www.jcb.org/cgi/doi/10.1083/jcb.201307020 JCB 11 JCB: Review

synapses are highly specialized, asymmetric inter cellular junction structures that are the basic units of neuronal communication. Proper development of synapses determines appropriate connectivity for the assembly of functional neuro nal circuits. Synaptic circuits arise during development through a series of intricate steps (Waites et al., 2005; McAllister, 2007; Jin and Garner, 2008). First, spatiotemporal cues guide axons through complex cellular environments to contact their appro priate postsynaptic targets. At their destination, synapse formation is specified and initiated through adhesive interactions between synaptic partner cells or by local diffusible signaling molecules. Stabilization of intercellular contacts and assembly into func tional synapses involves cytoskeletal rearrangements, aggregation, and insertion of pre and postsynaptic components at nascent synaptic sites. Maturation and modulation of these newl