We use the Drosophila visual neurons as a model to study dendritic development in the central nervous system. Like vertebrate cortex and retina, the medulla neuropil in the optic lobe is organized in columns and layers, suggesting that the fly medulla neurons and vertebrate cortex neurons confront similar challenges in routing their dendrites to specific layers and columns during development. In addition, the fly visual system has several unique advantages: (i) the medulla neurons extend dendritic arbors in a lattice-like structure, facilitating morphometric analysis;(ii) the presynaptic targets for many medulla neuron types have been identified from our anatomical studies;(iii) genetic tools for labeling specific classes of medulla neurons and determining their connectivity have been developed in our previous studies. We have developed novel techniques to analyze dendritic structures in 3D and to exploit the unique advantages of this system. First, to image reliably the slender dendrites of medulla neurons, we developed a dual imaging technique that generates isotropic 3D-images of dendrites by combining two confocal image stacks collected in orthogonal directions. Second, we developed an image registration technique that makes uses of the regular array structures of the optic lobe to standardize dendritic branching patterns. This, in combination with a series of statistical methods we established, allows us to analyze dendritic patterns in three-dimension space. Third, we have established an imaging technique (GRASP) to detect bona fide synaptic connections at the light-microscopic level. Using these techniques, we analyzed the dendritic morphologies of three types of medulla neurons, Tm2, Tm9 and Tm20. Tm9 and Tm20 receive input from R8 photoreceptors in their cognate columns and mediate color vision while Tm2 receives inputs from the achromatic R1-6 via L2 and L4 in its cognate columns and mediates motion detection. Morphometric analyses revealed (i) that the medulla neurons exhibit stereotypic dendritic arbors but the detailed branching pattern and topology are not conserved in each class;(ii) that the synaptic partnership between axons and dendrites are robust and specific;(iii) that the layer-specific routing and polarized extension of dendrites are two most critical determinants of type-specific dendritic patterns;(iv) that the dendritic arbors of these Tm neurons are largely confined to single medulla columns, consistent with their functions in processing retinotopic information. Based on these results, we hypothesize that dendritic development in the optic lobe neurons proceeds in two distinct processes: (i) routing dendrites in type-specific fashion, which, at least in part, serves to maximize the possibility of finding appropriate synaptic partners;(ii) matching different sections of dendrites with specific afferents, which likely requires specific interactions between axons and dendrites to ensure synaptic specificity. Using this approach, we uncovered the mechanism by which photoreceptor afferents directly regulate a key dendritic attribute of their target neurons, namely, the size of their receptive field. We demonstrated that TGF-beta/Activin derived from photoreceptors R7 and R8 selectively act on their respective postsynaptic targets, the Dm8 and Tm20 neurons, to promote dendritic termination and to restrict the dendritic fields to the appropriate size. We show that mutant Tm20 neurons devoid of Activin signaling lost their one-to-one correspondence with the R8s and formed aberrant synapses with neighboring photoreceptors. Given that stablishing a 3D dendritic field that matches an appropriate number of afferents is a general neural wiring problem, our findings are likely applicable to other systems, including vertebrates'.
