The bionic man (or woman), not so long ago a figment of the imagination, is quickly becoming a reality. Largely due to recent advances in biomedical engineering and microsurgery, our ability to introduce artificial materials into a human body to replace worn or damaged parts has improved dramatically. Transplants of human tissue and organs are also common, and in many cases promise a new lease on life for the fortunate patients who receive them. In this brief space, we will consider just a few types of transplantations--those involving skin, hearts, bone marrow, and fetal tissue.

Skin transplants, more often called grafts, have a long history of success. In this product, tissues from person's own body are harvested and introduced into areas where skin has sustained devastating damage to the regenerative layers. Transfer of one's own tissue is referred to as an autograft. Since the patient's own skin is being used, there are no complications of incompatibility and rejection.

In the earliest skin grafts, a nearby limb was temporarily attached surgically to the damaged area. The skin from a healthy arm or finger, for example, was loosened and stretched to cover the damaged tissue. When the new skin grew in to replenish the damaged cells, the limb was separated and released. An alternative technique that is more common today involves removal of sheets of skin. During the harvesting surgery, a sophisticated version of our kitchen carrot scraper is used to carefully lift sheer layers of epidermis. Part (or all) of the dermis is also removed; the depth of the sheet depends on the severity and depth of the injury. These sheer bits of skin are cut into small pieces that are not much larger than postage stamps. These fragments are carefully distributed on top of the exposed fascia. Once they establish a blood supply, the fragments begin to grow and replenish the damaged skin.

Recent advances in cell culture techniques have contributed further to the success of skin transplants. It is now possible to remove a small section of skin from a burn victim, and grow it under controlled laboratory conditions. From initial stamp-sized samples, large sheets of epidermis have been grown and used to cover burn areas. Cultured epidermis offers the added benefit of potentially scar-free healing.

Homotransplants, or allografts, are transplants between individuals of the same species who are not genetically identical. These transplants are more complicated than autografts because of the possibility of tissue rejection. The key to success is the tissue compatibility of the donor and the recipient. Medically, it is far easier to prevent rejection than it is to halt an immune system that has been activated. That is why meticulous matching of tissue is critical. The first step in matching involves the blood and the ABO and Rh systems, since the maker proteins (or antigens) on these cells are widespread throughout body cells. Once the blood groups have been matched, then other tissue matches are determined.

The role of the major histocompatibility complex (MHC) proteins is particularly important in transplantations. MHC-encoded proteins, located on the cell membranes of all body cells, make it possible for the immune system to know who the good guys are; that is, to recognize one's own cells as self and distinguish them from nonself cells. In mismatched transplantations, certain cells in the recipient, called cytotoxic T cells, see the transplanted cells as intruders, and act vigorously to destroy them. Hence, the fewer cell-marker differences (and the more matches) between the two individuals the better. (This is certainly a good time to have an identical twin handy.) Twin-twin, or isograft, transplants--with identical genetic material--have the greatest chance of success. Since few of us have twins, however, it is far more likely that cells with membrane markers like ours will be found in a close relative. If this is not possible, the hunt for compatible MHCs may be very long.

Heart transplants have probably received more public attention than any other organ transplant, perhaps because of the drama associated with the heart. When our hearts are failing, options are limited. The most superficially simple solution is to find a compatible donor heart that can be substituted for the diseased organ. First performed in 1987 in Maryland, the surgical technique has been vastly improved. What has not changed are the complicated and risk-filled procedures that precede and follow the operation. In order to be considered a candidate for transplant surgery, a patient must be evaluated by a team of physicians. They must determine whether the individual is first of all capable of surviving the actual surgery, and then of adopting a postsurgical lifestyle that would justify such an extreme measure. Once a patient passes the screening process, he or she must also be able to survive until a willing donor is found--a donor with compatible tissues. (The specific selection of candidates to receive heart transplants varies from institution to institution, but all hospital systems face similar difficult issuers.)

Following the surgery, doses of immune-suppressant drugs are administered in an attempt to keep the patient's body from rejecting the new organ. One such drug, cyclosporine, an extract of soil fungi, has proven particularly useful in this regard. Use of the immunosuppressants must be balanced against the risk of allowing the recipient to be vulnerable to pathogens, which could take full advantage of a compromised immune system.

Bone marrow transplants offer hope to a different type of patient. When disease (e.g., aplastic anemia and some types of leukemias) or radiation exposure destroys bone marrow, then transplanting healthy bone marrow into the malfunctioning bones is called for. As with heart transplants, there must be a match of blood type. Since the most likely source of compatible tissues is from individuals with similar DNA, close relatives are asked to become donors. Once the lengthy screening process has been completed, the patient's own bone marrow is destroyed prior to the surgery by means of massive doses of radiation or chemotherapy. During surgery, small amounts of healthy marrow tissue are removed from the donor. The sample is then mixed with anticoagulants and filtered to remove some types of lymphocytes before being introduced into the marrow recipient. Once inside the recipient, it is hoped that the new tissue will infiltrate the bone marrow with healthy cells--cells that will resume the functions of the ones that had to be destroyed.

The circumstances involving fetal tissue are quite different from skin, heart, or bone marrow transplants. The anguish over a miscarriage certainly cannot be mitigated. However, grieving parents may gain a small bit of comfort by knowing that tissue from the conceptus could possibly provide hope and relief for others. Parkinson's disease patients, diabetics, elderly individuals, and women with fertility problems may all be the grateful recipients of such fetal tissue. Fetal tissue is excellent for transplantation for several reasons. The fetal tissues lack most membrane markers, making it unlikely that the recipient's immune system will recognize them as foreign. Thus, the complications of rejection are avoided. Also, because the metabolic rate of fetal tissues is very high, the rapidly growing cells quickly merge with a recipient's cells. Since fetal cells are lately undifferentiated, they are plastic and capable of developing into the types of cells that the recipient needs.

On the other hand, there are those who argue that using fetal tissue for therapeutic purposes will increase the number of abortions. The question of whether this type of transplantation poses an ethical dilemma is still being hotly debated.

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