Figure 7.

Development of neural superposition axon projection pattern as observed in Drosophila. (A) Retinula cell axons arrive in the optic lobe during a temporal wave in the wake of photoreceptor differentiation in the developing eye disc (Tomlinson & Ready, 1987; Wolff & Ready, 1991, 1993). The intermediate target for retinula axons are two layers of glial cells in the optic lobe. (B) The arrival of retinula axon bundles is followed by a lateral growth cone sorting process. The growth cones form a new layer perpendicular to the axons, between the layers of glia cells, called the lamina plexus. Sorting the correct R1–R6 growth cones with the same field of view from six different ommatidia predetermines synaptic partners (Clandinin & Zipursky, 2002; Hiesinger et al., 2006). (C) After growth cone sorting into cartridges that receive input from the same field of view, each retinula terminal elongates proximally for up to 30 μm (Meinertzhagen & Hanson, 1993). (D) Lastly, synapses form between the postsynaptic lamina monopolar cells (red) and the presynaptic retinula cell columns (blue), obeying a minimal spacing rule (Meinertzhagen & Hu, 1996).