I've Been Thunderstruck

Neurons are the nerver cells that are the major constituent of the brain. At birth the brain has the maximum number of neurons – 20 billion to 200 billions neurons. Thousands are lost daily, never to be replaced, and apparently not missed, until the cumulative loss builds up in very old age.

The eye’s retina is part of the brain, and the retina’s cells are derived from brain tissue. Retinal cells and the cells that connect them to the brain are the least amenable to being manipulated outside the body. That is why we cannot currently do eye transplants.

[Source : The Handy Science Answer Book]