Right now, about 120,000 Americans are on a waiting list for an organ transplant and 22 will die today before any organs become available. The plain truth is there aren’t enough organ donors to meet the demand. And according to the U.S. Department of Health and Human Services, the number of available organ donors has remained static over the past decade. How can we overcome this crisis?
One answer may be stem cells. These “blank slate” cells can specialize into virtually any cell type in the body which has many scientists pursuing the holy grail of stem cell research: creating an unlimited supply of human organs. Today, a team of Salk Institute scientists report in Cell that they’ve taken an early but important step toward that goal by showing it’s possible to grow rat organs within a mouse. The results bode well for not only organ transplants but also for the study of human development and disease.
Chimera – monster or medical marvel?
Our regular Stem Cellar readers will be familiar with several fascinating studies using stem cell-based 3D bioprinters or bioscaffolds which aim to one day enable the manufacturing of human tissues and organs. Instead of taking this engineering approach, the Salk team seeks a strategy in which chimeric animals are bred to grow human organs. The term “chimeric” is borrowed from Greek mythology that told tales of the chimera, a monster with a lion’s heads, a goat’s body and a serpent’s tail.
The team’s first set of experiments explored the feasibility of this method by first focusing on rat-mouse chimeras. Reprogramming skin cells collected from rat tails, the scientists generated induced pluripotent stem cells (iPSCs) – cells with the embryonic stem cell-like ability to become any cell type – and injected them into very early stage mouse embryos. The embryos were then implanted into surrogate female mice and successfully carried to term. Examination of the resulting mouse pups showed that their tissues and organs contained a patchwork of both rat and mouse cells.
And for my next trick, I will make a rat pancreas in a mouse
Now, if the ultimate goal is to grow organs that are 100% human in a host animal, an organ that merely has a random patchwork human cells would miss the mark. To show there’s a way around this problem, the Salk team used the CRISPR gene-editing technique to generate mouse embryos that lacked a gene that’s critical for the development of the pancreas. Without the gene, no pancreas forms and the mice died shortly after birth. But when the rat iPSCs were integrated into the gene edited mice embryos, the rat cells picked up the slack as the embryo developed, resulting in chimeric mice with rat pancreases.
Using the same CRISPR gene editing strategy, the researchers also grew rat hearts, and if you can believe it, rat eyes in the chimeric mice. On top of that, the mice in these experiments were healthy with most reaching adulthood and one living two years, an elderly age for mice.
A first step toward growing patient-specific human organs in large animals
One small, actually big, problem is that mice are much too little to serve as chimeric hosts for human organs. So the team repeated these mixed species experiments in pigs which are much better matched to humans. In this case, they added human iPSCs to the pig embryos, implanted them into female pigs and let the embryos develop for four weeks. Although it wasn’t as efficient as the rat-mouse chimeras, the researchers did indeed observe human cells that had incorporated into the chimera and were showing the early signs of specializing in different cell types within the implanted pig embryos.
This work is the first time human iPSCs have been incorporated into large animal species (they also got it to work with cattle) and many years of lab work remain before this approach can help solves the organ shortage crisis. But the potential applications are spellbinding. Imagine a patient in need of an organ transplant: a small skin biopsy is collected to make iPSCs and, using this chimeric animal approach, a patient-derived organ could be grown.
Juan Carlos Izpisua Belmonte, the study’s team leader, talked about this possibility and more in a press release:
“Of course, the ultimate goal of chimeric research is to learn whether we can use stem-cell and gene-editing technologies to generate genetically-matched human tissues and organs, and we are very optimistic that continued work will lead to eventual success. But in the process we are gaining a better understanding of species evolution as well as human embryogenesis and disease that is difficult to get in other ways.”
Now, if the idea of breeding pigs or cows with human organs make you a little uneasy, you aren’t alone. In fact, the National Institutes of Health announced in 2015 that they had halted funding research that introduces human stem cells into other animals. They want more time “to evaluate the state of the science in this area, the ethical issues that should be considered, and the relevant animal welfare concerns associated with these types of studies.” To read more discussion on this topic, read this MIT Technology Review article from a year ago.