The biggest question, though, is whether the stembrids really are true ESCs. Verlinsky says the cells express a number of ESC markers. His team has also shown the cells can differentiate into a number of cell types, including heart muscle cells, neurons and blood stem cells, although the results have not yet been published.
Trounson, whose team has been experimenting with a similar approach, says far more detailed studies will be needed. Besides looking at further cell surface markers, Verlinsky's team must show that, like true ESCs, the stembrids are capable of forming cancers called teratomas, which contain a mixture of different cell types, when injected into immune-compromised mice, Trounson says. "Until they have done these studies, we must remain very sceptical."
Delegates at the meeting were dismayed that Verlinsky has applied for a patent on the stembrid method (US 2004/0259249). But he insists that he does not intend to stop other researchers using the method. "I have not done it so I can charge you, I have done it so no one else can charge me," he assured them.
Verlinsky's method would have huge advantages if it really does work. Obtaining the large numbers of fresh human eggs needed for therapeutic cloning is not possible for legal reasons in many countries and is very expensive in countries where it is legal, such as the US. By contrast, there is a virtually limitless supply of existing ESCs for fusion with adult cells.
Then there is the ethical issue, which is again exercising Congress. With Verlinsky's method, federally funded researchers could try to derive stembrids using already-approved ESC lines.
Verlinsky still has a lot to do to convince others of his claims, however. Mari