Onset of synaptogenesis in the plexiform layers of the chick retina: a transmission electron microscopic study.

The presently acknowledged onset of synaptogenesis in the chick retina from embryonic day 12 (E12) onward stands in contrast with the appearance of spontaneous electrical activity, of presynaptic proteins, or of neurotransmitters during early formation of the inner (E6-E8) and outer (E9) plexiform layers. Therefore, we investigated the chick retina from E6 to E12 at which age first synapses appear by transmission electron microscopy (TEM). The study provides evidence that synaptogenesis in the chick retina begins shortly after the plexiform layers have started to emerge. The first synapses are electrical synapses, which appear on E7, one day after the future inner plexiform layer emerged, and towards the end of E8 in the nascent outer plexiform layer. Conventional chemical synapses appear in both plexiform layers on E8, in the inner plexiform layer (stage 34) only a few hours earlier than in the outer plexiform layer (stage 35). The first synapses are formed close to the apex of the optic fissure and their frequency increases rapidly with age. The onset, the topography, and the developmental course of synaptogenesis correlate with the chronotopic course of maturation of retinal neurons and the age when spontaneous electrical activity occurs in the retina.