Extrinsic factors as multifunctional regulators of retinal ganglion cell morphogenesis

Neurons acquire a unique cell-type dependent morphology during development that is critical for their function in a neural circuit. The process involves a neuron sending out an axon that grows in a directed fashion to its target, and the elaboration of multiple, branched dendrites. The ultimate morphology of the neuron is sculpted by factors in the environment that act directly or indirectly to influence the behavior of the growing axon and dendrites. The output neuron of the retina, the retinal ganglion cell (RGC), has served as a useful model for the identification of molecular signals that control neuronal morphogenesis, because the entire development of the neuron, from the initiation of neurites to the establishment of synapses, is accessible for experimental manipulation and visualization. In this review we discuss data which argue that the visual system uses a limited number of signals to control RGC morphogenesis, with single molecules being reused multiple times to control distinct events in axon and dendrite outgrowth.