Injecting cells from one species into the embryo of another creates mixtures called chimeras. From left to right: an ordinary mouse, a mouse that’s partly rat, a rat that’s partly mouse, a white rat.
Credit: MIT Technology Review

Braving a funding ban put in place by America’s top health agency, some U.S. research centers are moving ahead with attempts to grow human tissue inside pigs and sheep with the goal of creating hearts, livers, or other organs needed for transplants.

The effort to incubate organs in farm animals is ethically charged because it involves adding human cells to animal embryos in ways that could blur the line between species.

Last September, in a reversal of earlier policy, the National Institutes of Health announced it would not support studies involving such “human-animal chimeras” until it had reviewed the scientific and […]