Fig. 4. Ureteral fibers contribute to the trigone. (A) Sagittal section through a Rarb2-Cre;R26RlacZ embryo at E14 showing lacZ-expressing mesenchymal cells surrounding the ureter (yellow arrowheads in all panels). Note the absence of lacZ-expressing cells in the bladder, trigone and urethra. (B) Higher magnification of a region of A. (C) Whole-mount of a newborn Rarb2-Cre;R26RlacZ urogenital tract showing lacZ-expressing smooth muscle cells lining the extra-mural and intramural ureter. (D) A section through the trigone showing lacZ-expressing cells surrounding the intramural ureter. (E) Smooth muscle uroplakin staining of a section serial to D, showing that the lacZ activity in D corresponds to smooth muscle. (F). Section through a human fetus at the same level as E, showing the ureteral muscle embedded in bladder muscle in the trigone. wd; Wolffian duct. Magnification: x100 in A; x200 in B-F.