In a science journal, a headline asserted that a permanent cure for genetic blindness had been developed. The article reported on a clinical study involving patients with Stargardt's macular dystrophy, an inherited condition where photoreceptor cells in the central part of the retina degenerate.
Describe the role of photoreceptor cells (rods and cones) in the retina and suggest why the degeneration of these cells leads to a loss of vision.
Photoreceptors rely on a constant supply of oxygen and nutrients from extremely narrow blood vessels, about 555 to 10 μm10\ \mu\text{m}10 μm in diameter, located in the choroid layer. Name this type of blood vessel.
In this study, researchers used retinal pigment epithelium cells derived from human embryonic stem cells to treat three patients with Stargardt's disease.
Two years after the treatment, all three patients showed an improvement in their ability to see. Explain why the headline claiming a "permanent cure for genetic blindness" may be misleading.
Suggest one other factor that researchers must investigate before this stem cell therapy can be approved for widespread clinical use.