Early embryonic expression ofTlx-1. In situ hybridization analysis ofTlx-1 mRNA expression (blue) in intact embryos (A–C) or transverse 50 μm sections (D–H). Embryonic stages are indicated numerically. The solid black lines inA–C indicate the approximate level and orientation of the corresponding sections shown in D,F, and G respectively. A,D, Ventral view (A) and section (D) through a stage 9⁻embryo. At this stage, Tlx-1 transcripts are detected medially beneath the developing neural plate in definitive pharnygeal endoderm as well as in endoderm of the anterior intestinal portal.E, Similar transverse section of a stage 11 embryo showing that Tlx-1 expression now extends into the mesenchyme. B, F, Lateral view (B) and section (F) through a stage 14 embryo (note that the rostral forebrain has been excised to prevent trapping of the in situ hybridization reaction product). Expression is concentrated within the developing branchial arches where it is restricted to the central mesenchyme and ventral medial endoderm and ectoderm. No expression is detected within the developing cranial ganglia. C, G, Lateral view (C) and section (G) through a stage 17 embryo. Expression persists within the developing branchial arches. At this stage, ganglia of the fifth and eighth (vii, viii) cranial nerves also express Tlx-1. H, Transverse section through a stage 21 embryo. Expression in the trigeminal ganglia (V) is restricted to cells that are distal from the nerve root. Within the hindbrain,Tlx-1 expression is confined to a bilaterally symmetrical population of cells located at the interface between the ventricular layer and the mantle. Scale bars: D, E, 100 μm; all other panels, 200 μm.