Bladder, urachus and urethra !

Embryonic structure

The bladder and urethra come from the front part of the cloaca; the urachus was a tube joining bladder and navel and closes before birth.

Updated 2026-11-01

See it in 3D: Urinary bladder · Trigone of bladder · Urethra (male) · Urethra (female) · Prostatic urethra · Membranous urethra · Spongy urethra · Prostate

What it is

Between weeks 4 and 7 the urorectal septum divides the cloaca into the rectum behind and the urogenital sinus in front. The upper sinus forms the bladder, the middle and lower parts form the urethra, and the bladder stays tied to the navel by the urachus, a remnant of the allantois. The trigone has a different origin from the rest of the bladder, which explains reflux and ectopic ureters.

Key facts

Weeks 4–7: partitioning of the cloaca.
As the bladder grows, the ureteric openings move up and out; the mesonephric ducts end lower, in the prostatic urethra.
The prostate begins as buds from the urethral epithelium driven by dihydrotestosterone.
In females the urethra buds off the paraurethral (Skene) glands, homologues of the prostate.
The most distal part of the glans urethra is classically described as ectodermal (glandular plate).

Structure

Urogenital sinus, vesical part: the bladder (except the trigone at its origin).
Urogenital sinus, pelvic part: prostatic and membranous urethra (male); female urethra and part of the vestibule.
Urogenital sinus, phallic part: spongy urethra (male), vestibule of the vagina (female).
Nearby splanchnic mesoderm: smooth muscle and connective tissue of the bladder wall.

What it becomes

Urinary bladder and bladder trigone.
Male urethra (prostatic, membranous, spongy) and female urethra.
Prostate and bulbourethral glands; in females, paraurethral and greater vestibular glands.
Median umbilical ligament (obliterated urachus).

Markers

Clinical notes

If the urachus fails to close: patent urachus, urachal cyst, sinus or diverticulum.
If mesenchyme fails to reinforce the cloacal membrane, it ruptures and the bladder lies open: bladder exstrophy.
A trigone with a short intramural tunnel favours vesicoureteral reflux.
Posterior urethral valves (males only) block the prostatic urethra and dilate bladder and ureters.
Urachal cancer (adenocarcinoma) arises at the bladder dome in adults.

Test yourself

Think of the answer, then tap the question to see it.

Which structure divides the cloaca, and what does its tip become?

The urorectal septum; it separates the urogenital sinus from the anorectal canal and its tip forms the perineal body.

Why does the bladder trigone have a different origin from the rest of the bladder?

It forms when the ends of the mesonephric ducts are absorbed into the bladder wall; endodermal epithelium from the urogenital sinus later covers it.

What adult structure does the urachus become?

The median umbilical ligament, a fibrous cord between the bladder dome and the navel.

What is the female homologue of the prostate?

The paraurethral (Skene) glands, which also bud from the urethral epithelium.

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See it in 3D

The structures of Durgical's 3D anatomy where it is found.

Vesicoureteral reflux

← All of embryology

Educational content on embryonic development; not a guide to prenatal diagnosis or to medicines in pregnancy and no substitute for a health professional's advice. No doses are given.

Sources: Embryology, Genitourinary - StatPearls - NCBI Bookshelf (StatPearls Publishing (NCBI Bookshelf), 2023); Embryology, Kidney, Bladder, and Ureter - StatPearls - NCBI Bookshelf (StatPearls Publishing (NCBI Bookshelf), 2023); Embryology, Urethral Folds - StatPearls - NCBI Bookshelf (StatPearls Publishing (NCBI Bookshelf), 2023).

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Representative model, for educational purposes only.