Fig. 3. Differential Hes1 expression between compartments and boundaries. (A) Lateral view of the developing central nervous system (CNS) in the mouse at E10.5. The CNS is partitioned into many compartments by boundaries, such as the zona limitans intrathalamica (Zli) and the isthmus. These boundaries and the roof and floor plates function as organizing centers by expressing Shh (pink), Wnt1 (green) or Fgf8 (purple). Cells in these compartments undergo active proliferation and neurogenesis, whereas those in boundaries undergo slower proliferation and no neurogenesis. Pth, prethalamus; Th, thalamus. (B) The Hes1 expression mode is different between compartments and boundaries: (Ba) variable expression (could be oscillatory) in compartments and (Bb) persistently high expression in boundaries. In compartments, when Hes1 levels are low, Mash1 levels are high, and vice versa. These cells finally lose Hes1 expression and differentiate into neurons. By contrast, in boundaries, Hes1 is persistently expressed at high levels, and neurogenesis is inhibited. This difference in the Hes1 expression modes may confer compartment versus boundary characteristics.