Their task was to set up computer models of evolving eyes to answer two questions. In their computer models, Nilsson and Pelger made no attempt to simulate the internal workings of cells. It would be nice, in the future, to do another computer model, this time at the level of the inside of the cell. But you have to start somewhere, and Nilsson and Pelger started after the invention of the photocell. Skin is a tissue, so is the lining of the intestine, so is muscle and liver.
Tissues can change in various ways under the influence of random mutation.
Sheets of tissue can become larger or smaller in area. They can become thicker or thinner. The beauty of simulating an eye, as distinct from, say, the leg of a running cheetah, is that its efficiency can be easily mea-optics. It would be much harder to come up with an equivalent numerical expression for the efficacy of a cheetah's leg or backbone. Nilsson and Pelger began with a flat retina atop a flat pigment layer and surmounted by a flat, protective transparent layer.