Flies with exogenous Runt expression in R1–R6 have disorganized lamina cartridges, but the axons of R1–R6 have terminals with features characteristic of those in the wild type. A, Wild-type organization of a lamina cartridge in UAS-HRP::CD2/+; MT14-GAL4/+ flies. A cartridge contains photoreceptor terminals (green) that originate from R1–R6 in neighboring ommatidia, which innervate five lamina neurons (L1–L5) including the axons of L1 and L2 (L) that lie at the core of the cartridge. All profiles are enveloped by a sheath of three epithelial glia (blue); additional details are in the study by Meinertzhagen and O'Neil (1991). Axons from R7 and R8 (R7/8, green), which terminate in the medulla, extend along the outside of the cartridge but are not synaptic in the lamina. B, Aberrant cartridges in UAS-runt/UAS-CD2::HRP; MT14-GAL4/+ flies are sets of photoreceptors and lamina neurons ensheathed in glia. Each contains variable numbers (<6) of expanded photoreceptor profiles populated with synaptic organelles, and also the unexpanded profiles of en passant photoreceptor axons. C–F, Consecutive EM sections of a wild-type tetrad synapse, with presynaptic T-bar ribbon (arrow) in the photoreceptor (R) and four postsynaptic partners (labeled 1–4). G–J, Photoreceptors in UAS-runt/+; MT14-GAL4/+ flies resemble wild-type terminals of R1–R6 by having capitate projections (*), and in containing synaptic vesicles (open arrowhead), and the presynaptic sites of tetrad synapses (G–I, arrows). G, R1–R6 synapse onto target cells with the cisternae and whiskers (arrowhead) characteristic of postsynaptic L1/L2 cells. H, I, Consecutive 60 nm sections through a synapse. Mutant synapses have four postsynaptic partners (labeled 1, 2, 3, 4) and thus formally are tetrads. J, Forming possible feedback synapses (arrow), other neurons are presynaptic to photoreceptors. Scale bars: A, B, 1 μm; G, 0.2 μm; I, J, 0.5 μm.