(from www.thalamus.wustl.edu/course/eye3.gif)There are always minor local repairs, for example, the rhodopsin-containing discs in the photoreceptors are replaced as needed. High school biology did teach us that there are two types of photoreceptors, the cones for day and color vision, and the rods for night vision. The cones reside in the central retina in the posterior pole (i.e., the macula). The rods, in the rest of the retina. In terms of visual acuity, the central part, i.e., the fovea, usually achieves 20/20; out in the periphery, around 20/200.
If the rods are wiped out, as in retinitis pigmentosa, then you have only the central part working, like looking through a small tube (i.e., tunnel vision). This is a true case of night blindness. And if the macula is lost, then you end up with loss of the central field, known as the central scotoma. When you look at a person's face, the nose area is now missing. Loss of different parts of the retina from, e.g., glaucoma or diabetes, then you'll have corresponding loss of the visual fields. Remember this is part of the central nervous system, left is right and up is down. A loss of the upper retina, you lose the lower visual fields.
These are generally what happened with wear and tear. And the loss of retina/visual fields is permanent. Attempts to transplant the retina are not ready for prime time just yet, if ever. Transplant of retinal cells, stem cells, or in fact gene therapy may hold more promise.
You can see why mapping the visual fields is so important in managing retinal diseases. And this is also where low-vision care comes in, so that some sight can be restored.