A Speculative Timeline for Xenotransplantation Trials
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Xenotransplanation is the use of animal organs, possibly genetically modified, in place of human organs for transplantation in cases of organ failure or damage. This is one of the developing technologies that will compete with electromechanical or bioartificial organs and tissue engineering of whole organs to make organ replacement a far more viable, effective, and low-cost prospect than it is today. To my eyes xenotransplantation was always going to be a transitional technology, economically viable for a period of years in which tissue engineering was still finding its feet, but the development of decellularization has made animal organs look like a far more interesting long term source of raw materials.

a valve from a human or animal donor is removed of all cells using tissue engineering, so that only its outer framework remains. This valve matrix is then colonised with cells that have been obtained from the blood of the recipient and propagated. Within a few weeks, a quasi-natural heart valve then emerges in this bioreactor, that exhibits no rejection response or other faults, but instead grows with the patient after the implantation. ... Recellularization makes xenotransplantation a much more viable technology to fill the tissue engineering gap prior to the ability to grow complex organs from scratch.

The use of the patient's own cells in a donor scaffold removes issues of immune rejection, wherein the patient's immune system attacks and destroys the donor organ. When immune rejection is removed from the equation, not only does the entire process become much safer and cheaper, we are left with the extracellular matrix organ scaffold as the actual raw material required. A full organ scaffold is presently too complex for researchers to construct from scratch, and even when this can be done at some point in the next decade or two, it will be expensive for a time thereafter. Obtaining the decellularized scaffolds from human organs puts you right back to where you started with the difficulties of sourcing donor human organs when needed, but using animal organs can work around that issue.

I noticed a recent article that discusses the timelines for present work on xenotransplanation without decellularization, which is largely focused on transferring cells and small sections of tissue rather than whole organ structures. Organs are clearly on the agenda, however:

During the past decade xenotransplantation, the use of animal organs, tissues or cells in humans, has made great advances. Due to the fact that more and more genetically modified pigs are available with genes to protect them from human immune response, has alleviated earlier problems in helping humans to accept such transplants. ... at this time the longest time of survival for pig organs in non-human primates varies from a few days in lung transplants to approximately 6-8 months in hearts transplants. Although research is still years away from conducting human trials of solid organ transplants of this nature, lifesaving transplants of a pig heart or liver could pose as an alternative solution until a human organ becomes available. At present researchers are investigating strategies to incorporate human anticoagulant or antithrombotic genes into genetically modified pigs, and additional genes to regulate the human inflammatory response.

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The authors also discussed in terms of organs, that stages of other strategies are currently more advanced than xenotransplantation, such as left ventricular assist devices for cardiac support. However, they agree that given time, transplanting a pig's heart will prove to be the better option compared to using a mechanical device.

...

Although remaining issues are delaying clinical implementation, experimental results obtained with pig islet, neuronal-cell, and corneal xenotransplantation have been encouraging. With new genetically modified pigs becoming available that are likely to improve the outcome of cellular and corneal xenotransplantation further, we believe that clinical trials will be justified within the next 2-3 years. No safety concerns that would prohibit such clinical trials have been reported...With regard to pig tissues and cells, as opposed to organs, it would seem that clinical xenotransplantation could soon become a reality.

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