By William Rimmer Link Japanese regulators have effectively given the green light to research involving human-animal chimera embryos, which are created by implanting human pluripotent stem cells into animals in early development. The revised guidelines, issued in early March, lift a previous requirement to terminate such embryos after 14 days.

The revisions now pave the way for Japanese scientists to study how to grow human organs in animals as an alternative to organ transplantation, and to produce better models to study human development and disease.

However, they also raise some ethical concerns, as a group of Japanese bioethicists at the Kyoto University note in a letter published today (April 4) in Cell Stem Cell. For instance, the guidelines would allow researchers to create chimeras with human cells populating animal brains—something that concerns the Japanese public. They also don’t explicitly prohibit chimera embryos made from human and nonhuman primate cells, which could take experiments into ethically uncharted territory. Read more.