Systems-matching by degeneration. I. A quantitative electron microscopic study of the generation and degeneration of retinal ganglion cells in the chicken.

The total number of optic nerve fibers of the chicken was determined at twenty sequential developmental stages from incubation day 5 to 104 days after hatching. It was found that the total number of optic nerve fibers increases from 4400 on incubation day 5 to about 4.0 million on incubation days 10 and 11. Thereafter, it decreases to a final value of about 2.4 million by incubation day 18 and remains constant from that time on until adulthood. Thus, 40% of optic fibers degenerate. Degenerating ganglion cells in the retina are first detectable by incubation day 9. Initially degenerating cells are located mainly in the central retina, but on subsequent days they can be found predominantly in peripheral zones. It is postulated that cell death occurs because of competition for adequate arborization space. If more retinal afferent fibers arrive than tectal termination sites are available, supernumerary fibers may degenerate. By degeneration the two systems retina and optic tectum, are matched in size.