Figure 2.

A model of Tbx18 action in radial patterning and urothelial differentiation. (A) Tbx18 is selectively expressed in ureteral mesenchyme, where it is required for smooth muscle differentiation and for differentiation of the urothelium, the specialized mucosa that provides barrier function. Tbx18 controls smooth muscle differentiation by regulating Shh responsiveness in subepithelial mesenchyme (3). Shh secreted from ureteral epithelia induces expression of Patched1 and Bmp4, a signaling molecule critical for recruitment of ureteral mesenchyme and for its differentiation into smooth muscle. (B) Tbx18 is selectively expressed in ureteral mesenchyme and is important for differentiation of the urothelium lining the ureters, but not the urothelium lining the bladder and urethra.