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 µm from the circular blood vessel (CBV) running parallel to the retinal rim. The peak density was about 1400/mm2 surface area at 90–130 µm 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/mm2at 130 µm from the CBV. It then decreased gradually to 228,000/mm2 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/mm2, compared to that at midday, 217/mm2. This increase was not seen in goldfish kept under either constant darkness or constant light.
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