allantois

**Embryology** - Arises ~day 16 (week 3) as an endoderm-lined outpouching from the caudal yolk sac that extends into the connecting stalk (future umbilical cord). - In humans it is vestigial functionally (unlike reptiles/birds, where it serves as a respiratory and waste-storage organ), but its mesoderm and vessels are important. - The allantoic blood vessels become the **umbilical arteries and vein** (the umbilical vein carries oxygenated blood to the fetus; the two umbilical arteries carry deoxygenated blood to the placenta). - The allantois is continuous with the **cloaca**. The urorectal septum divides the cloaca into the urogenital sinus (anterior) and the anorectal canal (posterior). The upper urogenital sinus forms the **bladder**, which stays connected to the allantois. - As the bladder descends, the allantois becomes a thick fibrous cord, the **urachus**, running from the bladder apex to the umbilicus. After birth it is the **median umbilical ligament**.

**Umbilical remnants (high yield)** - Umbilical vein → ligamentum teres hepatis (round ligament of the liver), in the free edge of the falciform ligament - Ductus venosus → ligamentum venosum - Umbilical arteries → medial umbilical ligaments (distal portions); the proximal portions persist as the superior vesical arteries - Allantois/urachus → median umbilical ligament - Vitelline duct → normally obliterated; persistence causes Meckel diverticulum

**Clinical correlations: urachal anomalies** - **Patent urachus:** the whole urachus stays open, so urine leaks from the umbilicus. It is associated with posterior urethral valves or other bladder outlet obstruction. - **Urachal cyst:** the middle segment stays patent, so a fluid-filled cyst forms between the umbilicus and the bladder. It is often asymptomatic but can become infected (lower abdominal pain, fever, palpable midline mass). Long-standing cysts carry a risk of **adenocarcinoma**. - **Urachal sinus:** the portion near the umbilicus stays open, causing umbilical drainage or infection. - **Vesicourachal diverticulum:** the portion near the bladder stays open and forms an outpouching at the bladder apex. - **Urachal carcinoma:** a rare bladder cancer, usually adenocarcinoma, arising at the dome. Suspect it with a midline dome mass and hematuria. - **Vitelline duct vs. urachal remnant:** umbilical drainage of urine suggests a urachal remnant. Drainage of bowel contents or mucus suggests a vitelline duct remnant.

**Other associations** - Allantoic fluid is not a major component in humans, but the term "allantoic cyst" can describe cystic remnants in the umbilical cord. - A single umbilical artery (2-vessel cord) is associated with congenital anomalies, especially renal and cardiac.

**Boards pearl** Umbilical drainage of urine in a neonate → patent urachus. The bladder arises from the urogenital sinus (endoderm), and the allantois is the source of the urachus.