Diversity of thalamic progenitor cells and postmitotic neurons.

The vertebrate thalamus contains multiple sensory nuclei, and relays sensory information to corresponding cortical areas. Moreover, the thalamus actively regulates information transmission to the cortex by modulating the response magnitude, firing mode and synchrony of neurons according to behavioral demands. The thalamus serves many other functions including motor control, learning and memory, and emotion. Such functional importance of the thalamus necessitates a better understanding of its developmental mechanisms. In this review, we will first describe the morphological organization of the developing thalamus. We will then discuss how neuronal diversity is generated and nuclei are formed during thalamic development. The first step in generating neuronal diversity is the formation of spatial diversity of thalamic progenitor cells, which is controlled by locally-expressed signaling molecules such as Sonic hedgehog (Shh), Wnt proteins and Fgf8. Lastly we will describe the roles of several transcription factors in specification of neuronal identity and nuclei formation in the thalamus. Our review will provide a molecular perspective for the organization of the thalamus prior to thalamus-cortex circuit formation.