ON ARTIFICIAL SKIN

If you hold this page between your fingers, a layer of cells about that thick, called the epidermis, is all that stands between you and a pretty cruel world. It is the top layer of the largest organ in the human body, the skin—a busy, multilayered defence system that keeps the world out and you in. It regulates body temperature, sheds waste products, produces chemicals like vitamin D, senses the world, and wards off invaders. You cannot live without it.

Serious burns that destroy even a small percentage of the skin are dangerous because skin cells can’t regenerate fast enough to keep fluid in and bacteria. out. Researchers have devised ways to ways to cover wounded tissue, using grafted skin from the patient, and from animals, human cadavers, and the test tube. But to understand how these techniques save lives, it’s necessary to look first at how human skin works.

Think of skin as a kind of obedient society of cells, in which each member works carefully and predictably at its job. The cells in the outer layer, the epidermis, form the first wall of defence. They are dead bricks of protein bound together by a kind of fatty mortar. Farther down is a layer of constantly dividing cells, the crew that is always replenishing the supply of bricks for this protective wall. These keratinocytes, named for the tough, stringy protein, keratin, from which they’re made, are programmed to find their way to the outer layer and promptly die, at which time they do their best work protecting the body. The layer that produces keratinocytes also provides melanocytes, pigment producers that send little packets of colour out to tan the surface cells and protect them from the sun.

Under the epidermis lies the dermis, a thick network of protein fibres, called collagen, which is produced by fibroblasts. These cells are part of a network that includes thousands of nerve endings, blood vessels, sweat and oil glands, and hair follicles. The capillaries in these blood vessels regulate body temperature by swelling blood to radiate heat away from the body or clamp down to conserve warmth. Some two thousand miles of tightly coiled sweat ducts regulate body temperature by flooding the surface with a slightly acidic and oily liquid that repels bacteria and cools the skin as it evaporates. The dermis, along with a layer of fat cells underneath it, acts as insulation and padding over the muscles and internal organs.

It takes several weeks for the keratinocyte, the major building block of skin, to develop, find its way to the outer wall, stand its post, and then flake away. It’s only when the skin has suffered an injury that this mild-mannered little cell and its fellow-citizen cells stir into action and realise their full potential. They have a powerful ability to mend their society.

An injury sets in motion a dramatic inflammatory response. Special cells influence the capillaries to flush the area with blood and other fluids, causing inflammation and swelling. Then, wave after wave of white blood cells scour the area, attacking microbes that have slipped in through the broken barrier. Meanwhile, fibroblasts start making scar tissue, which will draw the wound together and make it a smaller breach for the now wildly dividing keratinocytes to fill.

Sometimes the wound is just too great, and skin cells cannot grow fast enough to stop deadly infection and fluid loss from winning the battle. There are some 100 billion bacteria swarming over normal skin at any one time. Antibiotics are used until the swelling goes down, but the best defence is quickly covering a burn wound. But with what? It depends upon the seriousness of the wound.

A thin layer of skin taken from another part of the patient’s body is the best replacement. It is cut into a mesh like sheet that can be stretched to cover more area, and cells migrate to fill the gaps in the mesh. In the case of extensive burns, however, there isn’t enough good skin left. Until recently, people didn’t survive such burns.

Now, some sort of temporary skin replacement is used to buy time while surgeons take consecutive grafts from the remaining good skin, prepare cultures, or acquire synthetic skins. Cadaver skin, pig skin, and synthetic skins are used temporarily to cover the wound and encourage fibroblasts and blood vessels to create new dermis.

Human skin is the most immunogenic, or hostile to strange tissue, of all the body’s organs. However, a burn may suppress the immune system for as many as six weeks, a condition that can be prolonged by drugs, if necessary. Cadaver skin will work as a temporary covering for several weeks, and blood vessels may even begin to grow into it. But once that happens, unless the immune system is suppressed by drugs, the skin will be rejected. Pig skin is more likely to be rejected by the immune system and is used only as a biologic dressing that will last only a few days.

Cultured skin begins as tiny fragments salvaged from an unburned spot on the patient’s body. A shred of epidermis is broken up into individual keratinocytes, which are placed in a nutrient-rich solution and allowed to grow. Fibroblasts are included in the mix because they produce fluids that help keratinocytes grow. These cells will gather together in a dish just as they do on a wound, forming an intact sheet of epidermis. The new skin looks like a piece of wet tissue and is just as fragile. In about a month, postage stamp-size patches of new skin are available to cover a thousand times the area of the original piece. This new skin is not capable of growing new hair follicles or sweat glands, though.

Researchers have discovered that skin cultured from other sources works as well as that grown from a patient’s own cells. Apparently, keratinocytes produce chemicals that stimulate other keratinocytes to grow, no matter whose they are. Ultimately the graft is completely replaced by the patient’s own cells. Someday hospitals will keep frozen cultured tissue grown from such sources as the skin removed during circumcisions-to use right away when severely burned patients are brought in.

Another option is synthetic skins, which have a fibrous dermal layer made of collagen, usually taken from cowhide or even shark cartilage. A sheet of silicone or plastic serves as the protective top layer that is eventually peeled off and replaced with grafted skin over the newly grown dermis.

Skin grafts-human, animal, or synthetic-decrease the amount of scar tissue that grows over the wound. While the human body has all these mechanisms for healing itself, it sometimes doesn’t know when to quit. The deeper the wound and the longer it takes to heal, the worse the scarring will be. Scars can continue to grow and distort the wound for as long as two years. Age, skin colour, genetics, infection, and hormones affect the growth of scars. Growing children, dark-skinned people, and pregnant women tend to have thicker scars.

It is a tribute to the adaptability of this important organ-some 18 square feet of surface area and fully 17 percent of body weight-that it can put up with all the daily abuse we give it and also fight back so courageously when it’s injured. Scientists were hopeful that other human cells could be induced to grow in the laboratory and replace injured or diseased tissue in the same way skin cells do.