The epidermis is the outermost layer with different thickness at different sites. It is thinnest on the eyelids with only about 0.1mm, and, because of the need to sustain constant friction, is thickest on the soles of the feet and the palms of the hands, with about 0.7mm, which is about 3 –4 sheets of paper.

The epidermis consists of three interwoven types of cells:

  • Keratinocytes, which make the protein keratin;
  • Melanocytes, which produces the sun-tanning pigment melanin (which protects us from ultraviolet radiation and determines your skin color);
  • Langerhans cells, which are part of the immune system and intercept foreign substances that try to pass through the skin.

Growth Cycle of the Keratinocytes

They are the main cells forming the epidermis, accounting for about 80-90% of the cell population. They split at the bottom layer of the epidermis and migrate upwards, mature in the process and die off, forming the outer layer of dead, hard, tough cells that form the hard skin surface called the Stratum Corneum, which shed off at the end.

This process normally happens with sequences. Newly formed keratinocytes will replace those shed off at the top. The whole cycle takes about 40 days, and is particularly important for the healthy skin. Aged skin has a slower growth cycle, and this results in thick and rough skin.

Effective skin care products can normalise this cycle, thereby forming a thin and soft Stratum Corneum.

The keratincytes have different look when they are migrating upwards. They can be divided into 5 layers:

  1. Basal Cell Layer - It produces millions of new cells at the bottom of the epidermis every day.
  2. Stratum Spinosum
  3. Stratum Granulosum
  4. Stratum Lucidum
  5. Stratum Corneum

Stratum Corneum

It consists of piled up flat, dead and tough keratinocytes, basically a protein we call Keratin. Older people have thicker layers with about 17 cells thickness, whereas younger people have about 12. The cells are drier when nearer the surface, and too thick a layer will make the skin look dull. Some skin care products can moisturise this layer and make it thinner again, improving the tone.

The main function of this layer is to withstand damages from constant friction. Between them there is the Intercellular Lipid, which passes through the cells to the surface. It also helps the cells to stick together tightly to act as a barrier as well as prevent water loss. The quantity of this intercellular lipid will affect the water content in the Stratum Corneum. Another factor, the Natural Moisturising Factor, also affect the water holding capacity and therefore the water content.

When the skin ages, these two factors will decrease, and so the water content will drop. Researches show that the water content is about 75% when we are born, 70% when we are children, and only 60% when we are adults.

Connection with the Melanocytes

The melanocytes scattered amount the keratinocyts, mainly in the Basal Layer, at the border of the epidermis and dermis. Inside the melanocytes are structures called the Melanosomes, which are the factories for making Melanin. Melanin is passed through the connections to the surrounding keratinocytes, protecting them from the damages of the ultraviolet light, as well as absorbing and filtering excessive ultraviolet light so that less can penetrate into the deeper level to cause damages. When stimulated by ultraviolet light, its production escalates many folds and results in a darker skin complexion.

The number of melanocytes in different races is actually the same. It is the amount of Melanin that determine the skin colour. In black skin type, the melanosomes are larger and denser, with lager amount of melanin. The opposite holds for the white skin type.

When the melanocytes are not functioning right, many problems such as over pigmentation or uneven skin tone will arise. Sun, skin inflammation, hormonal changes can all interfere with the melanocytes metabolism and function. Some skin care products can effectively correct this by decreasing the production of melanin inside the melanocytes.

The Origin of Skin Allergy

The Langerhans cells act as the guard in the skin’s immune system. They look for foreign materials (Antigen) and pass them over to the lymph cells, which will start a whole cascade of rejection reactions. This is the basis of allergic contact dermatitis.

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