They used stem cells from a human embryo, which are capable of becoming any other type of cell in the human body from nerve to skin, muscle to kidney.

BBC: Deaf gerbils 'hear again' after stem cell cure

After all, to obtain these stem cells, you must destroy a human embryo.

NPR: Ethical Guidelines for Stem Cell Research Proposed

The success also provides encouragement that useable stem cells can be found from other sources apart from the human embryo, which remains controversial.

BBC: Stem cells repair heart attack damage