Tissue transplantation

 

While most accounts and arguments relating to transplantation focus on the solid organs, what is often overlooked is that many tissues can also be transferred between people. Indeed, not only was skin the first part of the body to actually be transplanted, but if we were to extend our criteria by a wide enough margin, we would be able to argue that blood transfusion is the most important and most widely practised transplantation procedure of all, with thousands of people daily receiving a “transplant” of blood matched to their own antigens through blood group typing. As in the previous section, we examine the rationale behind the grafting of different tissues as well as discuss why and how such measures are undertaken.

 

i) The cornea

The cornea, which is a transparent, dome shaped arc of tissue, forms the outermost layer of the eye, from where it performs two main functions. First, it protects the eye by acting as a barrier that prevents harmful material, including germs, dust fragments and ultraviolet light rays, from reaching the sensitive inner components of the eye, such as the internal lens and retina. Second, it acts like the outermost lens of the eye, with light passing through it before being focused onto the smaller inner lens, which sends this concentrated light to the retina for processing of images [1].

 

Many people suffer from corneal problems that can hamper their vision, resulting in astigmatism, near-sightedness and far-sightedness. Generally, these problems are rectified through the prescription of eyeglasses and contact lenses, or, in more complex cases, through corrective surgery. However, transplantation may sometimes be needed if the cornea becomes clouded up or is severely damaged [2]. In the first case, causes such as hereditary disorders, disease, and poor lifestyle factors cause a person to develop an opaque eye, which prevents light from passing through to the inner eye and results in blindness. In the second case, a cornea may be severely scarred if a person engages in activities that risk exposing the cornea to foreign elements without wearing eye protection, such as playing sports or working with power tools and chemicals.

 

In spite of being first performed in 1905, cornea grafting was not adopted to any great degree at first, although it is now the most widely undertaken form of transplantation in the world. In part, this is because corneas have a form of “immune privilege” that exempts them from having to undergo the same antigen matching rules as other organs and tissues [3]. Thus, they can be placed in another person’s eye regardless of the composition of the recipient’s immune system. While this exemption was originally attributed to the erroneous suggestion that corneas do not have antigens, research indicates that this immunity is due to several other factors that are only now becoming known, in particular, their lack of contact with blood vessels. If there is no contact with blood, then it is impossible for the immune system to send antibodies to destroy the foreign tissue, although in cases where contact does occur, rejection of the transplant may take place. In addition, just because corneas are usually safe from the immune system does not mean that there is room for complacency, as corneal grafts can transmit diseases, including rabies, Creutzfeld-Jakob disease, and possibly HIV to recipients [4].

 

ii) Bone marrow

Bone marrow, which is a living tissue located in the long bones and other dense bony parts of the body, serves as the host tissue in which the stem cells that form the blood components grow and develop. This tissue can be destroyed due to diseases such as leukaemia and thalassemia, or following exposure to toxic drugs and radioactive materials, with sufferers manifesting serious signs of illness that include anaemia (brought about by a lack of red blood cells) and an increased susceptibility to infection (arising from depletion of white blood cell levels). While some of these conditions may be handled with conventional medical treatments, the results obtained are not always desirable or sustainable over the long run. For example, while leukaemia may be treated with drugs or through irradiation, patients risk losing healthy cells along with diseased cells due to the non-discriminatory nature of these treatments, with the continuous provision of blood products and other supplements providing only limited comfort in what is often a drawn-out and painful healing process. Consequently, an alternative solution to treating these problems has been to restart local blood production in the patient through a bone marrow graft, where bone marrow cells are first removed by suction from within the appropriate bones of a donor before being transferred via infusion into the recipient.

 

The pioneer in this field has been E. Donnal Thomas, who was, along with Joseph Murray, a co-recipient of the 1990 Nobel Prize for Medicine. In addition to performing the first bone marrow transplant in 1956, where a patient with leukaemia received bone marrow donated by an identical twin, he was also, in 1970, to succeed in performing the more difficult task of transplanting bone marrow where the donor was not an identical twin of the recipient. This was an important achievement, for while the use of anti-rejection drugs in other fields of transplantation can largely overcome the need for a good antigen match, in this particular area, achieving a precise antigen match is of fundamental importance. This emphasis on antigen matching is due to the well-founded fear that patients may fall victim to a condition referred to as graft-versus-host infection should there be even a minor mismatch in the structure of their antigens. Under normal transplant conditions, there is always an expectation that the cells of the host may act to reject the new graft, but with bone marrow grafts, the reverse may also take place. Due to the concentrated presence of white blood cells in the transplanted bone marrow, the recipient’s own blood cells, tissues and organs may come under attack from the newly inserted cells, as these cells may interpret the host’s tissues to be foreign elements that need to be destroyed. In such cases, the patient is made considerably worse off than if no transplant had been performed at all [5].

 

Despite these problems, unrelated bone marrow donation has slowly become more common over time due to the fact that very few patients have identical twins capable of supplying identical tissue to them. Here, an important development has been the introduction of bone marrow banks, where people who wish to donate marrow are allowed to enrol with an agency that will match them up with a potential recipient should the need arise (which is highly unlikely, as the odds of a match between two random strangers is about one in sixty thousand) [6].

 

iii) Minor tissues [7]

Numerous other minor tissues can also be transplanted with varying degrees of clinical success. In some cases though, the grafting of these tissues is either highly controversial or still in an experimental stage.

 

1) skin – As was previously noted, layers of skin can be grafted as a form of plastic surgery to treat patients who have suffered disfigurement due to burns, radiation and disease, with the benefits being psychological as well as physical. While skin is usually obtained from healthy locations on a recipient’s own body (i.e. autografts), compatible skin provided by other people can also be used should the recipient have insufficient skin of his own, with special methods also existing to grow donated skin so that the amount that can be transplanted at any one time is greater than was previously possible;

 

2) blood vessels – patients with coronary artery disease or other conditions that affect the function of their blood vessels may require the replacement of these vessels if they are blocked or damaged. Here, clear arteries that are healthy and free of fatty deposits are used to bypass the obstructed pathways in order to ensure that the flow of blood between vital parts of the body continues without impediment. While blood vessels can be extracted from a recipient’s own body, usually from the leg, it is also possible to obtain these vessels from other donors;

 

3) bones – bone transplants may be performed if fractures that arise from injuries do not heal, as a form of reconstructive surgery, or as a means of ensuring that cancer does not spread to a patient’s bones and adjoining bone marrow, where eradication of a tumour is extremely difficult. While donated bones obtained from cadaveric donors may be shaped to suit the exact measurements of a recipient, they can provide the recipient with structural support only, as the cells that are located within them, such as bone marrow and regenerative cells, are usually dead once the transplant occurs;

 

4) heart valves – sometimes, the transplant of an entire heart into a patient may not be justified if all that this person has is a disease affecting the function of one or two heart valves. In such cases, a transplant that involves the replacement of the faulty valves with new ones may be performed, with this process having the peculiar advantage of ensuring that more than one person can benefit at the same time from the donation of a single heart;

 

5) connective tissues – investigations have been made to assess the feasibility of transplanting connective tissues, such as knees and muscle fibres, into people who have physical disabilities or disorders that prevent them from having proper mobility. While research into this field is still continuing, several experimental transplants have been performed, with the most notable ones being the transplant of an entire knee into a recipient in the late 1980s and of a whole working hand in the late 1990s;

 

6) scleral tissue – along with the corneas, another form of tissue that can be extracted from the eyes is scleral tissue, which makes up the core white matter in the eye. This tissue is useful for correcting several disorders, with up to 3 individuals being able to benefit from the tissue that can be extracted from a single eye;

 

7) sexual transplants – procedures such as artificial insemination and surrogate motherhood may be considered as forms of tissue transplants aimed at helping couples to conceive. In addition, the donation of eggs and sperm, which is undertaken in many countries, might also be classified as falling into the realm of sexual transplantation;

 

8) brain tissues – research is being conducted to determine whether cellular injections of brain tissue could retard or even eliminate the growth of what are currently incurable neurological disorders such as Parkinson’s disease. In addition, some scientists have proposed, only half in jest, that one day they will be able to transplant whole brains between individuals. While such a feat is presently impossible, someday it may be achievable, especially if ways to overcome obstacles, such as the preservation of nerve connections, can be overcome. Such research is controversial, as it brings into focus a range of questions as to how we define ourselves as well as to what a person actually should be seen as.

 

From the discussion provided in this section (and the previous section), it is clear that a wide range of tissues and organs can be transplanted to remedy a variety of human conditions. As shall be seen in the following section, the circumstances under which these body parts can be obtained is limited though, with clear restrictions being placed on the manner in which individual donors can be recruited and managed.

 

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