The cells of the retina of chick embryo at the 4th day of incubation were isolated by the trypsin disgregation tecnique. These cells were let reaggregate into spherules (about 200 $\mu$ diam.) and then cultivated in vitro for up to 25 days. By means of the electron-microscope the synapses were studied. The result is that the synaptic ribbons of photoreceptors and of the bipolar cells are specific structures, intrinsically determined in the early embryo, related to the modality of release of the chemical mediator, independently of any reconstruction of the specific synaptic model, termed dyad by Dowling (1966). On the contrary, the topographic orientation of the ribbons, along the bisetrix of the diedric prominence of the presynaptic terminal, appears to be related to the normal organization, as dyads, of the receiving buds (horizontal and bipolar cells or amacrine and gangliar cells). In fact in the reaggregated spherules a dyad model is never achieved and the ribbons are randomly distributed and frequently far from the cellular membrane. This fact means also that in the reaggregated spherules specific orientative forces on nerve terminals are absent.
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Con il contributo del Ministero per i Beni e le Attività Culturali