Spatial and temporal correlation between early nerve fiber growth and neuroepithelial cell death in the chick embryo retina.

The distribution of cell death in the ventral pycnotic zone of the chick embryo retina was studied in Hamburger-Hamilton's stages 16 to 25 (2 1/2 to 4 1/2 days of incubation). The number of fragments appearing in the retina increases notably from stage 20 at which stage they are limited almost exclusively to the optic disc region. At the same time optic fibers are seen in this area for the first time. In stage 24 cell death phenomena are numerous in the ventral retina, and become even more extensive in the following stage. Stage 25 meanwhile sees a drop in cell death in the dorsal retina. The overall picture presented by cell remains and young ganglion cells indicates that in stages 19-23 cell death occurs mainly in the zone between the ganglion cells of the posterior pole and the optic stalk. In the stage 25 retina most of the cell fragments of the ventral retina are found on either side of the fissure, while ganglion cells in the process of sending out axons toward the fissure appear laterally (nasally and temporally) to these zones of degeneration. Hence a spatial and temporal correlation is established between fiber growth and neuroepithelial cell degeneration, allowing us to construct a hypothesis with regard to the role that cell death might play in setting up an initial pattern of optic fiber growth.