Imagine a slightly flat cream cake slice with three main layers sandwiched together and lots of thinner ones within them. Our skin is just like that. The main layers are the epidermis. the top layer we see and touch; next comes the dermis; and underneath that, the subcutaneous fatty layer.
The top layer, or epidermis (epi means over and derma means "skin" in Greek - hence the term dermatologist) is a light-reflecting,
translucent covering for about 0.04 inches (1mm) over most of the body, twice as thin (about 0.02inches) on areas such as eyelids, and at its thickest (about 0.06 inches) on the soles of our feet or palms of our hands. The epidermis contains no blood vessels and is nourished by fine capillaries in the top layer of the dermis beneath it.
All day, everyday, skin cells (keratinocytes) are being born, developing and dying within the five levels of the epidermis. The cells begin life at the basal layer of the epidermis, then move up through the five levels, constantly changing shape and developing. 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 - paving the way for more "daughter" cells on their way up. The whole process takes 26 - 42 days, according to dermatologists. Over our adult life, however, the process gradually slows, and dead cells linger on the surface. This is why we tend to notice duller, less radiant skin in older people.
The Structure of the Epidermis
1. 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, and hair color. Melanin also helps protect your skin against sunlight. The skin's immune cells (Langerhan cells) originate here, to act as one of our first defence against invading bugs. Nerve endings reach up from the basal layer to the surface layer, where they respond to touch, heat, cold and pain.
2. Stratum spinosum or spinous layer: The cells, now irregular in shape and multi-sided, start to produce keratin, the main protein that makes up skin, nails and hair. Lipids (fats) appear, also making their way up to the surface to form the skin barrier.
3. Stratum granulosum: The busy shop floor of the skin factory, where keratin and lipids, include moisture-retaining ceramides, are developing further.
4. 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.
5. stratum corneum or horny layer: The top surface layer - like the frosting on a layered cake slice. The cells here, called corneocytes, are now 25 to 30 layers of dead, flattened discs, tghtly packed and cemented with lipids (fats) and proteins, forming a barrier structured much like a brick wall. its thickness varies enormously, from 0.01mm in fragile areas, such as the eyelid (making them very sensitive to harsh detergents such as soaps or SLS)
to 0.1mm on the soles of our feet.
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 chemical group is 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 dewy. In dry skin, however, there are reduced levels of NMF within the corneocytes and small cracks can develop - which is why 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, thus helping to rehydrate the corneocytes and restore skin's smooth, healthy surface.
If the stratum corneum barrier is damaged, it allows for transepidermal water loss (TWEL). Bad burns, for instance, cause massive water loss - but this loss also increases with dermatitis, eczema, psoriasis and other skin conditions. High TEWL is linked to raised permeability, making it easier for external irritants to enter the epidermis, resulting in increased sensitivity and dryness. Moisturizers work mainly by reducing TEWL.
pH and the Acid Mantle of the Skin
The term pH stands for Potential of Hydrogen, which measures the hydrogen concentration of any substance. We also know it as the acid/alkaline balance. It is graded from 0 to 14, with 0 being the most acidic, 14 the most alkaline, and 7 the neutral, or midpoint. The stomach has a pH of around 1, because you need stomach acid to break down food. The protective layer on the skin surface known as the acid mantle (a mix of sebum and sweat) has a mildly acidic pH, which helps kill bacteria and fungi; it's thought to function optimally at the skin's "normal" pH of 5.5. Many skin products are labeled pH balanced, meaning they have been formulated to have a pH close to 5.5. Soapy water has an alkaline pH of around 9.5. If you overwash your face with strongly alkaline soaps or cleansers, the acid mantle becomes more alkaline, and skin is left more prone to infection and damage.
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 0.01 inches on the eyelids up to 0.1 inches or more on the back. The thickness doubles between the ages of three and seven, and again at puberty.
The dermis is made up mostly of bundles of collagen - one of the strongest natural proteins, which gives skin elasticity and bounce - held together with fibres of another protein, elastin. It's the breakdown of collagen and elastin, due to sunlight and other external assaults as well as internal factors - most notably, stress - plus the natural changes of aging, that causes 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.
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 million to 4 million sweat glands, capable of producing up to 1 litre of sweat an hour, and even more in hot weather.
There are two types of sweat glands: eccrine and apocrine. Both began life in the dermis. The eccrine glands, which are rsponsible 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 that constitute 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 attracted to hair follicles, mainly in the armpit and in the genital area. Although they produce much less sweat, it's more concentrated and contains the proteins and fats responsible for your individual smell. Apocrine sweat glands can be turned on or off by the autonomic nervous system, but the system 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 extrmely nervous, but your face remains dry.
Tubelike structures called follicles - some containing a hair, some empty - orignate in the bottom of the dermis, extending up to the skin surface, where you see them as pores.
Branching off the follicles are grapelike 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 follicle has small muscle fibers called corrector pili attracted 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. In a healthy head of hair, average growth rate is around 0.02 inches a day.
Subcutaneous layer or hypodermis
At the bottom of the imaginary layered cake slice is a cushion of fat called the subcutaneous layer, which is filled with adipose (fat) cells, blood vessels and nerves. This varies in thickness: On the bottom, for instance, it may be a tenth of an inch thick, but it's entirely absent on eyelids, and considerable thinner on the neck. This layer helps plump up the skin, keep you warm, and protect your bones. It acts as a useful energy reserve 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 overexpands (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.
Page Source: Skin Care Secrets by Liz Earle