Japanese scientists were previously forbidden from allowing human cells to grow within other animals past a 14-day period, but in March the government relaxed the rules on embryonic stem-cell research aimed at creating human-animal hybrids, allowing such creations to be brought to term.
Bioethicists have raised the possibility that human cells might stray beyond development of the targeted organ, travel to the developing animal's brain and potentially affect its cognition, Nature reports.
To that end, Pacholczyk said that for any chimeras- third, hybrid animals- produced, care must be exercised to avoid the replication of major pillars of human identity in animals, such as the brain system.
In addition, chimeras which produce human sperm or human eggs should never be generated, he said, to avoid the production of the basic building blocks of human reproduction.
The Japanese scientist plans to begin with mice and rats, experimenting for two years, and said it is his hope to eventually apply for government approval to grow human-pig hybrid embryos for up to 70 days.
"Human cells generally do not grow very well in pigs or sheep, likely due [to] the evolutionary distance between us and them, so additional 'tricks' and genetic manipulations may be needed to help the human cells grow," Pacholczyk commented.
"There is also a chance of transmitting new viruses from, say, pigs into the human organs that the pigs are growing, so this will have to be carefully addressed to be sure that if such organs were ever used in transplants, humans would not become susceptible to new infections."
In the US, the NIH proposed in 2016 federal funding of projects to possibly create a human-animal hybrid, prompting serious moral and legal concerns from Catholic ethicists.
In comments submitted to NIH at the time, the National Catholic Bioethics Center stated that using human embryonic stem cells for research is wrong because "human beings at these vulnerable stages must be safeguarded, not exploited, in both clinical and research settings."