Emergence of clustered synapses during the development of a nervous system

Background: Synaptic organization is central for proper transmission of neural information. Studies in invertebrates and mammalian cortices show that synapses are clustered along neurite extensions, an organization that promotes key functional roles. Results: Here we studied how these synaptic clusters emerge during the development of a nervous system. Leveraging the available C. elegans connectomes that span all larval developmental stages, we show that clustered synapses are formed at the early stages of the neural network development and that their occurrence further increases throughout development. These synaptic clusters significantly constitute small neural circuits that endow the network with important functional roles, such as feedback between mutually synapsing neurons and information transfer in mutually regulated neurons. Moreover, clustered synapses emerge early on during the development of the head motor system, where they facilitate the crucial 3D head swings. Finally, the synaptic clusters within these key neural circuits are maintained throughout all developmental stages and are robustly found across different individuals, further accentuating their central functional roles in neural networks. Conclusions: Clustered synaptic structures emerge early on during the development of the neural network and are consistently observed among individuals. They appear significantly in motor circuits, possibly contributing to their function.