Two teams of American scientists believe they have for the first time found a way of cultivating human cells to produce any types of body tissue -- with enormous implications for transplant medicine and the treatment of disease.

The scientists use what are called stem cells from human embryos, which have the potential to become bone, muscle, nerves, or any body organ -- even brains.

They believe that eventually, they could lead to new ways of treating, for example, heart disease, many cancers and degenerative brain diseases such as Parkinson's.

But correspondents say there are serious ethical problems involved: the use of human embryo tissue for the reserach, and the danger that the technique could be applied to provide human clones.

They add there's also disquiet because the research is being done by a private company, which will therefore own the patents to any new process.