Development Of Bladder And
Ureter
Formation Of The Cloaca
The urinary bladder develops from the cloaca, a primitive
pouch that forms during the fourth week of gestation. At the beginning of the
fourth week, the embryo remains a trilaminar structure consisting of ectoderm,
mesoderm, and endoderm. The cloaca has not yet developed, but the cloacal
membrane is visible as a small depression near the caudal end of the embryo. At
this site, ectoderm from the neural plate merges with endoderm from the yolk
sac, without an intervening layer of mesoderm.
During the fourth week the embryo undergoes a folding
process, during which the yolk sac gives rise to the gut tube. The caudal part
of the gut tube, known as the hindgut, terminates at the cloacal membrane. The
most caudal end of the hindgut dilates to form the cloaca.
The cranioventral aspect of the cloaca is continuous
with a narrow tube, known as the allantois, that extends into the connecting
stalk. Meanwhile, the lateral walls of the cloaca receive the mesonephric
(wolffian) ducts.
Septation Of The Cloaca
By the sixth week, a septum divides the cloaca into an
anterior primitive urogenital sinus and posterior rectum. The exact mechanism
of the septation process has long been a topic of active debate and
investigation. Some have proposed that a septum oriented in the coronal plane
descends through the cloaca in a cranial-to-caudal direction, while others have
proposed that two lateral cloacal folds fuse in the midline to form a septum.
Still others have proposed various combinations of the two previous theories.
More recent investigations have rejected both of these theories, instead
arguing that septation results from advancement of the dorsal cloaca toward the
cloacal membrane as the embryo lengthens and rotates. During this process, the
urorectal fold, located between the allantois and the hindgut, passively advances
toward the cloacal membrane, causing effective septation. Subsequent apoptosis
of the cloacal membrane establishes two distinct openings that lead to the
primitive urogenital sinus and rectum. The tip of the septum lying between them
gives rise to the perineal body.
Maturation Of The Bladder
After cloacal septation is complete, the primitive urogenital
sinus contains three major parts. The most caudal part is known as the
definitive urogenital sinus, and it will become the penile and spongy urethra in
males, or the vestibule of the vagina in females. The neck, located just
proximal to the definitive urogenital sinus, will become the membranous and
prostatic urethra in males, or the urethra in females. The bulging area
proximal to the neck will become the urinary bladder in both sexes. The
allantois, which connects the bladder to the umbilical cord, will regress to
form the a thick, epithelial-lined tube known as the urachus, which in turn
will further degenerate into a simple fibrous cord known as the median umbilical
ligament. During subsequent weeks, the definitive urogenital sinus continues to
undergo structural changes as it becomes the mature
bladder. By the tenth week, the endodermal cells become a single layer of
cuboidal epithelium. Over subsequent weeks, additional cell layers appear,
which begin to assume the characteristics of differentiated urothelial cells.
Meanwhile, during the twelfth week, the surrounding splanchnopleuric mesoderm
differentiates to form the detrusor muscle, which lines the urothelium. As bladder
development proceeds, the mechanical distention associated with urine storage
appears to be essential for the development of normal wall compliance.
Maturation Of The Ureters
The ureteric buds appear during the fifth week of gestation
as small diverticula near the caudal ends of the mesonephric ducts (see Plate
2-1). They eventually give rise to the ureters, renal pelves, calices, and
collecting ducts.
Although the ureteric buds originally drain into the
mesonephric ducts, they are transferred to the future bladder in a process known
as mesonephric duct exstrophy, which occurs during cloacal septation. In this
process, the most caudal ends of the mesonephric ducts evert and balloon into
the lumen of the urogenital sinus. Eventually, the eversion of each duct is
extensive enough to bring the attached ureteric bud into the sinus. The buds
then separate from the mesonephric ducts and fuse with the posterior wall of
the urogenital sinus.
A ureteric bud with a more caudal position on the
mesonephric duct will not be drawn far into the bladder during the exstrophy
process, resulting in a more superior and lateral ureteric orifice, as well as a
short course through the bladder wall. In contrast, a ureteric bud with a more
cranial position on the mesonephric duct will be drawn deep into the bladder,
resulting in a more inferior and medial ureteric orifice, as well as a longer
intramural course.
Like the bladder, each ureter develops from a simple
epithelial tube into a complex, multilayered structure containing urothelium,
smooth muscle, and connective tissue. There is transient obliteration of the
ureteral lumen during the sixth week of gestation. Recanalization quickly
ensues, however, starting in the midureter region and progressing in both
directions until the entire lumen is once again patent.
Fate Of The Mesonephric Ducts
By the end of the exstrophy process, the mesonephric
ducts terminate in the bladder medial and inferior to the future ureteric
orifices. Although it was previously thought that the mesonephric ducts
contributed to the formation of the trigone, this long-held view has recently
been called into doubt.
In males, the mesonephric ducts become the ejaculatory
ducts, vas deferens, seminal glands (vesicles), and epididymis. In females,
in contrast, the mesonephric ducts largely degenerate, giving rise only to the
vestigial structures known as the epoöphoron and paroöphoron. Instead, the
paramesonephric (müllerian) ducts, which degenerate in males, are responsible
for formation of the female reproductive tract. These ducts appear lateral to
the mesonephric ducts during the sixth week, and in females they become the
uterine (fallo ian) tubes, uterus, and upper two thirds of the vagina.
