Trigone Formation

Above: A mesonephric duct (future ductus derferens) connects bilaterally to the urogenital sinus. Each duct gives off a bud that becomes a ureter. As the urogenital sinus enlarges, its wall incorporates both the ureters and the mesonephric ducts so they each open separately into the sinus. The ureters undergo cranial repositioning so they ultimately open anterior to the bladder neck sphincter. The repositioning produces the trigone (and urethral crest) which serves to anchor the ureters. Failure to properly reposition can result in an ectopic ureter, a source of postnatal incontinence when it opens distal to sphincters in the caudal urethra or vagina.

Below: Ureter migration is animated.

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