How do we keep our vision clear? And what happens if that view starts to cloud over? Our window onto the world is the cornea which lets light into our eyes. To keep the cornea in perfect condition, a fine layer of cells on its surface are constantly replaced, flowing in from a region called the limbus. Outside the limbus, a thick layer of cells called the conjunctiva protects the rest of our eye. The limbus acts as a barrier between the cornea and the conjunctiva but if it gets damaged, the conjunctiva can start to grow across the cornea, clouding the window and cutting out the light. Eye-drops or vitamin A supplements can help speed up healing for minor damage but if the damage is more severe, more drastic action might be necessary. Doctors can take some of the patient’s healthy limbus and transplant it, but take too much and they risk causing the same problem at the donor site. Instead, they can transplant cells or even entire corneas from other people. Corneal transplants are well understood procedures and they bring the whole package – cornea, limbus and all – but the risk of the body rejecting the transplant is high and the drugs needed to make the process work have dangerous side effects. If the patient has a small amount of healthy limbus left, doctors can still just take a chunk and amplify the cells in the lab before transplanting them back into the eye. That way there’s no rejection and far fewer cells are needed… Sounds good, but it requires specialist equipment and the patient still might not have enough healthy limbus left to work with. In these cases scientists can repair the damage using cells from a patient’s mouth, but these aren’t totally transparent and so cloud the cornea: better than nothing but far from ideal. In the most severe cases doctors can insert a prosthetic cornea but they try to avoid this as it brings with it a risk of infection and of a dangerous build-up of pressure behind the prosthesis. No one knows which of these treatments is most successful as no one’s studied it. Instead, treatment plans are often just based on each doctor’s preference or the facilities at the medical centre. In the future, researchers would like to grow bespoke corneas by engineering the patient’s own stem cells. These would be complete like a transplant but they wouldn’t be rejected. They’d be a perfect fit for the patient’s eye and doctors wouldn’t need to use any limbal cells to build them. However, research into these sorts of treatments is still in the very early stages and for now it remains way out of sight.