cloaca

**Embryology** - The cloaca is the dilated terminal portion of the hindgut (endoderm-lined). It receives the allantois ventrally and is sealed externally by the cloacal membrane (endoderm contacting ectoderm). - Weeks 4–6: the urorectal septum (mesoderm) grows caudally and fuses with the cloacal membrane. This divides the cloaca into: - Ventral urogenital sinus, which gives the bladder, urethra, and lower vagina (female) or prostatic/membranous urethra (male). - Dorsal anorectal canal, which gives the rectum and upper anal canal. - The cloacal membrane splits into the urogenital membrane (anterior) and the anal membrane (posterior). The point of fusion becomes the perineal body. - By about week 8–9 the anal membrane ruptures, creating a patent anus. - The pectinate (dentate) line marks the junction of endoderm (above) and ectoderm (below). This explains the differences in blood supply (superior rectal vs inferior rectal artery), venous drainage (portal vs systemic), lymphatics (inferior mesenteric nodes vs superficial inguinal nodes), innervation (visceral vs somatic), and cancer type (adenocarcinoma vs squamous cell carcinoma).

**Clinical correlates** - **Persistent cloaca**: occurs only in females. The rectum, vagina, and urethra join in a single common channel, with one perineal opening. It is often associated with Müllerian anomalies, urinary tract anomalies, and spinal anomalies. - **Anorectal malformations / imperforate anus**: caused by abnormal urorectal septum development or failure of the anal membrane to perforate. Fistulas are common: rectourethral in males, and rectovaginal or rectovestibular in females. Associated with VACTERL (Vertebral, Anal atresia, Cardiac, Tracheoesophageal fistula, Renal, Limb anomalies). - **Cloacal exstrophy**: a severe ventral wall defect caused by abnormal development of the cloacal membrane. It involves bladder exstrophy, an exposed hindgut, and an omphalocele. Often associated with imperforate anus and spinal defects (OEIS complex: Omphalocele, Exstrophy, Imperforate anus, Spinal defects). - **Urachal anomalies**: the allantois becomes the urachus, which connects to the bladder apex. It becomes the median umbilical ligament. Persistence causes a patent urachus, urachal cyst, or sinus. - **Down syndrome and Hirschsprung disease** also show associations with anorectal anomalies (Down syndrome has an increased incidence of imperforate anus).

**High-yield pearls** - Cloaca = Latin for "sewer". In birds, reptiles, and amphibians it persists in adults. In humans it is only embryonic. - Mesoderm of the urorectal septum is the key structure to remember. - Hindgut derivatives (distal 1/3 transverse colon to upper anal canal) are supplied by the inferior mesenteric artery.