A news story out yesterday exemplifies the "successes" of embryonic stem cells. The story proclaimed that scientists had "successfully used mouse embryonic stem cells to replace diseased retinal cells and restore sight in a mouse model of retinitis pigmentosa." Sounds pretty good? Later there is the requisite hyperbole about treatments, that "Once the complication issues are addressed" and a list of retinal diseases that will be treated with embryonic stem cells.

Wait a minute. Complication issues?

However, complications of benign tumors and retinal detachments were seen in some of the mice, so Dr. Tsang and colleagues will optimize techniques to decrease the incidence of these complications in human embryonic stem cells before testing in human patients can begin.

I would hope that they'd eliminate the complications first, not just decrease the incidence. And just how many of the mice are represented by "some"?

The abstract in the journal Transplantation gives a bit more detail:

Although more than half of the mice were complicated with retinal detachments or tumor development, one fourth of the mice showed increased electroretinogram responses in the transplanted eyes.

So, a quarter of the mice showed improvement, but more than half showed complications including tumors...

So much for an embryonic success.