Strategies for assembling columns and layers in the Drosophila visual system

The Drosophila visual system. (a) Anatomy of the Drosophila visual system (Adapted from Fischbach and Diettrich 1989). (b) Diagram illustrating the modular organization of the Drosophila visual system. Four topographically matched modules from the retina and each region of the optic lobe are shown. Ommatidia (retina), cartridge (lamina), column (medulla), lobula complex modules (lobula and lobula plate). (c) Illustration of a cross section through a lamina cartridge. The axons of R1-R6 photoreceptors synapse onto the dendrites of L1-L3 lamina neurons. The R cell axons form a ring around the dendrites, establishing a cylindrical structure that may optimize wiring efficiency. (d) R cell axons form tetrad synapses. At each R cell synapse, input is provided to four postsynaptic elements. L1 and L2 are present at every R cell synapse, but the other two components are variable and can include L3, amacrine (Am) or glial (not shown) processe