Figure 7.

Diagrams comparing how the domain of expression of Nkx2.2 (in blue) relates to the alar-basal boundary (red line) in the updated prosomeric model (A) and the columnar model (B). The neuromeres are marked for reference in (A), as well as the dorsal/middle/ventral diencephalic limiting sulci (ds, ms, vs) in (B). Note the transverse ZLI spike of the Nkx2.2 domain that separates thalamus and prethalamus is a secondary feature, due to the induction of this gene adjacent to the border of Shh expression, which is ectopically activated at the core of the ZLI. At neural plate and early neural tube stages, the expression band is strictly longitudinal (marked by dashes in A). In the columnar model (B), the correspondence of the boundary with the gene band is disrupted at the arbitrary deviation of the former into the telencephalon. Moreover, note this model cannot explain why the gene band extends into the hypothalamus, cutting it into two halves, which cannot be understood as alar and basal parts of the hypothalamus, as in (A) (question mark in B).