Fig. 6

A dynamic model of layer assembly in the medulla. Outer medulla layers are established in a stepwise manner during development through a precise sequence of interactions between specific cell types. To illustrate this, the figure concentrates on the stepwise targeting of L3 lamina neuron axons within the medulla during pupal development. (a) L3 axons (green) are prevented from innervating the serpentine layer and inner medulla by adhesive (CadN-dependent) and repulsive (Sema-1a/PlexA) interactions, that serve as a barrier to further extension. MeT = medulla tangential neurons. The gray neuron represents a potential CadN expressing target of L3 axons. (b) Prior to innervating the target layer, L3 axons (light green) terminate in a proximal domain of the outer medulla shared by the growth cone of another lamina neuron (dark green). Specificity for the proximal domain is regulated by dFezf (not shown). An additional lamina neuron subclass (blue-green) terminates in a distal domain of the outer medulla. (c) (left panel) L3 growth cones undergo a stereotyped structural rearrangement that segregates them into the developing target layer. Another lamina neuron (dark green) forms an additional arborization in the distal outer medulla. These events contribute to the emergence of discrete layers. (middle and right panels) DFezf activates Netrin expression in L3 neurons, and Netrin (purple) is secreted from L3 growth cones (green) providing an M3-specific cue for R8 photoreceptor growth cones (red). The sequential targeting of L3 and R8 growth cones contributes to M3 development. (d) Within the target layer, L3 axons (green) may distinguish between appropriate (dark orange) and inappropriate (light orange) synaptic targets through specific cell recognition molecules such as Dpr and Dip proteins