Urinary tract embryology

Key points

Kidneys

4th week Pronephros forms in cervical region and then regresses

Mesonephros gives rise to ureteric bud, which contacts the metanephros around 12th week to drive formation of kidney

Kidneys then ascend weeks 6-9 - horseshoe kidney + ectopias can arise

Bladder and urogenital sinus

Cloaca divided in 4th-7th weeks by urorectal septum

Anterior part is urogenital sinus

Upper sinus is bladder, middle is urethra and lower 1/3 vagina, prostatic & membranous urethra in males, lower is rest

Bladder absorbs caudal mesonephric ducts, to form trigone & ureters

Allantois -> urachus -> median umbilical ligament

Mesonephric (Wolffian) Ducts

Structures lateral to mesonephros become mesoneprhic ducts

Mesonephric duct in females is Gartners duct - regresses in absence of testosterone

In males - epididymis, vas, seminal vesicles

Prostate is formed by testosterone inducing cells around urethra

Paramesonephric (Müllerian) Ducts

Become females internal genitalia - regresses in males due to anti-müllerian hormone

Develop at 6 weeks, initially lying lateral to the wolffian ducts before shifting medially to reach the midline near the cloaca

By 9 weeks, they elongate caudally to insert at the urogenital sinus

At 12 weeks, the müllerian ducts fuse to form the uterus, cervix, and upper vagina

A bicornuate uterus results from failure of lateral fusion of the müllerian ducts

A septate uterus occurs due to incomplete resorption of the central septum after duct fusion, and has the highest risk of reproductive complications among müllerian anomalie

External genitalia

Develops from ectoderm

Genital tubercle

Labioscrotal fold - outer

Urogenital fold - inner

Urogenital membrane

Male anterior urethra is from tubularisation/closure of urogenital groove

References

Essentials of Pediatric Urology, 3rd edition, 2022, Chapter 1 Embryology