Stem cell-based therapies have been available for the eye for several years. In particular, diseases of the outermost layer of the eye, the cornea, can be treated with “limbal cell” transplantations.
The human eye is more or less spherical, but is rather asymmetrical. Our eyes are also one inch in diameter. The eye consists of a front and rear compartment. The front compartment consists of the iris, which is pigmented, the cornea, which is transparent, the pupil, which is the black, round opening in the iris that lets light in, the sclera or white part of the eye, and the conjunctiva, which is an invisible, clear layer of tissue that covers the front of the eye, which the exception of the cornea. Just behind the iris and the pupil lies the lens, which focuses the light on the back of the eye. Most of the eye is filled with a clear gel called vitreous.
The rear compartment is filled with vitreous humor, which is a liquid that is also rich in a slippery, acidic carbohydrate called hyaluronic acid and several types of proteins. The back of the eye is covered with special light-sensing cells that are collectively called the retina. The retina converts the energy from the visible spectra of light into electrical impulses, and behind the eye, the optic nerve carries these impulses to the brain. The macula is a small, sensitive area within the retina that gives central vision. It is located in the center of the retina and contains the fovea, a small depression or pit at the center of the macula that gives the clearest vision.
If we now focus on the cornea, we will see that there is a ring of tissue that connects the cornea and the sclera known as the limbus. The limbus possesses a stem cell population that replenishes the cornea and also serves as a barrier for the cells on the conjunctiva. If this stem cell population is damaged or depleted, then conjunctival cells invade the cornea, and disrupt its unique structure. The cornea is transparent and allows light to pass through it unperturbed. The conjunctiva, however, is translucent, and filled with blood vessels. If the cornea becomes invaded with conjunctiva cells, it will cloud over and vision will be obscured.
To fix this problem, scientists and eye surgeons have experimented with limbal stem cell transplants. The most successful forms of transplantation use limbal cells from one eye that are transplanted into the other eye. This procedure, however, has a few drawbacks. The removal of limbal cells from one eye can compromise the integrity of the donor eye. Secondly, the patient is now left with two eyes that are healing rom surgery.
A second procedure uses limbal cells from cadavers. This procedure provides a better solution, but the availability of the tissue is a problem. To solve the problem of insufficiency of tissue, several labs have tried to culture the limbal cells and grow them to larger quantities.
A paper in the British Journal of Ophthalmology by Basu and others from the Prasad Eye Institute in Hyderbad, India has examined many patients who received limbal cell transplantations from cadavers. They followed 28 eyes from 21 patients who suffered from limbal stem cell deficiency (LSCD). While this disease is relatively rare, it prevents patients from receiving limbal cell grafts from their own eyes, since both eyes are deficient for limbal stem cells. These patients were treated between 2001 and 2010, and all limbal cells were cultured in the laboratory first and then transplanted 10-14 days after their removal from the cadaver.
Each patient was followed up after surgery for about 5 years, and 71.4% of all patients showed eyes that were stable and clear. The eye sight in the treated eye improved to 20/60 or better in 19 of the 28 treated eyes. There were no serious ocular complications in any patients.
This paper shows that transplantation of cultured limbal cells from cadavers successfully restores the surface of the eye and improves vision in patients with blindness as a result of LSCD. This same technique can also be applied to patients with other types of eye surface disorders. Limbal stem cell transplantations seem to keep improving and they will hopefully become rather routine.
Beyond the Dish 