Fig. 5. Ureter differentiation occurs in zones of the UB surrounded by Bmp4-expressing mesenchyme. (A) Sections of E14.5 Bmp4^+/lacz embryos show abundant ß-gal staining, indicating that Bmp4 expression persists in the mesenchyme (arrow) surrounding the distal UB (dUB). ß-gal activity can also be detected in glomerular cells (g). (B,C) Immunofluorescent analysis of E15.5 frozen sections demonstrates that this zone of the UB network (red, E-cadherin staining) differentiates into the ureter, as determined by the presence of an SMA-positive connective tissue coat (B, green) and upregulated uroplakin expression (C, green). SMA-positive cells (B) are also detected around renal arteries (ra). At E12.5 (D), the E-cadherin-positive UB network lacks both UP-positive (D) and SMA-positive cells (data not shown). However, after E12.5 rudiments were cultured for 4 days (E,F) the distal-most domain of the E-cadherin-labeled UB (red) acquires a thick, SMA-positive connective tissue coat (E, green) and exhibits upregulated expression of uroplakins (F, green). (G) In situ hybridization detection of Bmp4 in cultured rudiments indicates that ureter morphogenesis occurs only in domains of the UB bounded by Bmp4-expressing mesenchyme. Bmp4 mRNA can also be detected in glomerular podocytes (g).