Pediatric Surgery, AlSadik Hospital, Qatif, Saudi Arabia
The urachus is a fibrous remnant of the allantois.
The allantois is a canal that drains the urinary bladder of the fetus and runs within the umbilical cord.
The urachus is a band of fibrous tissue extending from the dome of the bladder to the umbilical cord.
By 32 weeks, the urachus is obliterated and becomes a vestigial structure known as the median umbilical ligament (not to be confused with the medial umbilical ligament, which is a separate structure that lies laterally to the median umbilical ligament).
The urachus remnant lies in the space of Retzius, between the transversalis fascia anteriorly and the peritoneum posteriorly.
In 0.1–2.0 % of the population, the urachus remains patent and may result in urine leaking from the umbilicus or infected urachal cysts.
Persistence of urachal remnant can give rise to various clinical problems:
Urachal diverticulum (Vesicourachal diverticulum)
Because urachal remnants are uncommon and manifest with nonspecific abdominal or urinary signs and symptoms, definitive presurgical diagnosis is not easily made.
The urachus is also known to undergo malignant transformation and adenocarcinoma is the commonest.
A rare urachal tumor has been reported as a manifestation of IgG4-related disease.
Embryologically, the amniotic cavity bulges over the embryo and attaches to the yolk sac and the connecting stalk to form the umbilical cord.
Further caudal folding of the embryo incorporates the proximal yolk sac into the hindgut and the allantois which is a diverticulum of the yolk sac into the urogenital sinus.
The yolk and body stalks fuse to become the umbilical cord.
In the third week of gestation, the allantois, which grows into the body stalk is formed as a diverticulum from the yolk sac.
The allantois appears on about day 16 as a tiny, finger-like outpouching from the caudal wall of the yolk sac.
The bladder develops from the ventral portion of the expanded terminal part of the hindgut, the cloaca, which is contiguous with the allantois ventrally.
The cranioventral end of the bladder opens into the allantois at the level of the umbilicus; thus, the bladder initially extends all the way to the umbilicus.
By the fourth or fifth month of gestation, the bladder descends into the pelvis and its apical portion progressively narrows to a small, epithelialized fibromuscular strand, the urachus (Figs. 16.1, 16.2, and 16.3).
Figs. 16.1, 16.2, and 16.3
Diagrammatic representations of the embryo at 3, 4 and 5 weeks of intra-uterine life. Note the developing allantosis and omphalomesenteric duct
In late embryonic and fetal life and early postnatal life, the urachal portion, which is still microscopic, fails to grow; thus, its lumen remains narrow and is usually obliterated by fibrous proliferation.
In one-third of adults, it may be visible at microscopic examination as a structure communicating with the lumen of the bladder; however, in terms of function it can be considered closed by the latter half of fetal life.
The urachus varies from 3 to 10 cm in length and from 8 to 10 mm in diameter.
It is a three-layered tubular structure, the innermost layer being lined with transitional epithelium in 70 % of cases and with columnar epithelium in 30 %.
The structure is surrounded by connective tissue and an outermost muscular layer in continuity with the detrusor muscle.
Occasionally, the urachus may merge with one or both of the obliterated umbilical arteries, and there may be a slight deviation to the right or left of the midline.
As the distal hindgut and the urogenital sinus separate, the developing bladder remains connected to the allantois through a connection called the urachus.
Persistence of this communication leads to urachal remnants.
The urachal remnant anomalies include:
Subsequently, the yolk and body stalks fuse to become the umbilical cord.
Development of the abdominal wall narrows the umbilical ring, which should close before birth.
Persistence of the umbilical ring results in an umbilical hernia.
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There are four types of congenital urachal remnant anomalies.
Patent urachuss(50 %)
Urachal cyst (30 %)
Urachal-umbilical sinus (15 %)
Vesicourachal diverticulum (5 %)
A urachal cyst is a cyst which occurs in a persistent portion of the urachus between the umbilicus and the urinary bladder.
It presents as an extraperitoneal mass in the umbilical region.
Urachal cysts are usually silent clinically until infection, calculi or adenocarcinoma develop
It is characterized by:
Abdominal pain, and fever if infected
It may rupture, leading to peritonitis
It may drain through the umbilicus.
This is the commonest congenital malformation of the urachus.
In patent urachus, the whole urachus fails to obliterate and there is a patent communication between the urinary bladder and the umbilicus.
In umbilical sinus, the distal part of the urachus remains open to the umbilicus.
The usual presentation is persistent umbilical discharge.
In this, there is a wide patent urachal opening to the urinary bladder
Inflammation and malignancy are the commonest complications of urachal remnants.
Inflammation occurs more frequently in children and young adults.