Quantitative study of apoptotic cells in the goldfish retina

In the vertebrate retina, it has been shown that a large number of neurons produced at the onset of neurogenesis die early in development. Since this apoptotic cell death occurs in a short, limited time, little is known in detail on its histocytology. Using the terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling (TUNEL) method, we investigated the apoptotic cells in the adult goldfish retina in which the progenitor cell keeps proliferating. Most of the TUNEL-positive nuclei were found in the marginal retina, about 40-200 microm from the circular blood vessel (CBV) running parallel to the retinal rim. The peak density was about 1400/mm(2) surface area at 90-130 microm from the CBV. Overall retinal cell density in the marginal retina was also studied by toluidine blue staining. The retinal cell density increased toward the central retina, reaching a peak density of 278,000/mm(2)at 130 microm from the CBV. It then decreased gradually to 228,000/mm(2) in the mature retina. This 18% reduction seemed to be caused by apoptosis. We also found a significant increase in the mean density of apoptotic cells in goldfish sacrificed at midnight, 403/mm(2), compared to that at midday, 217/mm(2). This increase was not seen in goldfish kept under either constant darkness or constant light.