A pharmaceutical company press release claimed that a "complete and lasting reversal of diabetic retinopathy" had been achieved in a laboratory trial. In diabetic retinopathy, prolonged high blood sugar damages the neural retina, leading to the apoptosis (programmed cell death) of rod and cone photoreceptor cells in the macular region.
Explain the specific roles of rod and cone cells in converting light into a nerve impulse, and explain why their apoptosis prevents the formation of visual images in the brain.
The metabolic demands of these active photoreceptors are met by a dense network of microvessels, each with a lumen diameter of approximately 666 to 8 μm8\ \mu\text{m}8 μm and walls consisting of a single layer of endothelial cells. State the name of this type of blood vessel.
In this study, researchers injected induced pluripotent stem cell (iPSC)-derived photoreceptors into the vitreous humour of four patients with advanced retinopathy.
After an eighteen-month follow-up period, all four patients demonstrated a measurable increase in spatial contrast sensitivity. Explain why the press release's claim of a "complete and lasting reversal of diabetic retinopathy" is likely to be an overstatement.
Suggest one other biological or clinical factor that must be investigated before this stem cell therapy can be deemed safe and viable for general clinical adoption.