The top layer or epidermis (derma means 'skin’ in Greek hence a dermatologist is a skin specialist - and epi means 'over’) is a light-reflecting, translucent (like frosted glass) covering for the body. It's amazingly thin for something so tough - about 1mm (1000 micrometers) over most of the body, but at its thinnest on areas such as eyelids it is only about 0.05mm (50 micrometers), and at its thickest on the soles of our feet or palms of our hands it is 1.5mm (1500 micrometers). It contains no blood vessels and is nourished from capillaries in the top layer of the dermis below.
All day, every day, skin cells (keratinocytes) are being born, developing and dying in the five levels of the epidermis. The cells begin life at the basal layer of the epidermis, then move up through the five levels, changing shape and developing all the time. By the time they reach the surface, or horny layer, the cells have died, flattened and are being sloughed off -- a process called exfoliation or
desquamation - and more 'daughter' cells are on their way up. The whole process takes 26-42 days, according to American dermatologist Leslie Baumann. Over our adult
life, the process gradually slows and dead cells linger on the surface, which is why we notice duller, less radiant skin in older people.
The structure of the epidermis
-Bottom of the epidermis:
- Stratum Basale or basal layer: The skin cell factory, where millions of new, column-shaped cells are produced 24/7 from stem cells. As soon as they are formed, they're pushed up through the other skin layers by the constant production of new cells beneath them. Melanocytes, or pigmentation cells, start here too. The amount of melanin you produce controls skin color, how much we tan, hair color, and also helps protect your skin against sunlight. The skin's immune cells (Langerhans cells) originate here, to act as one of our first defenses against invading bugs. Nerve endings reach up from the basal layer to the surface layer, where they respond to touch, heat, cold and pain.
- Stratum spinosum or spinous layer: The cells, now irregular in shape and multi-sided start to produce keratin, the main protein which makes up skin, nails and hair. Lipids (fats) appear, also making their way up to the surface to form the skin barrier.
- Stratum granulosum: The busy shop floor of the skin factory, where keratin and lipids, including moisture retaining ceramides, are developing further.
- Stratum lucidum: Often referred to as part of the next layer the stratum corneum, this layer exists only on your palms and soles. The cells flatten and clump together to produce an extra layer of protection where they encounter most friction.
- Stratum corneum or horny layer: the top surface layer - like the icing on the cream cake slice. The cells here, called corneocytes, are now 25 to 30 layers of dead, flattened discs, tightly packed and cemented with lipids (fats) and proteins as a brick wall-like barrier. Its thickness varies enormously, from 0.01 mm (10 micromeres) in fragile areas such as the eyelid (making them very sensitive to harsh detergents such as soap or SLS) to 0.1 mm (100 micromeres) on the soles of the feet.
Natural Moisturizing Factor (NMF)
Although the corneocytes are technically dead by this stage they contain many chemicals that enable this dynamic barrier to respond to, and protect us from, the environment and prevent the loss of water. One important group of chemicals are called Natural Moisturizing Factor (NMF). NMF acts like a magnet for water, attracting it into the corneocytes so they swell up, preventing the formation of cracks in between them. This is why healthy skin is so smooth and shiny. In dry skin, however, there are reduced levels of NMF within the corneocytes and small cracks can develop - that's the reason dry skin feels rough and loses
its healthy luster. Good moisturizers contain constituents such as plant oils, glycerin and hyaluronic acid that mimic the deficient NMF and so help rehydrate the corneocytes, restoring the smooth, healthy skin surface. If the stratum corneum barrier is damaged, it allows 'trans-epidermal water loss' (TEWL). Bad burns, for instance, cause massive water loss - but this loss also increases with dermatitis, eczema, psoriasis, etc. High TEWL is linked to raised permeability, making it easier for external irritants to enter the epidermis, resulting in increased sensitivity and dry skin. Moisturizers work mainly by reducing TEWL.