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Fig. 1. Differential gene expression at the ventral MFB. (B-E) Cross-section through the mesencephalon of HH20 chick embryos, showing the area depicted in the schematic view (A). (F) Schematic lateral view of a HH25 chick brain, indicating the location of major tracts. Highlighted are the mlf in red and the pc in green. Black circles at the ventral MFB represent the respective nuclei. (G-J) Lateral view of the ventral mesencephalon and pretectum of HH25 chick embryos, focussing on the ventral MFB (see boxed area in F). The position of the MFB is marked by a line. Red staining in B-E,G-I indicates the expression of Isl1 (arrowhead in B,G; ventral stripe, marking the oculomotor nucleus) and Pax6 (arrow in B,G; dorsal stripe). (B,G) Blue staining indicates the expression of Sax1 in a broad domain dorsal and rostral to the oculomotor nucleus. Sax1 and Pax6 signals overlap. (C,H) Expression of Six3 around the ventral MFB. Six3 staining in the ventral mesencephalon (blue) is split into two longitudinal domains by the intervening Pax6 stripe (arrow). (D,I) The mRNA for Emx2 (blue) is also largely localised in two longitudinal stripes in the ventral mesencephalon, divided by the Pax6 signals (arrow). Emx2 and Isl1 signals partially overlap at the MFB (arrowhead). (E,J) Pax6 is expressed in a single longitudinal stripe in the ventral mesencephalon (arrow in J), dorsal to the oculomotor nucleus (red Isl1 signal in E). Pax6 is also expressed in the pretectum, in a ventral patch of cells (arrowhead in J), and dorsally. (K-O) Schematic representation of nuclei position (K) and expression domains for Sax1 (L), Six3 (M), Emx2 (N) and Pax6 (O) in the ventral midbrain and pretectum. Numbers in K represent the midbrain arcs 1-3. III, oculomotor nerve; INC, interstitial nucleus of Cajal; llf, lateral longitudinal fascicle; mes, mesencephalon; mlf, medial longitudinal fascicle; nIII, oculomotor nucleus; nPC, nucleus of the posterior commissure (ventral part); pc, posterior commissure; pt, pretectum; RN, red nucleus; tpoc, tract of the postoptic commissure.