The urinary bladder is a temporary storage reservoir for urine. It is located in the pelvic cavity, posterior to the symphysis pubis, and below the parietal peritoneum. The size and shape of the urinary bladder varies with the amount of urine it contains and with pressure it receives from surrounding organs.

The inner lining of the urinary bladder is a mucous membrane of transitional epithelium that is continuous with that in the ureters. When the bladder is empty, the mucosa has numerous folds called rugae. The rugae and transitional epithelium allow the bladder to expand as it fills.

The second layer in the walls is the submucosa that supports the mucous membrane. It is composed of connective tissue with elastic fibers.

The next layer is the muscularis, which is composed of smooth muscle. The smooth muscle fibers are interwoven in all directions and collectively these are called the detrusor muscle. Contraction of this muscle expels urine from the bladder. On the superior surface, the outer layer of the bladder wall is parietal peritoneum. In all other regions, the outer layer is fibrous connective tissue.

There is a triangular area, called the trigone, formed by three openings in the floor of the urinary bladder. Two of the openings are from the ureters and form the base of the trigone. Small flaps of mucosa cover these openings and act as valves that allow urine to enter the bladder but prevent it from backing up from the bladder into the ureters. The third opening, at the apex of the trigone, is the opening into the urethra. A band of the detrusor muscle encircles this opening to form the internal urethral sphincter.

Referrence:Bladder(Chinese Version)

Normal Development of the Bladder David A. Hatch, M.D.

In the early stages of development, there is no separation of the urinary and alimentary tracts. A common chamber, known as the cloaca, forms in the caudal region of the fetus. At the caudal end of the cloaca, ectoderm lies directly over endoderm forming the thin cloacal membrane. As development progresses, a septum forms (Toureux's fold) dividing the hind gut from an anterior chamber, the urogenital sinus. This septum extends in a caudal direction. Two tissue folds arise from the lateral sides of the cloaca (Rathke's plicae). These folds move medially toward each other to complete the separation of the hind gut from the urogenital sinus. Tourneux's folds and Rathke's plicae together form the uro-rectal septum.

At 10 weeks gestation, the bladder is a cylinder. The cranial portion of the cylinder tapers to become the vesico-allantoic canal. By 12 weeks, the vesico-allantoic canal closes completely leaving the median umbilical ligament. What happens if this tract fails to close?

Normal Bladder Development (Contd.)

The mesonephric (Wolfian) duct descends from the mesonephros to meet the urogenital sinus. Once this connection is made, fetal urine drains into the urogenital sinus. The ureteric bud arises from the mesonephric duct and progresses laterally to invade the metanephrogenic blastema (precursor of the mature kidney). The caudal end of the mesonephric duct (past the ureteric bud is called the common excretory duct. As renal development proceeds, the common excretory duct is incorporated into the urogenital sinus. Progressive incorporation of the common excretory duct eventually leads to separate openings of the ureter and mesonephric duct into the urogenital sinus. By 37 days of gestation, the ureter empties into the urogenital sinus cephalad to the mesonephric duct. The urogenital sinus is divided between the orifices of these two tubes. The cephalad portion of the urogential sinus will become the bladder while the caudal portion will become the urethra.

The caudal end of the developing bladder thickens with smooth muscle in a triangle between the two ureteric orifices and the urethra, the trigone.

Abnormal Development of the Bladder/Proximal Urethra David A. Hatch, M.D.

The ureter normally traverses the base of the bladder, traveling through the ureteral hiatus (the hole through which the ureter enters the detrusor muscle), coursing within the bladder between the detrusor muscle and the bladder mucosa a distance of 1.5 - 2 cm before opening into the bladder. Occasionally, the ureteral hiatus is so large that the distal ureter and surrounding bladder mucosa herniate through the detrusor muscle during voiding. This is called a peri-ureteral diverticulum.

Separation of the bladder from the rectum and the development of the anterior abdominal muscles normally occurs before the cloacal membrane regresses. This leaves these two chambers (bladder and hind gut) as separate structures contained within the abdomen. Rarely, the cloacal membrane ruptures before mesoderm has separated the anterior bladder from the abdominal wall (bladder exstrophy) or before the separation of the bladder from the hind gut by the uro-rectal septum (cloacal exstrophy).

If the cranial end of the developing bladder (vesicoallantoic canal or urachus) fails to fuse and close, urine can drain out of the bladder at the umbilicus. This condition is known as a patent urachus. Infections can occur along this tract. Tumors (usually adenocarcinoma) can arise in a patent urachus. This is thought to result from chronic inflammation. See a case history.

The mesonephric duct terminates in the prostatic urethra in males. Normally, the distal end of the mesonephric duct becomes the ejaculatory duct. However, in some boys, the distal end of the mesonephric duct persists as flaps of tissue that can obstruct the flow of urine out of the bladder. These tissue flaps are called posterior urethral valves.

Bladder Exstrophy ©David A. Hatch, M.D., 1996 http://www.meddean.luc.edu

The cloacal membrane normally ruptures leaving only the urogenital sinus (the urethra and vaginal introitus) open. If mesoderm (which will become the abdominal muscles) has not separated the ectoderm from the endoderm between the allantois and the genital tubercle, rupture of the cloacal membrane leaves the urethra and bladder open as a plate of mucosa on the lower abdomen. In bladder exstrophy, the rupture occurs after the uro-rectal septum has separated the urogenital sinus from the hind gut. What happens if the cloacal membrane ruptures before the uro-rectal septum has divided the bladder from the hind gut? See answer.

Bladder Exstrophy

This male infant was born with bladder exstrophy. The bladder mucosa is everted and lies on the abdomen. Both ureteric orifices lie on the exstrophic bladder. Notice that the penis is shortened and that there is no urethral meatus. His urethra is actually a plate of deep red mucosa lying on the dorsal penis. This is called epispadias.

His bladder was closed by inverting the bladder plate and sewing it closed into a sphere. Later, his urethra will be reconstructed.

Bladder Diverticulum

A 5-year-old girl comes to your clinic because she has had several urinary tract infections. During these episodes she has fever, nausea, dysuria (painful urination) and left flank pain. Her renal and bladder ultrasound shows two normal kidneys. You order this voiding cystourethrogram.

This periureteral diverticulum causes vesico-ureteral reflux because it prevents the normal valve mechanism of the distal ureter. When the child voids, the increased pressure within the bladder causes the entire distal ureter and some of the adjacent bladder mucosa to herniate. After her ureter was surgically reimplanted into her bladder she had no more reflux.

Cloacal Exstrophy

The cloacal membrane normally ruptures leaving only the urogenital sinus (the urethra and vaginal introitus) and the rectum patent. If mesoderm (which will become the abdominal muscles) has not separated the ectoderm from the endoderm between the alantois and the genital tubercle, exstrophy occurs. If rupture of the cloacal membrane occurs before the uro-rectal septum has separated the bladder from the hind gut, the urethra, bladder and large bowel lie open as plates of mucosa on the lower abdomen.

This infant has cloacal exstrophy. Notice that the bladder is actually two patches of mucosa, each with its own ureteric orifice, lying on either side of a patch of large bowel.