HISTOLOGY OF THE URETERS, URINARY BLADDER AND URETHRA Pamela BL

The excretory passages for the urine for each kidney are the major and minor calyces, renal pelvis, the ureter, the urinary bladder(where the urine is temporarily stored), and the urethra.

With the exception of the urethra, the calyces, the renal pelvis, ureter and bladder have the same basic histologic structure namely an adventitia(or in some regions a serosa), a muscularis and a mucosa.

URETERS The ureters conduct urine from the renal pelvis to the urinary bladder. They serve not only as a route for the urine, but also contribute to its flow by means of their regular peristaltic contractions.

The ureters enter the bladder obliquely so that contraction of the bladder to expel urine tend to close off the ureteral openings and force it out through the urethra. The muscle of the ureters and in other urinary excretory passages is smooth.

This muscular layer is arranged in bundles generally disposed as an inner longitudinal layer, a circularly arranged middle layer and an outer longitudinal layer.

The renal pelvis is partly inside the kidney and partly outside. In a unilobar kidney it collects urine directly from the collecting ducts which open at the apex of the pyramid. In multilobar kidneys the pelvis connects to tributaries, the major calyces, and by these to sub tributaries, the minor calyces, which collect the urine from the pyramids.

URINARY BLADDER The urinary bladder is situated in the pelvic region of the abdominopelvic cavity. It contains three openings, two for the ureters and one for the urethra. The triangular region between these three openings, the trigone, is relatively smooth and constant, whereas the remainder of the bladder wall is thick when the bladder is empty and thinner when the bladder is distended.

The arrangement of muscle layers in the muscularis is usually more difficult to ascertain than in the ureter. Muscle fibers around the urethral opening form an internal sphincter for this aperture.

On its superior aspect the bladder is covered by peritoneum, and thus this part of the bladder wall shows a serosa. Elsewhere, the outer layer of the urinary bladder consists of an adventitia.

The mucosa of the calyces, renal pelvis, ureters and bladder consists of transitional epithelium and a lamina propria of loose to dense connective tissue. Surrounding the lamina propria of these organs is a dense woven sheath of smooth muscle.

The transitional epithelium of bladder in undistended state is 5 or 6 cells in thickness, and the superficial cells are rounded and bulge into the lumen.

When the epithelium is stretched, as when the bladder is full of urine, the epithelium is only 3 or 4 cells in thickness, and the superficial cells become squamous.

Transitional epithelium displays two unique features. It forms an osmotic barrier between the hypertonic urine and the tissue fluids, and its cells possess a large reserve of surface membrane that is brought into position when the bladder is distended.

Structurally, the special feature of transitional epithelium is the presence of modified areas of plasma membrane designated as plaques. These plaques, measuring about 12 nm, are thicker than the normal membrane.

When the bladder is empty, the plaques fold inwards, occupying space in the superficial reaches of the cell. When the bladder is distended, the plaques are returned to a surface position.

The muscle fibers of the bladder run in every direction(without distinct layers) until they approach the bladder neck, where 3 distinct layers can be identified: the internal longitudinal layer, distal to the bladder neck, becomes circular around the prostatic urethra and the prostatic parenchyma in men.

It extends to the external meatus in women. Its fibers form an involuntary urethral sphincter. The middle layer ends at the bladder neck, and the outer longitudinal layer continues to the end of the prostate in men and to the external meatus in women.

The urinary passages are covered externally by an adventitial membrane, except for the upper part of the bladder which is covered by serous peritoneum.

URETHRA The male urethra is divided into three parts: prostatic, membranous and penile. The prostatic portion of the urethra passes through the prostate gland, and numerous ducts from the gland open into this portion of the urethra. The prostatic urethra is lined by transitional epithelium.

The membranous portion of the urethra is short, it penetrates the urogenital diaphragm. The urogenital diaphragm contains striated muscle which serves as the external sphincter of the urethra.(the smooth muscle in the bladder wall surrounding the origin of the urethra serves as the internal sphincter).

The membranous portion of the urethra is lined by pseudostratified columnar epithelium. The penile portion of the urethra is the longest portion, extending through the corpus spongiosum of the penis. It is lined mostly with pseudostratified columnar epithelium, but near its opening the epithelium becomes stratified squamous.

The female urethra is short, measuring from 3 to 5 cm in length. It is lined partly with pseudostratified columnar epithelium and partly with stratified squamous epithelium

The female urethra also penetrates the urogenital diaphragm, and the striated muscle of this structure constitutes the external urethral sphincter. Smooth muscle fibers, chiefly around the origin of the urethra from the bladder, constitute the internal urethral spnincter.

REFERENCES Histology: a text and atlas Ross/ Reith Basic histology: text and atlas Junqueira/Carneiro tenth edition.