Researchers in Spain have transformed mature cells into a primitive, stem-cell state inside the bodies of mice. If the approach can be refined for humans, it may be possible to treat diseases such as diabetes and heart ailments by creating fresh tissue inside a living patient.

The long-term dream of stem-cell science has been to transplant lab-made tissue into patients, but despite years of experiments the quality of such tissue remains poor. An alternative approach is to make the fresh tissue inside the bodies of patients...