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.