Development of neuronal connections in chick embryonic retino-tectal system: an overview

Synaptogenesis has been examined in the retino-tectal system of the chick embryo both biochemically and morphologically. We have evaluated the amount and the rate transport of glycoproteins synthetized in retinal ganglion cells and axonally transported to retinal nerve endings. The distribution of glycoproteins inside the tectum shows that they are part of the synaptosomal membrane either because they are incorporated in it "en route" or because they flow through an axolemmal flow. The role they might play in the building up of the synaptic membrane is discussed in relation to the maturation of synaptic contacts, observed either with E-PTA staining method or with routine fixation methods. The distribution of a carbohydrate binding protein was followed with histochemical techniques. This protein is synthesized by the matching tectal neurons, and its synthesis is developmentally regulated. It distribution is affected by innervation. All of these findings are discussed in relation to the hypothesis that three different steps are involved in embryonic synaptogenesis. These three steps are: 1) interneuronal recognition; 2) formation of initial contact, and 3) formation of synapses. The possibility that different kinds of cell surface macromolecules might play a different role is also discussed.