Fig. 3

BC Axons Form Presynaptic Structures without Postsynaptic Neurons

(A) WT ath5:GAP-RFP retinas stained with anti-HuC/D (to label ACs/RGCs) and anti-RibeyeA (to label ribbon synapses). Obvious punctate RibeyeA staining is visible in the IPL, whereas weak staining is seen in the BC cell bodies in the apical half of the INL and the OPL.

(B) When ACs and RGCs are absent in the ath5:GAP-RFP;ath5^−/−;ptf1a^−/−;ptf1aMO embryos, the weakly RibeyeA-staining BC cell bodies span the entire INL, and punctate RibeyeA staining is visible in the BC-only IPL. Scale bar in (A) and (B), 10 μm.

(C) Examples of vesicle-filled bouton-like structures (asterisks) containing ribbons near the retinal basal lamina of animals with severely reduced numbers of ACs and RGCs. Left: Higher magnification of the two such structures, presumed to be BC axonal boutons (arrowhead) shown in the inset. Right: Three consecutive sections (s1–s3) of processes in basally located neuropil in another animal. Arrows indicate ribbons juxtaposed to appositions with other processes, one of which can be another BC terminal (s1–s3).

(D and E) WT BC synapses exhibit readily apparent postsynaptic densities (thickenings, arrow) that are not seen in the mutant/morphant plexus (s1–s3).