Differential expression of Crb1, Crb2a and Crb2b in the diencephalon and the optic vesicle during early development of the CNS. A-C. At 10-somite stage (-ss), only Crb2a (B) was detectable at the apical surfaces of the optic vesicle (B, arrowheads) and the diencephalon epithelium (arrows), with weaker Crb2a expression in the dorsal diencephalon neuroepithelium than in the ventral region (B, arrow). By contrast, Crb1 (A) and Crb2b (C) expressions were not detectable. D-F. At 15-ss, Crb1 expression was detectable at the ventral diencephalon (D, arrow), and Crb2a expression was detectable throughout the apical surfaces of the diencephalon (arrows) and the optic vesicle (E, arrowheads). Weak Crb2b expression was detected at ventral diencephalon (F, arrow). G-I. At 20-ss, the Crb1 expression extended to the entire apical surface of the diencephalon epithelium (G; arrows), Crb2a expression persisted in the retina (H, arrowheads) and the diencephalon (H, arrows). Crb2b expression remained restricted to the most ventral region of the diencephalon (I, arrow). The ventral diencephalon (VD); dorsal diencephalon (DD); optic vesicle (optic v.); and the retina (R) are marked in panel A and G. Scale bars, 50 ! m.