Fig. 1. (A) Dorsal schematics of the E9.5-10.5 neural tube (left) and E12.5 CNS (right); rostral is towards the top and caudal is towards the bottom. In the neural tube, the dorsal midline (DM, blue) of the hindbrain has a rhomboidal shape owing to unique morphogenetic movements (Alexandre and Wassef, 2003; Awatramani et al., 2003). Choroid plexus (CP, orange) forms at or near the midline in the hindbrain (4th ventricle), diencephalon (3rd ventricle) and dorsomedial telencephalon bilaterally (lateral ventricles). Whether the diencephalic and telencephalic CP are separate or continuous was uncertain. Ovals designate the telencephalon. (B) Schematics of dorsal and medial views of E12.5 forebrain, showing the positions of sections 1-3 on the right. The medial view, adapted from in situ hybridization images (Grove et al., 1998), illustrates one telencephalic hemisphere with overlying diencephalon (gray). The dorsomedial telencephalon (DMT) consists of telencephalic CP epithelium (CPe) and cortical hem (brown). At anterior DMT levels (section 1), these bilateral structures are separated by the choroid plaque and occupy a midline position. After reaching the diencephalon (section 2), the bilateral DMT are dorsomedial rather than midline and contiguous with both telencephalic and diencephalic neuroepithelium. Once past the interventricular foramina (section 3), the DMT loses its diencephalic connection and becomes contiguous solely with other telencephalic structures. (C) Schematics of the transgenic mating strategies used in this study; see text for details. A, anterior; CP, choroid plexus; CPe, choroid plexus epithelium; DM, dorsal midline; DMT, dorsomedial telencephalon; n.e., neuroepithelium; P, posterior.