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.
This middle layer is connected to the epidermis by the ‘basement membrane', a soft, pliable but strong layer of tissue. The dermis is thicker than the epidermis, again varying over different parts of the body from about 0.3mm on your eyelids up to 3mm or more on your back. The thickness doubles between the ages of three and seven, and again at puberty.
The dermis is made up mostly of bundles of collage none of the strongest natural proteins, which gives skin elasticity and bounce - held together with fibers of another protein, elastin. It's the breakdown of collagen and elastin, owing to sunlight and other external problems, as well as internal factors - notably, stress - plus the natural changes of ageing, that cause skin to lose its tone and start to sag. All around the collagen and elastin is hyaluronic acid, a sticky, slippery substance (because it attracts and holds water), which is essential for skin moisture.
this layer has lots of sweat glands and follicles, nerves and also blood vessels, which supply oxygen to the epidermis and take away waste products.
Sweat is vital for regulating body temperature and keeping the skin moist. As you perspire, the moisture on your body evaporates and cools the skin, regulating the temperature of the whole body. These glands are also activated when you're nervous or frightened - as in the sweaty-palm response. You have between 2-4 million sweat glands, capable of producing up to a liter of sweat an hour, more when in a hot climate.
There are two types of sweat gland, eccrine and apocrine, both of which begin life in the dermis. The eccrine glands, which are responsible for most output and are found all over the body, are long, hollow tubes with a coil at the bottom where the mixture of water and salts which makes up sweat is produced. It then flows up the tube to the opening (pore) on the skin's surface.
Apocrine glands are similar, but they're attached to hair follicles, mainly under the armpits and in the genital area.
Although they produce much less sweat, it's more concentrated and contains proteins and fats which are responsible for your individual smell. Apocrine sweat glands can be turned on or off by the autonomic nervous system,
but this operates as a whole so when one gland is activated, all glands spring into action. The eccrine glands are more precisely controlled and different parts of the body work independently, which is why your palms can sweat when you're extremely nervous while your face remains dry.
Tube-like structures called follicles - some containing a hair, some empty - originate in the bottom of the dermis, extending up to the skin surface, where you see them as pores.
Branching off the follicles are grape-like clusters of sebaceous (oil) glands, which produce the oily substance known as sebum. Sebum makes its way into the follicle and then up to the skin surface, where it mixes with sweat to become what's known as the 'acid mantle’
Each hair follicle has small muscle fibers called arrector pili attached to it, which contract when you're cold or frightened causing the follicle to protrude slightly above the surface. This is technically called piloerection -- more commonly known as 'goose bumps'. Stem cells located around the arrector pili are mainly responsible for our hair growth and the average growth rate for a healthy head of hair is around 0.4mm (400 micrometers)in a day
Subcutaneous layer or hypodermis
At the bottom of the imaginary cream cake slice is a cushion of fat, called the subcutaneous layer, which is filled with adipose (fat-filled) cells, blood vessels and nerves. This varies in thickness: on your bottom, for instance, it's several millimeters thick, but entirely absent on eyelids, and thinner on the neck. This layer helps plump up the skin, keep you warm, protect your bones and acts as a useful energy reserve tank in times of famine. It's also involved in synthesizing vitamin D from the sun, which is vital for healthy bones and teeth, boosting the immune system and reducing inflammation in the body.
Normally, the skin is quite loosely attached to muscle and bone with connective tissue, but if the fatty layer over expands (because you put on weight, for example), then the fibers in the connective tissue are pulled tight,
resulting in the mattress-like dimpling of our favorite skin villain, cellulite.