The transplant procedure is attracting much research interest as a possible treatment for Parkinson's disease, epilepsy, stroke and other ailments. It provides new, young cells which mature into replacements for older, damaged cells.

But the poor survival rate of the cells, combined with the shortage of embryonic material, limits the possible usefulness of the therapy.

However, the Swedish and German scientists have increased the survival rate of the transplanted brain cells in rats. They believe their method "may reduce the number of embryos required to one-fourth the current number."

Human cloning

Embryos provide a source of immature cells which can replace damaged ones

The provision of embryonic material for medical purposes is also a principle reason given by scientists wishing to clone human embryos. However, this particular development is unlikely to slow the pace of scientific research in that direction.

The researchers, based in the Wallenburg Neuroscience Centre, Lund, Sweden and Konstanz University, Germany began by noting that the transplanted cells perished as a result of apoptosis, a natural process of cell suicide. Most cells are dead a week after an operation.

The scientists knew that caspase enzymes are fundamental to apoptosis. Therefore, before transplantation, they incubated the embryonic cells in a drug which inhibits the production of caspase enzymes.