Abstract
Abstract: :
Purpose: Apoptosis in retinas of warm–blooded animals occurs in three stages during development. The third of these stages coincides with the synaptogenesis between GC axons and their targets, i.e., with the time when electrophysiological activity becomes important. In turtle retina, we have shown that emergence of amacrine synapses are necessary for triggering the appearance of spontaneous waves of activity. This occurs at Stage 22 (S22) of embryogeny. In turn, emergence of ganglion–cell light responses need the maturation of both photoreceptors and ribbon synapses at S23. The aim of this work is to establish the temporal relationship between apoptosis, synaptogenesis, and electrophysiological activity in turtle retinas. Methods: Retinas from Trachemys scripta elegans embryos of S15, 16, 18, 19, 20, 22, 24, and 26, were fixed in 4% paraformaldehyde for TUNEL technique. We prepared other retinas from S18, 20, 22 and 24 for transmission electron microscopy (TEM). We fixed them with 1% paraformaldehyde and 1.6% glutaraldehyde. Results: TUNEL–positive cells (TUNEL+) were observed from S16 to S24 in both INL and GCL, with peak densities occurring at S19 and S20, and vanishing by hatching (S26). At all stages, the TUNEL+ density was greater in central retinal areas than in peripheral ones. Electron micrographs gave similar density results, showing four signs of cell apoptosis: First, there was early nucleolar disorganization (S18 and S20), with amorphous and homogeneous structure. Second, we observed different patterns of nuclear chromatin distribution. Third, there were masses of ribonucleoprotein–like material (RNP) dispersed in the cytoplasm at S23 and S24. Fourth, necrotic cytoplasmic areas appeared, specially at S18–S22. Conclusions: We propose that the surge of apoptosis at S19 and S20 is a remodeling phenomenon necessary for normal retinal synaptogenesis in the IPL. Hence, this surge may be necessary for the emergence of normal electrophysiological activity. In turn, electron–microscopic data suggest that in turtle retinas, apoptosis is a complex, multi–stage process, involving both the cytoplasm and the cell nucleus. In this context, the masses of RNP–like material dispersed in the cytoplasm may correspond to heterogeneous ectopic RNP–derived structures (HERDS). These structures may be morphological signs of transcriptional arrest observed in early embryonic development.
Keywords: apoptosis/cell death • retinal development • synapse