Fig. 1. The medial arc contains three molecularly distinct territories. In each row, pairs of E5 embryos are shown in whole-mount preparations (left column) and in cross-sections (right column). In the left panel, rostral is towards the top and the ventricular surface faces the viewer. In the right panel, the ventricular surface lining the midbrain vesicle is towards the top. Markers are shown in color-coded text at the bottom of each panel. (A,B) ACHE gene expression (purple) distinguishes arcs (labeled 1-3, medial to lateral). Two-color demonstration of ACHE, PHOX2A and ISL1 gene expression establishes that the PHOX2A+ and ISL1+ cells are restricted the medial arc (arc 1), but do not fill it completely. Arrowhead in A indicates the rostral limit of PHOX2A labeling. Arrows in B indicate ISL1-negative territories within the ACHE-rich medial arc (see below). (C) TH (arrow) and PHOX2A gene expression demonstrates that some midbrain DA neurons are also found in the medial arc. (D) The PHOX2A+ and the TH+ neurons occupy discrete ventricular-pial positions within the medial arc. (E,F) A population of BRN3A+ cells is detected in the medial arc rostrally. (E) These cells overlap the PHOX2A labeling caudally and conform to the ACHE+ medial arc territory that lies rostral to the arrowhead in A. (F) When viewed in cross-section, the BRN3A+ cells lie pial to the PHOX2A+ territory. (G,H) EMX2 gene expression also identifies this rostral medial arc territory. Relationship of medial arc EMX2 labeling to the ISL1+ OMC is illustrated. Within the medial arc, BRN3A and EMX2 are expressed exclusively within this rostral territory. BRN3A and EMX2 expression is not, however, restricted to the medial arc in midbrain development. Both genes are expressed in developing tectum (E, and data not shown), EMX2 identifies arcs 2-4 (G,H) and by E7 BRN3A marks out other ventral midbrain structures as well (not illustrated) (Fedtsova and Turner, 2001; Fedtsova and Turner, 1995). 1,2,3, arcs 1, 2 and 3; III, third ventricle; IS, isthmus; OMC, oculomotor complex; rFP, rostral, or midbrain, floor plate; tec, tectum.

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