Engineering Animals to Grow Human Organs — But Not Human Minds

In 2010, scientists at the University of Tokyo and other institutions reported a startling achievement: They had successfully grown a pancreas made almost entirely of rat cells within the bodies of mice. The research was directed by Hiromitsu Nakauchi, a physician-scientist who also holds an academic appointment at Stanford University. Nakauchi specializes in developing “chimeric” animals made up of cells from different species. This unprecedented feat proved that an entire organ from one kind of animal can be grown within another. The chimeras had been generated as early-stage embryos that, after being implanted into surrogate mothers, came to term “just as in normal mouse pregnancy,” Nakauchi wrote in an emailed response to questions.

Nakauchi’s Japanese lab, now at the Institute of Science Tokyo, a public research university, hit another milestone this year by producing litters of baby mice that were partly human. Currently under review at the journal Nature Biomedical Engineering, the experiment is the first instance of pregnancies involving human-animal chimeras resulting in live births. The human cells were integrated with the animals’ blood vessels and heart muscles, showing...