Credit: Christian Darkin/Photolibrary

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The thick mattress that underlies the epidermis is called the dermis. While the epidermis is predominantly composed of cells with a thin smear of extracellular matrix in between, in the dermis, things are reversed. The extracellular matrix becomes a gelatinous sea within which floats the occasional cell and blood capillary. Woven though is a protein mesh with rope-like collagen fibres that provide tensile strength, while feathery coils of elastin give skin its elasticity.

If collagen and elastin provide the strength and springiness of the dermal mattress, then the stuffing is made of gigantic molecules called glycosaminoglycans (GAGs), the most common of which being hyaluronic acid. GAGs absorb huge amounts of water to form the jelly consistency of the dermis. This gives the dermis its ability to resist compression and provides a medium through which nutrients can diffuse from capillaries to nourish the cells. The mesh, springs and stuffing of the dermal mattress are produced by fibroblast cells. As the skin ages, both the epidermis and dermis thin out and the orderly architecture disintegrates. Instead of regularly arrayed ropes and coils, the collagen frays and the elastin clumps.

The skin is constantly being regenerated by resident stem cells. Some 300 million out of the total two billion skin cells are replaced daily. With all that regeneration going on, what causes skin to age? Judith Campisi at the Buch Institute for Ageing, in California, says researchers haven't nailed the culprits yet. But there are prime suspects.

Sunlight is a major factor. UV light generates highly reactive free radicals that can damage the fabric of the dermis. For evidence of this process, simply compare the skin on your bottom with that on your face. "Dermatologists tell us it's about 20 years younger", for a middle-aged or older individual, says Campisi. Free radicals also directly damage DNA. Cells with damaged DNA are potentially cancerous and therefore dangerous. As if aboard a sabotaged starship Enterprise, inbuilt safety circuits command damaged cells to self-destruct. Stem cells are among those occasionally hit, and when one of their number goes down, the skin's ability to regenerate is compromised. But a decline in stem cells is probably not the only cause. Some cells don't self-destruct when damaged; they merely shut down. The technical term is 'senescence', the cell's equivalent of drifting into old age.

Until 10 years ago, researchers had only observed senescence in the culture dish, and the relevance of the phenomenon to living tissue was unclear. But in 1995, Campisi and her colleagues identified senescent cells in aged human skin. The older the person, the greater the number. It turns out these senescent cells are not just harmless pensioners: they have a nasty tendency to release enzymes that digest collagen. Melanocytes are also damaged. For some reason, when these cells age, they lose their fine control; instead of drip-feeding melanin to keratinocytes, they release it in large dollops which leads to the unflattering liver spots (solar lentigines) that appear on the face and the backs of hands of older people.

There is one substance that, when smeared onto skin, has been proven to reverse ageing. However, it's not a cosmetic, but a drug. That substance is retinoic acid, also known as Retin-A. It can only be obtained on prescription, and for good reason; it affects the early stages of embryo development and, even when applied as a cream, can produce birth defects in pregnant mothers. Not surprisingly, it has multiple effects on skin: it stimulates the epidermis to thicken, stimulates the fibroblasts to produce more collagen and hyaluronic acid (fortifying and restuffing the dermal mattress), and puts a damper on the activity of melanocytes (reducing liver spots). It even treats acne by reducing the production of sebum, the skin's natural oil.

But there are drawbacks. It takes at least four months of treatment before the desired effects are visible, during which time skin can become tender, red, flaky and sun-sensitive. Cosmeceuticals aiming to copy the effects of retinoic acid use weaker relatives such as vitamin A (retinol), which is converted to retinoic acid in living cells. The effects are not as irritating – but not as effective either.

The active ingredients in the vast majority of cosmeceuticals can be roughly divided into four categories: those that neutralise free radicals; those that regenerate components of the dermis; those that coax senescent cells from retirement; and those that relax the tiny face muscles that cause wrinkling.

Anti-oxidant molecules employed to neutralise free radicals include vitamin E, vitamin C, coenzyme Q10, niacinamide (B3), biotin (B8), and a vast array of plant extracts.

One example of a contemporary cosmeceutical is Lancôme's new product, 'Primordiale Neutral', which targets women in their thirties.
Its six key ingredients, including biotin, promise to absorb 99 per cent of free radicals generated in the skin. According to Lancôme's scientific director Véronique Delvigne, based in Paris, "it is a preventative treatment that reduces the first lines".

So do antioxidants work to prevent skin ageing? They certainly soak up free radicals when added to cells in the test tube, but just how effective they are in a cream applied to skin is another matter. For example, vitamin C is unstable and not very good at penetrating skin. According to Monash University's Finnin, "there's a much more reliable way to get vitamins to the skin: just eat them".

Refurbishing the skin's mattress is another key goal of cosmeceuticals. Yesterday's creams contained collagen, elastin and hyaluronic acid, and marketing led the consumer to believe that these gigantic molecules could somehow penetrate the surface of skin, find their way into the dermal mattress, and weave themselves into the fabric. Hardly likely, according to Finnin. More likely they exert their effects at the surface, where their water-absorbing properties create a filling effect, "like Spakfilla" – a product used to smooth out cracks in walls – he says.

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