Expansion
The urorectal septum divides the cloaca into urogenital sinus in front and anorectal canal behind
Expansion
The hindgut ends in the cloaca, a common chamber shared with the allantois. The urorectal septum grows down to divide it by about week 7:
- Anteriorly - the urogenital sinus, becoming bladder and urethra
- Posteriorly - the anorectal canal
The septum meets the cloacal membrane at the future perineal body, dividing it into urogenital and anal membranes, which then break down.
Failures produce:
- Imperforate anus - the anal membrane fails to break down
- Rectovesical, rectourethral or rectovaginal fistula - incomplete septation
- Persistent cloaca - no septation at all, with a single perineal opening
Because the pectinate line marks the junction of endoderm and ectoderm, a “high” lesion above the levator ani has a different prognosis for continence than a “low” one. Look for associated VACTERL anomalies.