Spatio-temporal patterns of retinal ganglion cell death during Xenopus development

During development of the retina in mammals and birds, most retinal ganglion cells (RGC) that are produced are eliminated later in development by cell death. In lower vertebrates, however, such massive cell death has not been observed; total ganglion cell number increases linearly during most of development. Using 3H-thymidine or 5-bromodeoxyuridine labeling of retinal cell nuclei, we have been able to identify postmitotic RGC populations in Xenopus central retina at different developmental stages and follow their fate during development to postmetamorphic stages. RGC populations that become postmitotic between embryonic stages 32 and 49, during the initial stages of retinal growth, lose 40-77% of their cells during metamorphosis (approximately 4,000-5,000 cells). Twenty percent of the RGC present at stage 54, which later disappear, represent the same population of dying RGC that were present at stage 49. This suggests that the ganglion cells that became postmitotic between stage 49 and 53/54 show no apparent decline in numbers during metamorphosis. Since thyroxine is known to stimulate an increase in RGC number as well as the extent of fiber projection on the tectum, we suggest that this reduction in RGC numbers is not due to thyroxine-induced neuronal cell death. After stage 54, however, binocular vision develops in Xenopus (Keating, '74) and ipsilateral fibers begin to grow into thalamic visual neuropils (Hoskins and Grobstein, '85). We suggest, therefore, that as in mammals, in which RGC elimination correlates with binocular segregation of contralateral and ipsilateral retinal axons in visual centers, a similar process may occur in the frog among those RGC projecting to thalamic visual neuropils.