Zona limitans intrathalamica

The zona limitans intrathalamica (ZLI) is a transverse boundary located between the prethalamus (previously also known as ventral thalamus) and the functional distinct thalamus (dorsal thalamus; Kuhlenbeck, 1930; Shimamura et al., 1995). Besides its morphological characteristics, it bears the hallmarks of a signalling centre. Fate mapping experiments in chick have shown that the ZLI is cell lineage restricted at its boundaries and therefore can be termed a true developmental compartment in the forebrain (Zeltser et al., 2001; Garcia-Lopez et al., 2004). Besides morphological characteristics, the ZLI is the only structure in the alar plate of the neural tube that expresses signaling molecules belonging to the Hedgehog (Hh; Figdor and Stern, 1993; Puelles and Rubenstein, 1993). In mouse, the function of Hh signaling at the ZLI has not been addressed directly due to a complete absence of the diencephalon in Shh mutants (Ishibashi and McMahon, 2000). Studies in chick have shown that Shh is both necessary and sufficient for thalamic gene induction in vitro Hashimoto-Torri et al., (2003), whereas Kiecker and Lumsden, (2004) have shown that Shh signaling from the ZLI is required for both prethalamic expression of Dlx2 and thalamic expression of Gbx2 and Sox14 in vivo. In zebrafish, the structure corresponding to the Hh expression has been termed as mid-diencephalic boundary (MDB; Macdonald et al., 1994, Scholpp et al., 2006) but subsequently referred to as ZLI (Barth and Wilson; 1995), in keeping with pre-existing terminology for other vertebrates. In zebrafish, it was shown that the expression of two shh genes, shh-a and shh-b (formerly described as twhh) mark the ZLI territory, and that ZLI development is accompanied by expression anteriorly of dlx2a, a marker of the prethalamus, and posteriorly of dbx1a, a marker of the thalamus (Scholpp et al., 2006). Shh signaling is sufficient for the molecular differentiation of both the prethalamus and the thalamus but is not required for their maintenance and Shh signaling from the ZLI/alar plate is sufficient for the maturation of prethalamic and thalamic territory while ventral Shh signals are dispensable. File:/Users/scholpp/Desktop/Zona limitans intrathalamica.jpg