Skin Layers has two main structures. The outer epidermis is chiefly protective, and the underlying dermis contains many different tissues with varied functions. Under the dermis is a layer called subcutaneous fat.
The outer epidermis continually renews and replaces itself by cell division. It consists of five skin layers of cells:
- Basal layer (Stratum germinativum)
- Prickle Layer ( Stratum spinosum )
- Granular Layer (stratum granulosum)
- Clear layer ( stratum lucidum)
- Horney Layer (stratum corneum)
The basal layer consists of box-like cells that multiply quickly and gradually move up to the surface.
As the cells travel upwards, they develop tiny prickles, which bind them together tightly. Then, they begin to flatten and fill with a waterproofing protein known as keratin. Finally, the cells die and reach the surface fully keratinized, resembling untidy, scale-like interlocking tiles on a roof. As they flake away with daily wear and tear, more cells arrive from below to replace them.
The journey from epidermal base to surface takes about four weeks.
The epidermis holds a large amount of water, with the basal cell layer holding the highest percentage (approximately 80 %). Water is held in the spaces between the cells; each subsequent layer has less water, with the horny layer containing approximately 15 %.
The skin’s capacity to retain water decreases with age, making the ageing skin more vulnerable to dehydration and wrinkles. The intercellular lipids in between the epidermal cells are responsible for hydration, epidermal firmness and smoothness. They protect against trans epidermal water loss (TEWL).
They also provide protection against any offending or injuring substances invading the skin. Very important groups of these lipids are ceramides and fatty acids, which are also contained within the ingredients of some skincare products . It is important to recognise that the cell renewal process is responsible for the production of these essential lipids. If the cell cycle slows down, the production of lipids slows down and results in dryness and dehydration.
The horny layer of the epidermis is an important layer in relation to understanding skin problems. It is the part of the skin that is directly affected by the external environment.
It also plays a key role in helping to contain moisture in the rest of the skin. Without adequate retained moisture, skin can become dry and unhealthy.
Under normal conditions, up to 15% of the horny layer consists of water. Which is vital to enable the stratum corneum to work. The natural functions of the skin do not work as efficiently, when the horny layer contains less than 10 % of water.
The predominant cell of the epidermis is the keratinocyte. In their upward metabolic process, they undergo a series of chemical changes. Transforming from soft cells into flat scales that are constantly rubbed off. An essential factor in beautiful skin is the healthy metabolism of keratinocytes. If the keratinocyte formation is not functioning properly in the epidermal layers, it cannot generate an aesthetically pleasing horny layer. The healthy balance of all the essential elements (water, lipids, etc.) is needed in order to ensure the health of the keratinocytes and the skin is not impaired.
The dermis has two layers:
- A superficial papillary layer
- A deeper reticular layer
The papillary dermis lies just beneath the epidermal junction. It is relatively thin and is made up of loose connective tissue. Which includes: capillaries, elastic fibres, reticular fibres, collagen. The reticular dermis is the deeper and thicker layer of the dermis. Which lies above the subcutaneous layer of the skin. It contains dense connective tissue, which includes: blood vessels, elastic fibres (interlaced), collagen fibres (in parallel layers), fibroblasts, mast cells, nerve endings, lymphatics. Additionally, there is a substance surrounding the components in the dermis. This has a gel-like consistency, because it contains mucopolysaccharides, chondroitin sulphates, and glycoproteins.
Fibroblasts are the primary cell type present in the dermis and are responsible for the production and secretion of procollagen and elastic fibres. Procollagen is then catalysed by enzymes to form collagen, which is finally cross-linked to create strong parallel layers. The collagen accounts for up to 70% of the weight of the dermis, primarily Type I collagen with some Type III collagen. Elastic fibres, on the other hand, account for less than 1% of the dermal weight, although they still play an important functional role to resist forces that may deform the shape of the skin. The two types of fibres are bound together by a mucopolysaccharide gel, through which the nutrients and waste products can diffuse to other areas of tissue.
GUIDELINES FOR SKIN LAYERS
When you apply skincare products the best practice is to go from more watery to thicker substances. The reason is that if you apply a thicker, richer formula first, it forms occlusive and could prevent thinner , more liquid ones from penetrating the skin.
However, there would be some exceptions to the rule. If you treat skin conditions with a prescription product, then it is better to apply it on the skin right after cleanser and toner. This is because you want the active ingredients that are treating the skin condition to have the best chance of going deep. If you choose to use face oil, it should always go last, because oil is occlusive and it creates a barrier. While oil should be the last step in your night-time routine, when it comes to your morning regimen, sunscreen – whether your formula of choice is chemical or physical – should always go last. Other products have a mission to get to the skin and get absorbed and be efficacious, but sunscreen is a shield. You want it on the top because it is blocking the rays from penetrating the skin.