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. Author manuscript; available in PMC: 2008 Nov 25.
Published in final edited form as: Dev Dyn. 2008 May;237(5):1359–1372. doi: 10.1002/dvdy.21536

Figure 8. Summary of ventral midbrain specification by SHH signaling.

Figure 8

Schematic representation of wild-type (A, E, G) and Shh-/- (B-D, F, H) midbrains in wholemount (A-F) and in transverse section (G-H) views. Wholemounts (open book preparation) are presented with rostral to the top and the ventricular surface facing the viewer. Cross-sections are shown with dorsal to the top and ventral toward the bottom of each schematic. A. Wild-type mouse showing that by E8, Shh expression (tan) is seen at the ventral midline of the midbrain and the expression of Pax7 (blue) is dorsally restricted. B-D. The initial specification of ventral midbrain (demonstrated by the absence of Pax7), occurs in the absence of SHH (B, C) and SMO (data not shown), but becomes dependent upon HH signaling by E9 (D). The ventral-dorsal transformation of the Shh-/- midbrain (Pax7+) displays a rostral-caudal sequence of progression, with Pax7 appearing first rostrally at E8.5 (C) and then ubiquitously by E9 (D). The midbrain-hindbrain boundary (MHB) is specified and is spatially coherent between E8.5-E9 in the Shh-/- midbrain. The mutant midbrain is significantly smaller than the wild-type by E9 (see text). E. At E12.5, the ventral midbrain of wild-type mice is organized into a series of arcuate territories labeled medial to lateral by the expression of Phox2a (arc1: pink) Pax6 (purple) and Evx1 (red). Midbrain arcs are arrayed parallel to Shh (tan) expression in the rostral floor plate (rFP). The expression of Pax7 (blue) continues to be restricted to dorsal midbrain (tectum, tec). The position of the MHB is indicated by Wnt1 (yellow) and Fgf8 (green) expression. F. E12.5 wholemount demonstrating the continued presence of Pax7 throughout the Shh-/- midbrain. The MHB is severely perturbed, displaying a complete absence of Fgf8 and severely reduced and disrupted Wnt1 expression. Midbrain size is dramatically reduced. G. Cross-section through the wholemount shown in E demonstrates the relationship between ventral midbrain arc-specific cell-fates (color coded as in E), the dorsal midbrain expression of Pax7 and Shh. Note that all Evx1+ neurons are located in the mantle (differentiated) layer of ventral (Pax7-negative) midbrain. Note also the spatially restricted expression of Wnt1 in the roof plate (RP). H. Cross-section through the E12.5 Shh-/- midbrain shown in F, displaying the complete absence of Phox2a and Pax6 and the sparing of 25% of the normal number of Evx1+ neurons. The spared Evx1+ neurons are obscured from view in the wholemount view shown in F by the ventricular layer expression of Pax7. The RP is broadened and disrupted. Abbreviations: MHB: midbrain-hindbrain boundary; rFP: rostral floor plate; RP: roof plate; TEC: tectum.