Skin Grafts and Artificial Skin

Third-degree burns leave no dermal tissue to regenerate what was lost, and therefore they generally require skin grafts. The ideal graft is an autograft—tissue taken from another location on the same person's body—because it is not rejected by the immune system. An autograft is performed by taking epidermis and part of the dermis from an

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undamaged area such as the thigh or buttock and grafting it to a burned area. This method is called a split-skin graft because part of the dermis is left behind to proliferate and replace the epidermis that was removed—the same way a second-degree burn heals.

An autograft may not be possible, however, if the burns are too extensive. The best treatment option in this case is an isograft, which uses skin from an identical twin. Because the donor and recipient are genetically identical, the recipient's immune system is unlikely to reject the graft. Since identical twins are rare, however, the best one can hope for in most cases is donor skin from another close relative.

A homograft, or allograft, is a graft from any other person. Skin banks provide skin from deceased persons for this purpose. The immune system attempts to reject homografts, but they suffice as temporary coverings for the burned area. They can be replaced by autografts when the patient is well enough for healthy skin to be removed from an undamaged area of the body.

Pig skin is sometimes used on burn patients but presents the same problem of immune rejection. A graft of tissue from a different species is called a heterograft, or xenograft. This is a special case of a hetero-transplant, which also includes transplantation of organs such as baboon hearts or livers into humans. Heterografts and heterotrans-plants are short-term methods of maintaining a patient until a better, long-term solution is possible. The immune reaction can be suppressed by drugs called immunosuppressants. This procedure is risky, however, because it lowers a person's resistance to infection, which is already compromised in a burn patient.

Some alternatives to skin grafts are also being used. Burns are sometimes temporarily covered with amnion (the membrane that surrounds a developing fetus) obtained from afterbirths. In addition, tiny keratinocyte patches cultured with growth stimulants have produced sheets of epidermal tissue as large as the entire body surface. These can replace large areas of burned tissue. Dermal fibroblasts also have been successfully cultured and used for autografts. A drawback to these approaches is that the culture process requires 3 or 4 weeks, which is too long a wait for some patients with severe burns.

Various kinds of artificial skin have also been developed as a temporary burn covering. One concept is a sheet with an upper layer of silicone and a lower layer of collagen and chondroitin sulfate. It stimulates growth of connective tissue and blood vessels from the patient's underlying tissue. The artificial skin can be removed after about 3 weeks and replaced with a thin layer of cultured or grafted epidermis. At least two bioengineering companies have developed artificial skins approved in 1997-98 by the U.S. Food and Drug Administration for patient use. The manufacture of one such product begins by culturing fibroblasts on a collagen gel to produce a dermis, then culturing ker-atinocytes on this dermal substrate to produce an epidermis. Such products are now being used to treat burn patients as well as leg and foot ulcers that result from diabetes mellitus. This is one aspect of the larger field of tissue engineering, which biological technology companies hope will lead, within a few decades, even to engineering replacement livers and other organs.

Saladin: Anatomy & Physiology: The Unity of Form and Function, Third Edition

6. The Integumentary System

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© The McGraw-H Companies, 2003

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