Eye Disease

There are a number of diseases and conditions that cause visual impairment or blindness. There are diseases that affect the cornea and the retina, as well as injuries to the surface of the eye and the optic nerve. All of these conditions are promising targets for stem cell treatment.

Some eye diseases can be traced to a genetic defect, such as Stargardt’s disease and retinitis pigmentosa (RP). Others begin because of another illness, such as diabetic retinopathy, not an uncommon complication of diabetes. The most common eye diseases are associated with aging, most notably cataracts, glaucoma and age-related macular degeneration (AMD).

In many of these conditions, early diagnosis is critical to prevent permanent and irreversible damage, since drug treatment or surgery is available.  In other conditions, the prognosis is poor.

AMD is a retinal degenerative disease associated with defects in the retinal pigment epithelial cells or in the photoreceptors themselves, which are responsible for supporting vision in the retina. The failure of these cells (especially the cones in the center or macula of the retina) leads to progressive loss of central vision and sometimes scarring in the back of the eye.  New blood vessels can start to grow in the retina and then leak fluid. This advanced, rarer form is known as “wet” as opposed to “dry” AMD, and accounts for 10 per cent of cases.

Dry AMD, which accounts for 90% of cases, while usually considered not as dramatic, presents the spectre of progressive disability for millions of people resulting ultimately in blindness. It is now the leading cause of blindness in people over 60, and there is no treatment. About 25 per cent of this population has some degree of visual loss due to AMD, a figure that is expected to triple within the next 10-20 years. There is an urgent need for prevention and treatment strategies for age-related macular degeneration.

The cornea can also be injured or damaged due to disease. One of the most common indications for corneal transplant is blindness caused by infection with herpes simplex virus. With approximately 500,000 new cases per year in the US, it is a major concern, since transplantation depends on the availability of replacement corneas. Corneal transplants, which number about 40,000 per year in the US, have traditionally relied on retrieving a healthy cornea from a cadaver.

The cornea

These statistics convey the urgency for scientists to find new ways to generate healthy corneal tissue – for either intrinsic repair (repairing the damaged cornea from within) or extrinsic repair (transplanting a new cornea created in the lab).

Advances are coming quickly in this field, with evidence that human embryonic stem cells can generate new corneal tissue, which can then be grafted into place. Intrinsic repair, however, requires better understanding of the cell signals that would differentiate the stem cells within the eye to replace the cells that have been damaged by disease.

There are several different kinds of stem cells in the eye, each of which serve a different function. Limbal stem cells (sometimes called corneal stem cells) support the cornea and protect the eye from wear and tear by refreshing the cells on the surface of the eye. The conjunctiva, which covers the surface of the eye, is a thin layer of tissue that nourishes and lubricates the eyes. Conjunctival stem cells play a role in continually bathing the eye in tears and mucous. Both limbal and conjunctival stem cells can be grafted onto existing tissue to repair damage.

However, what about the ability to see?

The Retina

The retina is the part of the eye that receives light and discerns images, words on a page, faces. Retinal stem cells, the cells responsible for generating all of the retinal cells that are necessary for sight, are found in the thin black ring around the coloured iris, and they are among the easiest human cells to grow in a laboratory. These adult retinal stem cells are the source of photoreceptors, known as rods and cones, and of the cells that support them, known as retinal pigment epithelial cells. Once retinal stem cells have created their differentiated progeny during fetal development, retinal stem cells apparently become dormant. Therefore, unlike the blood and some organs, the retinal stem cells responsible for sight do not regenerate differentiated retinal cell types like photoreceptors in the adult. Thus, any damage had long been considered irreversible.

The discovery of stem cells in the retina by Dr Derek van der Kooy in Canada in 2000, and the subsequent progress in generating them from human embryonic stem cells, has led to realistic hopes that blindness and vision impairment from degenerative eye diseases can be reversed using stem cell therapy.

A team in Scotland recently announced a small clinical trial in Edinburgh and Glasgow that will remove the limbic stem cells from the cornea of deceased donors to grow sufficient replacement tissue in the laboratory, which will then be transplanted onto the surface of the cornea of patients suffering with chronic corneal disease.  This blends the traditional approach to treatment with stem cell technology to grow additional tissue.

Many obstacles must be overcome before the therapeutic use of stem cells to treat eye disease is routine, but researchers agree that a cure for blindness may be on the horizon.

Our thanks go to the Stem Cell Network in Canada for their work on this information