Xenotransplantation has been viewed as a plentiful source of organs and tissues for transplantation (Table 1).[1,2] However, xenotransplantation may even be preferred over allotransplantation in certain circumstances. Where organ failure is caused by a viral infection, e.g., hepatitis, xenotransplantation might be preferred because the transplant would resist reinfection by the virus that caused organ failure. Xenotransplantation might also be preferred as a way of delivering genes of therapeutic importance. For example, an animal source might be genetically engineered to express a gene at a high level or under regulated conditions.
An important consideration today is how to weigh xenotransplantation against other potential approaches to treating organ failure (Table 2). Though some new technologies, such as stem cells, tissue engineering, and cardiac assist devices, have received much attention, they have also received less scrutiny than xenotransplantation because their development is so recent. Most likely, these technologies will be applied in ways that fill therapeutic niches, such as repairing local defects or injury of tissues. Devices may eventually be used to replace the heart, but application for other organs is more remote. On the other hand, cell transplantation, stem-cell transplantation, and tissue engineering seem less promising for replacement of the function of structurally complex organs such as the kidney, lungs, and heart. For replacement of these organs, organogenesis (the de novo formation of organs) or xenotransplantation may be necessary. Xenotransplanta-tion may also find application in conjunction with organogenesis. For example, one might envision growing human organs, perhaps derived from stem cells, as a xenograft in an animal host and then transplanting the organs to human patients.
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