Each year, hundreds of Canadians die waiting for an organ transplant. Organ recipients often have to wait several years to receive an organ. The problem is acute, and it’s not going anywhere. 

One solution to this crisis is to create new organs and tissues. From coating pig kidneys with human cells to bioprinting new skin for burn patients, researchers at U of T are approaching the problem from a myriad of angles.

Background: Organs from stem cells

Decades ago, scientists thought the creation of new organs for transplant fits squarely within reach. James A. Thomson and colleagues at the Wisconsin Regional Primate Research Center at the University of Wisconsin-Madison first isolated human embryonic stem cells in 1998. Certain stem cells can make copies of themselves and specialize into any cell type in the body, like blood or kidney cells. 

In their paper, Thomson and colleagues proudly declared that “the standardized production of large, purified populations of… human cells… will provide a potentially limitless source of cells for drug discovery and transplantation therapies.”

While certain transplantation therapies, like bone marrow transplants, are now commonplace, growing organs in a lab has proven more difficult. So, scientists had to start small. In 2013, Takanori Takebe and colleagues at Yokohama City University Graduate School of Medicine created 3D liver-like structures from stem cells that recapitulated the function of the liver in mice. 

These structures have become a powerful tool in research and are similar to 3D structures of cells scientists call organoids. But creating organs from the ground up, particularly at the human scale, isn’t always practical.

This is especially true for kidneys, the most greatly needed organ for transplant. “[The kidney has] such a complex microarchitecture, where the urine filtrate has to go through each nephron, and then into the ureter,” explained Tonya Bongolan, who completed her doctoral studies in biology studying kidney regeneration at U of T last year under the supervision of Professor Ian Rogers — whose lab studies stem cells and the formation of organs — in an interview with The Varsity.

Decellularizing organs

After years of research, the Rogers Lab has now ‘decellularized’ and ‘recellularized’ a whole mouse kidney, which functions much like a kidney in a body. In the Rogers Lab’s studies of recellularized mouse kidneys, the kidneys filtered proteins and glucose, producing a urine-like substance.

To decellularize an organ means to remove all of the cells, leaving behind only the extracellular matrix, the organ’s structure without any of the cells. The extracellular matrix contains specific molecules and proteins that provide structure to the organ, facilitate cells’ movement, and tell unspecialized cells what to become. 

As the decellularization process proceeds, a kidney loses its pinkish colour and becomes “ghost-looking,” Bongolan described. After decellularization, scientists can recellularize — or add new cells to — the kidney using human adult primary renal epithelial cells, a type of kidney cell. This allows for kidneys from pigs to be decellularized, recellularized with human cells, and transplanted instead of having to create a kidney from scratch. 

Scientists at the Tissue Repair and Regeneration Lab at the University Health Network (UHN) perform similar research about the lungs and trachea, which present unique challenges. For instance, entirely decellularizing tracheal tissue can compromise its structure. To combat this, Fabio G. Aoki and colleagues at the Tissue Repair and Regeneration Lab at UHN and U of T only partially decellularized tracheal tissue. Using this method, the tissue retains its integrity and supports the attachment of cells.

Zooming out of decellularization, The Burn Research and Skin Regeneration Lab at Sunnybrook Hospital — led by Dr. Marc Jeschke, a U of T professor in the Department of Surgery, Department of Immunology and the Institute of Medical Science — has worked on improving outcomes for burn patients. To treat patients with large burns, Richard Y. Cheng and colleagues at the lab, and at U of T’s Institute of Biomaterials and Bioengineering, designed a handheld device that creates skin precursor sheets, essentially printing a skin-like material on the spot. These sheets were composed of fibrinogen — a protein that can recruit host cells that heal burns — and mesenchymal stromal cells, cells isolated from bone marrow, adipose, and other tissue sources that help with inflammation, the immune response, and healing. 

The out-of-body experience of livers

Moving to the liver, Dr. Nazia Selzner, a transplant hepatologist in the Multi-Organ Transplant Program at UHN, works with the ex vivo, or “outside of the body,” platform. The platform allows scientists to maintain organs, like livers, outside the body by pumping blood through them, setting the appropriate temperature, and overall mimicking the body’s environment. 

“[The liver] feels very happy because it’s outside the body, but it has the impression that it is within the body,” said Dr. Selzner in an interview with The Varsity. Currently, the ex vivo platform allows scientists to check whether a donor’s liver is functioning well enough to be eligible for a transplant. 

In the future, the ex vivo platform could help scientists do even more. As researchers find ways to extend the amount of time organs can be kept outside of the body, new avenues could open for treating diseased livers and re-transplanting them. “For example, if a liver is too fatty, we can de-fat it on the machine, and then use it for transplant. If there is cancer, you can treat the cancer and then re-transplant it,” explained Dr. Selzner. 

Researchers still need to overcome challenges before many of these technologies can reach the clinic. For instance, despite success in the decellularization field, many current methods aren’t able to get rid of all of the cellular material on an organ, creating a risk of transplant rejection, when a transplant recipient’s immune system attacks a transplanted organ. Whether this will be an important problem clinically is an open question. 

For now, thousands of people continue to wait for organ transplants. “I believe that clinic-wise, solid organs [like the kidney] are going to be the final frontier,” said Bongolan. We’re closer to that frontier now than ever before.