Introduction to the Bladder

Embryology and Anatomy

The bladder is a hollow distensible viscus with a strong muscular wall. Embryologically, it forms from the urogenital sinus, which is contiguous with the allantois (a hindgut diverticulum that extends to the umbilicus). The allantois normally involutes by the 2nd month of gestation, forming the median umbilical ligament. Any persistent segments of the allantoic channel are called urachal remnants.

The distal ureters are incorporated into the posterolateral portion of the bladder wall at the trigone. The urethral orifice forms the distal apex of the trigone at the bladder base.

The bladder wall consists of 4 layers. The lumen is lined by uroepithelium, which consists of 3-7 layers of stratified flat cells. These cells are flexible and can change shape from cuboidal to flattened as the bladder distends (hence the term transitional epithelium). The 2nd layer is the lamina propria, which is very vascular. The 3rd layer is the detrusor muscle (muscularis propria). The detrusor muscle is a complex network of interlacing smooth muscle fibers. The inner and outer muscle fibers tend to be oriented in a longitudinal fashion, but distinct layers are usually not discernible. Fibers from the detrusor muscle merge with the prostate capsule (or anterior vagina in females) and pelvic floor muscles. A 4th adventitial layer is formed by connective tissue. A serosal covering, formed by the peritoneum, is present only over the bladder dome.

The bladder is located within the extraperitoneal space and is surrounded by loose connective tissue and pelvic fat. The perivesical space contains the bladder and urachus. The prevesical space (also called space of Retzius) extends anteriorly to the pubic symphysis and communicates posteriorly with the presacral space. These spaces can expand to contain large amounts of fluid as in an extraperitoneal bladder rupture or hemorrhage from pelvic fractures.