Fig. 3. Fate change of the metencephalon to the mesencephalon by Ras^S17N misexpression. (A) Repression of ERK activity by misexpression of Ras^S17N. (B-D) Views from the dorsal (B), left (C) and right (D) of the E10.5 chick brain after misexpression of Ras^S17N. The tectum differentiated ectopically in place of the cerebellum (arrow on B,D). Posterior part differentiated into the cerebellum according to the original program (arrowheads on D). (E) A transverse section of the brain at the level of (E) indicated on (A). (F,G) Higher magnification of the cerebellum and ectopic tectum indicated as (F,G, respectively), on (D). The external granular layer is differentiating on the control side (arrowhead on F). (H) Higher magnification of the proper tectum. The ectopic tectum has nine layers (G), while the proper tectum has ten layers (H). (I,J,L,M) Whole-mount immunohistochemistry with anti-neurofilament antibody on E4 embryos. (K,N) Bright field of (J,M, respectively). An arrow and arrowhead on (K) indicate a large ectopic swelling and a small one in the metencephalon, respectively. In normal embryos (I), oculomotor (III) and trochlear (IV) nerves are discernible. In Ras^S17N-transfected embryos (J), trochlear nerve fibers arise from the posterior part of the large swelling and run dorsally (arrowheads). Additional nerve fibers arise from the posterior part of the small swelling (arrow). These fibers merge while running dorsally. In some cases, a nerve trunk originates from the ventral metencephalon and run a similar course to the oculomotor nerve (arrowheads on L). In another embryo that is transfected with Ras^S17N, several additional nerve fibers could be discerned posterior to the proper trochlear nerve. In some embryos, the nerve trunk that resembles oculomotor nerve arose from the metencephalic region (M, arrowheads). tel, telencephalon; di, diencephalon; tect, tectum; cer, cerebellum; cont, control side; exp, experimental side; egl, external granular layer; ne, neuroepithelium; III, oculomotor nerve; IV, trochlear nerve. Scale bars: 4 mm (B,C,D); 2 mm (E); 200 µm (A,F,G,H); 400 µm (I-N).