Creating chimeras, especially those with human cells, may prove controversial, given the possibility that test animals could be humanized in undesirable ways. One would be if human cells should be incorporated into a pig’s brain, endowing it with human qualities. Insertion of human stem cells into the early embryos of monkeys was prohibited in 2009, and remains so because monkeys, given their evolutionary closeness to humans, might easily have their brains altered by human cells. All-purpose human stem cells were first derived from human embryos in 1998 and from ordinary adult tissue cells in 2007. To achieve the goal of growing human organs for transplant, researchers must first engineer pigs that cannot make the organ of interest.

Source: New York Times January 26, 2017 17:08 UTC