cloacal membrane

**Embryology** - The cloacal membrane forms at the caudal end of the embryonic disc during gastrulation (week 3). It is a region where ectoderm and endoderm are directly apposed with no intervening mesoderm. The buccopharyngeal (oropharyngeal) membrane is the cranial counterpart and has the same bilaminar structure. - With cephalocaudal and lateral folding (week 4), the cloacal membrane is drawn onto the ventral surface. It lies at the caudal end of the hindgut, which dilates to form the cloaca. - The cloaca is an endoderm-lined chamber. The allantois arises from its ventral wall.

**Partitioning** - The urorectal septum is mesoderm that grows caudally between the allantois and hindgut. It fuses with the cloacal membrane at the future perineal body (weeks 6-7). - This divides the cloaca into two parts. The ventral part is the urogenital sinus, which gives rise to the bladder, urethra, and in females the lower vagina. The dorsal part is the anorectal canal, which gives rise to the rectum and upper anal canal. - The cloacal membrane is split into two parts. The ventral part is the urogenital membrane and the dorsal part is the anal membrane.

**Rupture** - The urogenital membrane ruptures first (about week 7). The anal membrane ruptures about week 8, which opens the anal canal to the amniotic cavity. - The pectinate (dentate) line marks the junction of the endodermal upper anal canal and the ectodermal lower anal canal (proctodeum). The anal membrane lay at this site. - Above the pectinate line: hindgut (endoderm) origin, visceral innervation, supplied by the superior rectal artery, drains to the internal iliac nodes, and the epithelium is columnar. Below the line: ectoderm origin, somatic innervation (inferior rectal nerve), supplied by the inferior rectal artery, drains to the superficial inguinal nodes, and the epithelium is stratified squamous.

**Clinical correlations** - **Imperforate anus:** the anal membrane fails to rupture, or the anorectal canal fails to develop. It is associated with VACTERL (Vertebral defects, Anal atresia, Cardiac defects, Tracheoesophageal fistula, Renal anomalies, Limb anomalies) and with Down syndrome. Rectourethral or rectovaginal fistulas may occur. - **Persistent cloaca:** the urorectal septum fails to form or fuse. The urinary, genital, and GI tracts share a single common channel. - **Cloacal exstrophy:** the cloacal membrane overdevelops, which prevents mesoderm from migrating in and forming the lower abdominal wall. The membrane then ruptures, which exposes the bladder halves and the bowel. It is associated with omphalocele, imperforate anus, and spinal defects (OEIS complex). - **Bladder exstrophy and epispadias:** these result from abnormal cloacal membrane development or an abnormal position of the membrane. Overextension of the membrane prevents mesenchymal ingrowth and leaves the abdominal wall defect. They are associated with pubic diastasis. - **Hypospadias:** this is a related urogenital malformation. It results from incomplete fusion of the urethral folds and is not a membrane defect.

**High-yield for boards** - Bilaminar membranes without mesoderm: the buccopharyngeal membrane, the cloacal membrane, and the thin region where the future thin-walled structures form. - Urorectal septum, together with the cloacal membrane, gives the perineal body. - The anal canal above the pectinate line is endoderm and below it is ectoderm.