The scientists successfully transplanted clusters of human cells, called brain organoids, into the brains of rat embryos. It was a striking demonstration of how brain organoids, once only grown in labs for research, could work in real life. The results offer a tantalizing opportunity to model the human brain with more life-like precision than ever and find cures to terrible brain diseases. Brain organoids were developed to solve a central dilemma in neuroscience—there aren’t realistic models of the human brain. In 2021, the National Academies gathered a group of scientists, ethicists, and religious leaders to discuss the ethical concerns with human brain organoids.