The urachus is a fibrous remnant of the allantois, a canal that drains the urinary bladder of the fetus that joins and runs within the umbilical cord. The fibrous remnant lies in the space of Retzius, between the transverse fascia anteriorly and the peritoneum posteriorly.
Vertical section of bladder, penis, and urethra. Urachus is seen at top
Urachus is #1
The part of the urogenital sinus related to the bladder and urethra absorbs the ends of the Wolffian ducts and the associated ends of the renal diverticula. This gives rise to the trigone of the bladder and part of the prostatic urethra.
The remainder of this part of the urogenital sinus forms the body of the bladder and part of the prostatic urethra. The apex of the bladder stretches and is connected to the umbilicus as a narrow canal. This canal is initially open, but later closes as the urachus goes on to definitively form the median umbilical ligament.
Failure of the inside of the urachus to be filled in leaves the urachus open. The telltale sign is leakage of urine through the umbilicus. This is often managed surgically. There are four anatomical causes:
- Urachal cyst: there is no longer a connection between the bladder and the umbilicus, however a fluid filled cavity with uroepithelium lining persists between these two structures.
- Urachal fistula: there is free communication between the bladder and umbilicus
- Urachal diverticulum [Vesicourachal diverticulum]: the bladder exhibits outpouching
- Urachal sinus: the pouch opens toward the umbilicus
The urachus is also subject to neoplasia. Urachal adenocarcinoma is histologically similar to adenocarcinoma of the bowel. Rarely, urachus carcinomas can metastasise to other regions of the body, including pelvic bones and the lung.
- Larsen, "Human Embryology," 3rd ed., pg. 258
- Le, Tao; Bhushan, Vikas; Vasan, Neil (2010). First Aid for the USMLE Step 1: 2010 (20th Anniversary ed.). USA: The McGraw-Hill Companies, Inc. p. 122. ISBN 978-0-07-163340-6.
- Guray, Sogut, et al. (2000) Urachal Cyst. Eastern Journal of Medicine 5(2):76-78.
- Elser, C; Sweet, J; Cheran, S K; Haider, M A; Jewett, M; Sridhar, S S (February 2012). "A case of metastatic urachal adenocarcinoma treated with several different chemotherapeutic regimens". Can Urol Assoc J. 6 (1): E27–E31. doi:10.5489/cuaj.11109.
- Travis W. Dum; Da Zhang; Eugene K. Lee (2015). "IgG4-Related Disease in a Urachal Tumor". Case Reports in Urology. pp. 1–3. doi:10.1155/2014/275850. PMC 4151357. PMID 25202466. 275850.