Morphological features of developing synapses as revealed by the E-PTA staining technique.

Synaptogenesis was studied in the optic tectum of chick embryos from the stage 40 to the hatching (last week of incubation). Our object was to characterize the morphological changes occurring in pre- and postsynaptic elements during this period of the development. Specialized interneuronal contact zones have been examined after conventional fixation procedure, or after staining of glutaraldehydefixed blocks with ethanolic phosphotungstic acid (E-PTA). Some mature synapses are seen also at the 14th incubation day (stage 40), and they increase in number to the hatching. In addition, immature synapses are seen in which presynaptic elements seem to be the less differentiated. Also the study of the evolution of the PTA-stainable paramembranous densities at the synaptic sites confirms these observations. These findings suggest a considerable prenatal synaptogenesis, as already demonstrated in various brain areas of the chick embryo, and also that the maturation of the postsynaptic site may precede and determine that of the presynaptic contact and vesicle accumulation.