Retina Transplant Difficulties

Question

Why is retina transplant not as easy as the normal eye donation and transplant (I think the latter involves the cornea ) ?
This says that a new method has come up but why isnt the process similar and simple as the normal transplant ?
What are the difficulties in retina transplant ?

Answer 1

This is a diagram of a cross-section of the cornea:

It is an amazing but relatively simple structure, the shape of which is responsible for about 60% of our focusing power, and the clarity of which allows light to enter. It is avascular (no blood vessels), and the non-mylenated nerve endings are very tiny and present in the epithelium.

Transplanting this relatively simple tissue is easy (well, not do-it-at-home easy, but easy.) The only problem with cutting all those nerve endings is that the new cornea will feel nothing if there's dust in the eye, etc. This is a problem, but nothing compared to blindness.

This is a diagram of a cross section of the retina:

As you can see, it is not nearly as simple as the cornea. The nerves (to the left: yellow, light blue, medium blue and darker blue), which collect information from the rods and cones (pink and purple) are very numerous, and absolutely vital to the ability of the brain to interpreting information from these photoreceptors. Therefore, a cut piece of retina immediately loses the ability to send any information to the brain. To transplant it into an eye with a damaged retina would do nothing to improve vision. Also, it is not easy surgery. It's very difficult.

What has been called "retinal transplants" in the recent past have been tiny pieces of embryonic retina, about...

4 millimetre square of retinal tissue, complete with retinal progenitor cells and the retinal pigment epithelium that nourishes them. The tissues were placed in the sub-retinal space beneath the fovea, the area of the retina responsible for sharp central vision.

Note, they didn't try to connect nerves or blood vessels. And, it came from an embyro (so the supply is a problem.) The improvement in vision - which was significant in 7 out of 8 recipients (one patient going from 20/800 to 20/160) - is not long lasting, and believed to have been not from the transplanted retina connecting to the existing retina, but from the release of growth factors from the embryonic tissue improving the health of the degenerating retina.

The newest in "retinal transplants" (which might be better called stem-cell-supplied growth factors implanted in the retina) actually derives from the patient's own stem cells:

A Japanese woman in her 70s is the first person to receive tissue derived from induced pluripotent stem cells, a technology that has created great expectations since it could offer the same regenerative potential as embryo-derived cells but without some of the ethical and safety concerns. [No news yet on whether it worked.]

In a two-hour procedure... a team of three eye specialists lead by Yasuo Kurimoto of the Kobe City Medical Center General Hospital implanted a 1.3 by 3.0 millimetre sheet of retinal pigment epithelium cells into an eye of the Hyogo prefecture resident, who suffers from age-related macular degeneration, a common eye condition that can lead to blindness.

TL;DR: It's not as easy as taking cadaver retina and replacing a patients retina (that would result in immediate and permanent blindness). Whereas corneal transplantation is very easy.

_{Corneal Innervation and Cellular Changes after Corneal Transplantation: An In Vivo Confocal Microscopy Study
Retinal transplants see fleeting success
Japanese woman is first recipient of next-generation stem cells}