Patients in need of life-saving organ transplants can spend years waiting for a suitable donor. The ultimate solution to the organ shortage crisis may be the development of processes to grow healthy transplant-able organs in the lab – and for some, this means growing human organs inside animal hosts.

Chimeras, creatures containing parts from multiple animals, are not a new concept in biology. Chimeras can even arise naturally: on occasion, two fertilized eggs occupying the same womb can fuse and eventually develop into a single organism. However, the creation of hybrid chimeras with cells from two different species tends to give us pause, especially when the hybrid is part human. Such experiments blur the boundaries between human and animal that we use to justify animal experimentation in the first place, raising difficult ethical and philosophical questions.  

Earlier this year, Japan lifted its ban on bringing human-animal hybrid embryos to term (scientists had previously been required to abort such embryos after 14 days), and Japanese scientists are taking advantage of the new legality of hybrid experiments to attempt to grow human organs. The strategy is to deprive the host animal embryo of its ability to form a specific organ of its own, instead providing it with human cells, with the hope that the host animal will then use the human cells to build the organ it lacks. 

Time will tell whether this approach will be successful, but there's so much to consider here ethically. For instance, it is possible that the human cells get integrated into other organs inside of the host animal. We need to somehow reconcile our cultural concepts of humanity with our views on other animals before biotechnology forces us to take positions on fraught ethical issues.