Differential developmental strategies by Sonic hedgehog in thalamus and hypothalamus

The traditional concept of diencephalon (thalamus plus hypothalamus) and with it the entire traditional subdivision of the developing neural tube are being challenged by novel insights obtained by mapping the expression of key developmental genes. A model in which the hypothalamus is placed in the most rostral portion of the neural tube, followed caudally by a diencephalon formed by prethalamus, thalamus and pretectum has been proposed. The adult thalamus and hypothalamus are quite unlike each other in connectivity and functions. Here we review work on the role of the secreted morphogen protein Sonic hedgehog (Shh) in the developing diencephalon and hypothalamic region to show how different these two regions are also from this point of view. Shh from the prechordal plate (PCP) induces and patterns the hypothalamus but there is no evidence that this role is fulfilled by a morphogen gradient. Later, the hypothalamic primordium itself expresses Shh and a large part of the hypothalamus belongs to the Shh lineage, including the ventral domains. Neural Shh is necessary to complete the specification (lateral hypothalamus), differentiation and growth of the hypothalamus. Although Gli2A is the major effector of Shh in this region, hypothalamic specification also depends on the suppression of Gli3R by Shh secreted by the PCP as well as the neuroepithelium. The thalamus is patterned by an Shh morphogen gradient originated in the ZLI following similar mechanisms to those in the spinal cord. The thalamus itself does not belong to the Shh lineage. Gli2A is necessary for appropriate growth and specification of the thalamic nuclei, to the exception of the medial and intralaminar groups (limbic-related), whose development depends on Gli3R. Beyond specification and patterning, the scarce data available about cell sorting and aggregation in these two regions shows key differences between them as well. In summary, not only expression patterns but also developmental mechanisms support a separation of the traditional thalamus and hypothalamus into different prosomeric domains.