Fig. 5. The trigone is formed predominantly from bladder muscle. (A) A sagittal section through a Sm22-Cre-R26RlacZ embryo at E14. lacZ-expressing mesenchymal cells are visible in the bladder, urethra and trigone (white arrow), but not in the ureter or Wolffian duct. (B) Section through the bladder and urethra of an adult Sm22-Cre-R26RlacZ mouse showing descendents of the urogenital sinus mesenchyme that have differentiated in the bladder and urethra muscle. (C) Section through an adult Sm22-Cre-R26RlacZ mouse showing the ureter, which has few if any lacZ-expressing cells, and its path through the bladder muscle that is extensively labeled by the Sm22-Cre transgene. (D) A section through the intramural portion of the ureter in an Sm22-Cre-R26RlacZ adult. (E) A section from the same sample as in D, stained for smooth muscle alpha actin to reveal muscle of the intramural ureter, unlabeled by the Sm22-Cre transgene. (F) Section through a comparable level of a human embryo showing the path of the intramural ureter through the bladder muscle of the trigone. Magnification: x100 in A-C; x200 in D-F.