Stem cells

Researchers who study developing human embryos have long limited their experimentation to lab embryos that are no more than 14 days into development. Some scientists are now pushing that boundary.

Clinics are using stem cells to treat problems ranging from arthritis and torn tendons to paralysis and stroke. But researchers say there's no evidence that the treatments work or are safe.

The National Institutes of Health proposed lifting its moratorium on funding for research on part-animal, part-human embryos — which raises a huge dilemma, says bioethicist Insoo Hyun.

The National Institutes of Health is proposing to fund experiments that create chimeras of human and animal cells while also imposing restrictions in response to ethical concerns.

Researchers experimenting with chimeric embryos say they could develop into adult pigs, sheep or cows with human organs that one day might be suitable for transplantation in people.

The National Institutes of Health has issued a moratorium on funding work that puts human stem cells into nonhuman embryos. The concern is that hybrids might develop human brain cells, sperm or eggs.