Credit: mali maeder, Pexels
A team of Chinese researchers has achieved an unprecedented feat: growing and beating tiny human hearts inside pig embryos. An achievement that, beyond the technical data, raises profound questions about what it means to create hybrid life between deeply distant species.
Chimera embryos have been studied for years: organisms generated by combining human cells with animal embryos in hopes of growing transplantable organs. Pigs are ideal candidates for these experiments because of the structural similarity of their organs to human organs, which also makes it possible to transplant organs.
But obtaining a functioning human heart within another species remains a delicate undertaking, requiring a biological balance yet to be deciphered.
The group led by Lai Liangxue of the Chinese Academy of Sciences worked on pig embryos by deactivating two genes critical for heart development. At that point, it injected genetically modified human stem cells to increase the chances of survival and growth.
The transfer occurred at the morula stage, just after fertilization, and the resulting embryos were implanted into surrogate sows.
The hearts, surprisingly, began to beat. For three weeks, within those embryos, human cells gave rise to heart tissue compatible in size and structure with human tissue at the same stage of development.
A luminescent biomarker confirmed the human origin of those cells. However, how much of the heart was truly human was not disclosed: too low a percentage would compromise the organ's effectiveness for eventual transplantation; too high, on the other hand, could interfere with the development of the host embryo.
Despite stopping growth after 21 days, the step taken is far from negligible. Growing a complete human organ in an animal requires not only biological compatibility, but also precise management of ethical, technical and physiological constraints.
https://www.nature.com/articles/d41586-025-01854-x
