The renal pelvis is the funnel-shaped, expanded upper portion of the ureter.
Within the renal sinus it divides into 2 or 3 major calyces, which in turn
divide into minor calyces. The pelvis extends from the renal hilum and narrows
outside the kidney to become the ureter. The ureter is a tubular structure,
which descends more or less vertically in the retroperitoneal space, cross
the iliac vessels at the pelvic brim and turn medially to enter the urinary
bladder, running obliquely through its wall.
The urinary bladder is a hollow muscular organ shaped roughly like
a tetrahedron when empty and spherical when filled with urine. The posterior
triangle of the tetrahedron forms the base of the bladder, the anterior
the apex, the inferior angle the apex, the inferior angle leadin to
the urethra is called the neck and the two posterolateral angles admit
the ureters. The smooth triangular area outlined by the ostia of the ureters
and the internal orifice of the urethra is called the trigone.
The uvula is the smooth, small ridge at the inferior corner of the
trigone just above the internal urethral orifice.
The mucosa of the renal pelvis, ureter and urinary bladder is lined by stratified
epithelium, the urothelium or transitional epithelium (from
the erroneous concept that this epithelium constitutes a transition between
stratified squamous and columnar epithelia). The epithelium is 2-3 cells thick
in the pelvis, up to 5 in the ureter and up to 7 cells thick in the bladder
in the undistended state. The basal layer is formed by cuboidal to columnar
cells while the superficial layer is made up of relatively larger cells, which
may take on an "umbrella-like" shape due to their outer convex shape. Interposed
between the two layers are polyhedral, intermediate cells. The urothelium
lies on a thin, non-discernable basement membrane beneath which is the connective
tissue of the lamina propria. There is no muscularis mucosae and the
lamina propria rests on the muscle coat.
Normal bladder mucosa showing transitional epithelium and lamina propria.
Bladder wall. From left to right - lumen, transitional epithelium, lamina
propria, thick layer of smooth muscle.
Transitional epithelium of the bladder. Transitional epithelium is a stratified
epithelium characterized by the fact that the most apical cells are the roundest
and largest in diameter. T.E. is designed to be able to increase its surface
area as the lumen is dialated by urine.
The pelvis and ureter have two continuous thin muscle layers that originate
in the minor calyces and form a spiral, helical arrangement with a loosely
spiraled internal layer and a more tightly spiraled external layer in the
ureter. In the lower third of the ureter, a third outer longitudinal layer
is present. All 3 layers merge with the 3 layers (inner longitudinal, middle
circular and outer longitudinal) in the bladder wall, which run longitudinally,
transversely and obliquely.
Cross-section of ureter. It has a very irregular lumen, which is lined by
transitional epithelium. Under the epithelium is a connective tissue layer
and beneath that two layers of smooth muscle, inner longitudinal and outer
circular.
Transitional epithelium is a stratified epithelium characterized by the fact
that the most apical cells are the roundest and largest in diameter. T.E.
is designed to be able to increase its surface area as the lumen is dialated
by urine.
Beneath the outer muscle coat is the serosa, made up of loose connective
tissue and containing blood vessels and lymphatics.
