CS17 OPT model showing 3D gene expression domains. (a) Digital sagittal section through the CS17 OPT model, with the GAP43 gene expression domain shown in red. The plane of this sagittal section is shown by a line on the corresponding transverse section in (b). The GAP43-negative region in the hindbrain floor plate is shown on both sections by an arrow. (c) The same digital sagittal section, with PAX6 gene expression displayed in green. (d) High power images of GAP43 and PAX6 expression near the diencephalon/midbrain boundary. The upper two panels correspond to the rostral midbrain, where there is no expression of GAP43 or PAX6. The lower two panels correspond to the caudal diencephalon, in the region of the posterior commissure. Here both genes are expressed (brown chromagen).