The introduction of human embryonic stem cells (hEBCs) in another human being has two significant clinical problems. First, there is the problem of immune rejection. However, this may not be an issue in the eye, in which immune rejection does not frequently occur. However, there is also the problem of teratoma formation. Although teratomas are benign (meaning only that they do not spread to other tissues), you can easily see why growing tumors may be a bit of a problem in the small, closed environment of the eye. So far this problem has alluded investigators. You can see how this issue will be handled way down in the article:
Pete Coffey of UCL, the director of the project, said he was confident the procedure would work in humans but the team needed to ensure the safety and quality of batches of cells, which would take time.Which is a bit like saying that we are confident that we will soon be visiting other distant galaxies, but the team needs time to work out that "light speed" issue. Its amazing how the media falls for this.