Since years, patterning and function of some brain parts such as the cortex in the forebrain and the optical tectum or cerebellum in the midbrain/hindbrain region are under strong investigation. Interestingly the diencephalon located in the caudal forebrain has been ignored for decades. Consequently, the existing knowledge from the development of this region to function in the mature brain is very fragmented. The central part of the diencephalon is the thalamus. This central relay station plays a crucial role in distributing incoming sensory information to appropriate regions of the cortex. The thalamus develops in the posterior part of the embryonic forebrain, where early cell fate decisions are controlled by local signaling centers.

In this Research Topic we discuss recent achievements elucidating thalamic neurogenesis - from neural progenitor cells to highly specialized neurons with cortical target cells in great distance. In parallel, we highlight developmental aspects leading from the early thalamic anlage to the late the organization of the complex relay station of the brain.

First we will address the very early events in neural plate patterning which leads to the subdivision in forebrain, midbrain and hindbrain primordia. This is followed by the specification of the diencephalon. One main aspect of the issue will be the induction and specification of the thalamic anlage. Patterning within elaborate brain regions, such as the neocortex or the cerebellum, is known to require instructive cell populations – ‘local organizers’. The work of several labs has identified a similar organizing structure within the thalamus - the mid-diencephalic organizer (MDO). Organizers are located at prominent morphological discontinuities or boundaries in the neural primordium. Indeed, the MDO is localized at the zona limitans intrathalamica – the border between the prethalamus (formerly known as ventral thalamus) and the thalamus (formerly known as dorsal thalamus). Organizers are needed to establish concentration gradients of morphogenetic signal molecules in adjacent responsive tissues. The most prominent of the organizer’s signals, Sonic hedgehog, is necessary for conferring regional identity on the prethalamus and thalamus and for patterning their differentiation. Several articles will focus on different aspects of the induction and function of the MDO in zebrafish, chicken and mouse. Recent advances have been made to understand the function of other major signaling pathways here the Fgf pathway and the canonical Wnt / ß-catenin pathway. Similarly, the MDO is also a potent source for Fgf ligands and canonical Wnt ligands. We will elucidate the function of these signaling pathways and show that these pathways are required to establish integrity of the tissue. A further aspect will be the influence of the embryonic roof plate on thalamus development. This aspect has been completely overlooked in the last years. After patterning and specification, the thalamus becomes parcellated into several nuclei – independent functional units, which are specialized on transmitting information from a specific sensory organ to areas in the cortex. How these cells cluster form these entities will be discussed in several articles. Then, we will address the question how do neurons from a thalamic nucleus find their correct target area in the cortex? The area of the formation of the major nerve bundles the thalamo-cortical connection is under investigation from several labs. We will elucidate this in detail with the focus on intrinsic cues in thalamic neurons, but also on extrinsic cues released from tissues through which the axons have to navigate. In the last part of the issue we will add two articles, which will discuss similarities and differences within thalamic development across species. We feel that a comparative summary of the issue will have a great benefit as it will bundle common genetic and morp

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