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Although the two studies recruited a very limited number of patients, the news is a real boost for the nascent field of stem cell therapy for ocular disease (Table 1). At the very least, the transplanted cells appeared not to result in any untoward cell replication or growth. What's more, the unexpectedly solid signs of efficacy in a population with very poor vision to begin with provide a basis for further refining the therapy for use in patients who could obtain even greater benefit. “Overall the median visual acuity improvement was three lines,” says Robert Lanza, CSO at Marlborough, Massachusetts–based Ocata. It corresponds, Lanza adds, to a doubling of the visual angle. Pete Coffey, of University College London (UCL) is more circumspect, given that the patients involved in the study had already lost their central vision. “The RPE aren't going to make the photoreceptors come back,” he says. “The best you can hope for is to slow down the development of the scotoma, that blind area.”
Dry AMD is characterized by the appearance of drusen, which are yellow or white deposits of cellular debris that accumulate in Bruch's membrane, a five-layered extracellular matrix sandwiched between the RPE and the choroid, the vascular layer of the eye. Geographic atrophy, the advanced form of the condition, involves the widespread loss of RPE cells, which perform multiple housekeeping functions in support of the retina's light-sensitive photoreceptors, including the control of the flow of nutrients and metabolites to and from the photoreceptors and phagocytosis of the cellular debris they shed continuously. The death of photoreceptors generally follows RPE cell loss. Dry AMD is far more prevalent but is generally less severe than wet or neovascular AMD, which is a major cause of blindness. A subset of dry AMD patients progresses to the wet or exudative form of the disease, however, and there is, as yet, no drug therapy to prevent this. No drugs are available either for Stargadt's macular dystrophy. It most commonly arises from mutations in the gene encoding ATP-binding cassette, sub-family A, member 4 (ABCA4), which protects photoreceptor cells from toxic retinoid species following photo transduction.