is a thin hydro-film on the surface of the stratum corneum. It is composed of secreted sweat and sebum and has the function of providing the skin with an acidic pH (normally 4.5-5.5 and up to 6.5 if using alkaline products and moisturisers). Constantly increased pH can distrupt the acid mantle and skin barrier and is one of the causes of sensitive, better known as sensitised skin.
Under normal circumstances, there appears to be no difference in trans-epidermal water loss in both the young and the old. However, there is delay in recovery when the skin of an old person is unduly disturbed unlike that of a younger person. This is a point of consideration when making cosmetic choices as an older person or when offering guidelines to such a client.
The deeper layers of the epidermis are more hydrated and the stratum corneum is the least hydrated of all the five layers. Ageing decreases hydration to the upper layers of the skin due to slower functions, decrease blood flow, diffusion and an increased layer of dead skin cells makes it harder to diffuse water to the surface and soak the stratum corneum. Moisturisers and humectants applied from the top are a temporary solution, however true hydration comes from improving elements deep in the skin layers.
- Decrease thickness of unwanted layers of stratum corneum by:
- Decrease excessive and irregular melanin distribution due to extrinsic causes such as free produced by sun exposure.