Researchers are making progress in the tissue engineering of components of the eye, an area of development that is less hindered by the challenge of producing blood vessel networks than is the case for most other tissue types:
Discs made of multiple types of eye tissue have been grown from human stem cells - and that tissue has been used to restore sight in rabbits. The work suggests that induced pluripotent stem (iPS) cells - stem cells generated from adult cells - could one day be harnessed to provide replacement corneal or lens tissue for human eyes. The discs also could be used to study how eye tissue and congenital eye diseases develop. A second, unrelated paper describes a surgical procedure that activates the body's own stem cells to regenerate a clear, functioning lens in the eyes of babies born with cataracts.
In the first study, a team cultivated human iPS cells to produce discs that contained several types of eye tissue. The cells grew in distinct regions so that researchers could extract and purify specific types, including those found in the cornea, retina and lens. The team was able to remove cells from one region of a disc to grow sheets of corneal epithelium that the researchers then successfully transplanted into rabbits with defective corneas. Previous studies have generated retinal or corneal tissue using iPS cells, but none has produced such different types of eye cell in a single experiment. Cells made from the recipient's own cells using the disc method could one day supply tissue to repair damaged eyes without the threat of rejection by the immune system. But much remains to be done before any such therapy could be tested in humans.
By contrast, the cataract paper could have an almost immediate impact on treatment. The technique described does not involve culturing cells outside the body or transplanting material that would require regulatory approval. "This is just a change in a surgical procedure. They are not putting in an artificial lens: they are just letting the lens regrow." The research was inspired by a typical side effect of implanting artificial lenses to treat cataracts: the new lenses often become cloudy as the recipient's own cells grow over them. A team decided to find out whether this regrowth signalled that the body is capable of regenerating an entire lens. The scientists began a series of animal studies to assess whether lens epithelial stem/progenitor cells (LECs) that exist naturally in a fully formed mammalian eye can produce a new lens. Encouraged by the results, the team developed a surgical technique and tested it in rabbits, macaque monkeys and, finally, 12 human infants.
In the new method, surgeons slice a 1.5-millimetre opening in the side of the lens capsule to remove the diseased lens, prompting the eye's LECs to grow a new one. In initial tests, this approach produced a much lower rate of complications - 17% - than the 92% seen after typical cataract surgery. And the lenses generated did not grow opaque as artificial lenses tend to do. The first infant treated using the method underwent surgery two years ago and still has good vision.