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Updated: Jul 17, 2026
In Ovo Electroporation in Embryonic Chick Retina
Published on: February 5, 2012
Onset of synaptogenesis in the plexiform layers of the chick retina: a transmission electron microscopic study
Ulrich Drenhaus1, Tilman Voigt, Günter Rager
1Department of Medicine, Division of Anatomy, University of Fribourg, Fribourg, Switzerland.
Insights
Synaptogenesis in chick retinas begins earlier than previously thought, starting with electrical synapses on embryonic day 7 and chemical synapses by embryonic day 8. This early development aligns with neuronal maturation and electrical activity onset.
Area of Science:
- Neuroscience
- Developmental Biology
- Ophthalmology
Background:
- Current understanding places chick retina synaptogenesis onset at embryonic day 12 (E12).
- This contrasts with earlier observations of electrical activity, presynaptic proteins, and neurotransmitters during inner (E6-E8) and outer (E9) plexiform layer formation.
- A discrepancy exists regarding the precise timing and initial forms of synaptic development in the chick retina.
Purpose of the Study:
- To investigate the precise onset and initial characteristics of synaptogenesis in the developing chick retina.
- To reconcile the timing of synaptic formation with the emergence of retinal layers and early neuronal activity.
- To characterize the types and distribution of the earliest synapses in the chick retina.
Main Methods:
- Transmission electron microscopy (TEM) was employed to examine chick retinas from embryonic day 6 (E6) to E12.
- High-resolution imaging allowed for the identification and characterization of nascent synaptic structures.
- Age-specific analysis focused on the emergence and development of both electrical and chemical synapses.
Main Results:
- Synaptogenesis in the chick retina commences shortly after the formation of the inner and outer plexiform layers.
- The first observed synapses are electrical, appearing on E7 in the inner plexiform layer and by the end of E8 in the outer plexiform layer.
- Conventional chemical synapses emerge in both plexiform layers on E8, with a slight temporal precedence in the inner plexiform layer.
Conclusions:
- Synaptogenesis in the chick retina begins earlier than previously established, initiated by electrical synapses.
- The timing of initial synaptogenesis correlates with the chronological maturation of retinal neurons and the onset of spontaneous electrical activity.
- These findings provide a refined timeline for synaptic development in the vertebrate retina.
Abstract:
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.
