Apoptotic cell death of proliferating neuroepithelial cells in the embryonic retina is prevented by insulin

The role of programmed cell death is well established for connecting neurons. Conversely, much less is known about apoptosis affecting proliferating neuroepithelial cells. Chick retina from day 4 to day 6 of embryonic development (E), essentially proliferative, presented a defined distribution of apoptotic cells during normal in vivo development, as visualized by TdT-mediated dUTP nick end labelling (TUNEL). Insulin, expressed in the early chick embryonic retina as proinsulin, attenuated apoptosis in growth factor-deprived organotypic culture of E5 retina. This effect was demonstrated both by TUNEL and by staining of pyknotic nuclei, as well as by release of nucleosomes. Application of a 1 h [methyl-H-3]thymidine pulse in ovo at E5, followed by organotypic culture in the presence or absence of insulin, showed that this factor alone decreased the degradation of labelled DNA to nucleosomes by 40%, as well as the proportion of labelled pyknotic nuclei. Both features are a consequence of apoptosis affecting neuroepithelial cells, which were in S-phase or shortly after. In addition, when the E5 embryos were maintained in ovo after the application of [methyl-H-3]thymidine, 70% of the apoptotic retinal cells were labelled, indicating the in vivo prevalence of cell death among actively proliferating neuroepithelial cells. Apoptotic cell death is thus temporally and spatially regulated during proliferative stages of retinal neurogenesis, and embryonic proinsulin is presumably an endogenous protective factor.