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It's a wrap

It is the largest and most revealing of the body's organs, and contains everything we are. Yolanda Brooks lays bare the secret life of skin.

Let's be ghoulish for a moment. If you skinned an average-sized man and unfurled his hide it would measure about two square metres - roughly the size of a single bed. If you then weighed it, you would find the skinless corpse had shed between 3.2kg and 5kg.

Not only is skin the largest organ of the body, it is also the most revealing. One glimpse and you can have a good guess at someone's age, sex, race, general well-being and body temperature. It is a communication tool that starts to tell a story before we have opened our mouths.

Skin, like the mythical perfect man, is strong yet sensitive. At its most basic level it stops us from leaking. It also acts as a waterproof barrier and protects us from infection, ultraviolet light and chemical and physical damage. It aids to cool us down, tries to keep us warm, helps in the excretion of toxic substances, produces vitamin D to help strengthen our bones, reveals our feelings and desires and gives us our sense of touch.

Surface tension

Skin might not have the romance of the heart or the mystery of the brain, but it is probably the organ that causes us the most grief - with its freckles and wrinkles, birthmarks, stretch marks, scars, acne and problematic complexions. Visit any high-street pharmacy and you will see that the skin-care section is given much more floor space than the vitamins and potions that are supposed to enhance our internal well-being. Count the different brands of spot cream and gasp at the price of a pot of the latest anti-wrinkle treatment. And if you happen to be standing in the Harrods beauty emporium, your gasping may turn to fainting when you realise that Natural Bisse Diamond anti-ageing cream costs pound;300 per 100ml.

But unguents to counteract the appearance of wrinkles don't work quickly enough for some, so they have resorted to all-out chemical warfare.

Botulinum toxin - popularly known as Botox - was originally developed in the 1970s to treat eye muscle disorders. Since then it has been given a new lease as a restorer of youthful looks. It works by paralysing the underlying facial muscles, but it is the skin that benefits, with the easing of frown lines, crow's feet and forehead creases. In 2002, Botox was injected into some 1 million American foreheads, and in the UK tens of thousands of people are sporting rigid but wrinkle-free faces.

The Isolagen process is another therapy being used to maintain wrinkle-free skin. Rather than relying on concoctions cooked up in a laboratory, the process takes collagen-producing cells known as "fibroblasts" from behind the ear. These cells are reproduced in a culture over a period of six to eight weeks before being re-injected back into "problem areas" over two or three sessions.

In the long term, Isolagen Inc - the company behind the new process - hopes that customers will be donating their cells in their youth and taking them out of cold storage for future usage. The removal of cells is a one-time procedure, and the new cells can be stored to provide rejuvenating injections throughout the recipient's lifetime.

At the moment, Botox and Isolagen remain a minority pursuit for the wealthy, but tanning is the cosmetic skin practice for the masses. Figures from the Sunbed Association show that there are an estimated 28,000 sunbeds humming away in tanning salons around the UK. Indeed, the bid to get the perfect all-over tan is so widespread that it could be classed as a leisure pursuit.

At the other end of the scale are skin-lightening creams, many of which contain the toxic hydroquinone. They are used by those who want to fade their Asian or African complexion. But perhaps we should dwell no further on the aesthetics of the integumentary system - the skin and associated appendages such as hair, nails and sweat glands. Instead, let us marvel at how our most public organ keeps us alive.

A damn good hiding

The skin is adapted to perform slightly different functions, depending on its precise location on the body. It is thickest on the palms of our hands and the soles of our feet, where the epidermis (the top layer) can be up to 4mm deep. The skin around our eyelids is the most delicate area, where it is 0.5mm thick. Overall, the average skin depth across the body is 0.6mm.

Our fingertips and toes have ridges to give us a better grip when handling objects. Around most of the body, the skin is taut, but the skin protecting the elbows, knees and lips is loose and folded to allow flexibility. A healthy skin usually replaces itself approximately every 30 days, but this process quickens if there is damage such as a cut or sunburn. This replacement process will also speed up in response to abrasions caused by skin peeling or exfoliation. For people suffering from the chronic skin disease psoriasis, the cell renewal rate is as short as four days. This causes the skin to become inflamed, red and flaky.

As well as keeping out water, the skin prevents viruses, bacteria and fungus from getting in; hence the need for plasters and bandages to cover wounds. With partial-thickness burns, the epidermis is damaged and there are varying degrees of damage to the dermis. But with full-thickness burns, both the epidermis and the dermis are usually destroyed.

Hard graft

In the first few hours after a serious burn, large amounts of fluid are lost because the body's natural immune response is to direct fluid from the blood vessels into the damaged tissues. This accounts for the blister that develops shortly after you get a small burn. Imagine that response a hundred-fold in the case of a serious burn. Untreated, a patient who has suffered serious burns will quickly go into shock. The body can no longer regulate its heat loss and the immune function of the skin is compromised, leaving the patient vulnerable to serious infection.

With full-thickness burns, plastic surgeons will usually cut away all the damaged skin and try to replace it using skin grafts. Sometimes, in the case of extensive burns, there will not be enough healthy skin to graft on to all the burned areas, so temporary skin coverings are used until the skin-graft donor site can be re-harvested.

Surgeons obtain temporary skin coverings from a variety of sources - often human organ donors and pigs. There are also some artificial preparations made from a range of substances, including silicone, shark's cartilage and cow collagen. But grafting techniques create a number of problems. Most obviously, taking skin from a donor site causes more damage to healthy areas of skin, at least temporarily. Also, once wounds such as these have healed, the quality of the grafted skin will remain compromised and more delicate, less elastic and more sensitive to sunlight.

Artificial skins buy valuable time for burns patients, but they will have to become a lot more sophisticated before they can substitute for the real thing.

Malleable membrane

At the UK Centre for Tissue Engineering (UKCTE), a collaboration between Liverpool and Manchester universities, scientists are working on a range of projects such as a natural alternative to artificial skin for wound healing. One area of work is stem-cell research. Stem cells have generated a lot of interest in the medical research community on account of their ability to develop the form of any type of cell. Scientists hope that if these cells can be manipulated in certain ways, it might be possible in future to rejuvenate parts of the body, cure disease and even generate new organs.

It is widely known that stem cells are found in abundance in human embryos and the umbilical cord, but each of us also carries stem cells for maintenance and repair. Researchers at UKCTE are investigating ways to activate and steer these cells to the site of a wound - a leg ulcer, for instance - to facilitate the healing process. The research is in its early stages, but one possible outcome could be a "patch" containing chemicals to promote self-healing.

Thermostatic throwback

Your skin won't stop you from getting hypothermia but it does have a temperature control function. When it is hot, the blood vessels near the skin's surface dilate (vasodilation) to release heat. When we sweat, moisture evaporates from the skin and takes the heat with it. When it is cold, the vessels constrict (vasoconstriction). This keeps the blood away from the surface, and explains why the skin has a slightly blue appearance in low temperatures.

When we are cold, the body also raises the fine vellus hairs on our skin to trap warm air, which may cause "goose bumps". Interestingly, these are quite useless as a heat-preserving mechanism; in fact, they are a throwback to our cave-dwelling days when we had considerably more body hair.

A little of that human touch

It is through our skin that we experience our sense of touch - and a life without touch would be one fraught with danger. All skin is sensitive to touch, but it is on the hands, fingertips, eyelids, genitals and lips where we have receptors known as Meissner's corpuscles which detect light touch.

It is through these corpuscles that we experience our greatest tactile acuity.

Hands and fingers are particularly sensitive. Run your finger across a smooth surface and you will be able to sense the difference between a gap of 1mm and one that is 1.5mm. Without this fine sense of touch, we would have problems developing our fine motor skills. Through our skin we can also detect pressure, pain and temperature, and in combination these will give us information about an object's contours, temperature, density and texture. We do not have receptors to distinguish between wet and dry; it is our pressure and temperature sensors working together that tell us whether objects are solid or liquid.

Weathering heights

Like many parts of the body, the skin becomes less efficient with age.

Older skin has fewer collagen fibres and a thinner epidermis, making it more sensitive. The subcutaneous fat which plumps out the skin reduces, making it harder to retain moisture, and so the skin becomes drier.

From the outside, this decreased efficiency becomes evident in jowls, creases, wrinkles and general sagginess. If you gently pinch the skin of a child, it will instantly spring back to its original position. But try the same thing with someone whose birthday suit has been worn for a good few years and the skin will reposition itself much more slowly.

Next time you take a look in the mirror, remember that your skin is a lot more than an index of your personal gorgeousness. Spare a moment to acknowledge this champion multi-tasker of the human body.

Thanks to Professor Tim Hardingham, director of the UK Centre for Tissue Engineering, and Michael Tyler, director of the burns unit at Stoke Mandeville Hospital, Buckinghamshire.For more information about skin conditions see the British Association of Dermatologists website:

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