Thalamic neuronal specification and early circuit formation.

The thalamus is a central structure of the brain, primarily recognized for the relay of incoming sensory and motor information to the cerebral cortex but also key in high order intracortical communication. It consists of glutamatergic projection neurons organized in several distinct nuclei, each having a stereotype connectivity pattern and functional roles. In the adult, these nuclei can be appreciated by architectural boundaries, although their developmental origin and specification is only recently beginning to be revealed. Here, we summarize the current knowledge on the specification of the distinct thalamic neurons and nuclei, starting from early embryonic patterning until the postnatal days when active sensory experience is initiated and the overall system connectivity is already established. We also include an overview of the guidance processes important for establishing thalamocortical connections, with emphasis on the early topographical specification. The extensively studied thalamocortical axon branching in the cortex is briefly mentioned; however, the maturation and plasticity of this connection are beyond the scope of this review. In separate chapters, additional mechanisms and/or features that influence the specification and development of thalamic neurons and their circuits are also discussed. Finally, an outlook of future directions is given.