Development Of Urinary Bladder &Urethra Dr. Nimir Dr. Safaa

Objectives: Describe the development of the cloaca and urogenital sinus. Discuss parts of the urogenital sinus. Understand the origin of different parts of the bladder. Congenital anomalies of the bladder. Understand the origin of different parts of the urethra. Congenital anomalies of the urethra.

The lower urinary system composed of the: Bladder and the urethra. Is formed from the endoderm of the hindgut. During the fourth to seventh weeks of development, the Cloaca divides into the urogenital sinus anteriorly and the anal canal posteriorly.

Three portions of the urogenital sinus can be distinguished: 1. The upper largest part(vesical) gives the urinary bladder. 2. The pelvic part in male gives rise to the prostatic and membranous parts of the urethra, in female it gives the whole urethra and part of the vagina. 3. The phallic part differs between male & female.

The bladder develops mainly from the vesical part of the urogenital sinus, but its trigone region is derived from the caudal ends of the mesonephric ducts. The entire epithelium of the bladder is derived from the endoderm of the vesical part. The other layers of its wall develop from adjacent splanchnic mesenchyme.

Initially, the bladder is continuous with the allantois. The allantois obliterates and form a thick fibrous cord, the Urachus. The urachus connects the apex of the bladder with the umbilicus. In the adult, it is known as the median umbilical ligament.

The caudal portions of the mesonephric ducts are absorbed into the wall of the urinary bladder. The distal parts of the mesonephric ducts in the male become the ejaculatory ducts. The mucosa of the bladder formed by incorporation of the ducts (the trigone of the bladder) is mesodermal. With time is replaced by endodermal epithelium.

Development of the urethra : The urethra forms itself from the lower part of the urogenital sinus (UGS). In a male the prostate and membranous part of the urethra arise from the pelvic part of the UGS while the spongy urethra comes from the phallic part (urethral plate). In female the whole urethra and part of the vagina arise from the pelvic part of the UGS while the phallic part (urethral plate) forms the vestibule and the labia minora. The connective tissue and smooth muscle of the urethra in both sexes are derived from splanchnic mesenchyme.

Development of the urethra in female The female urethra develops from the pelvic part of urogenital sinus

The epithelium of the urethra in both sexes is endoderm. The epithelium of the prostatic urethra forms the prostate gland in male. In the female, the cranial part of the urethra gives rise to the urethral and para-urethral glands.

Bladder Defects: Urachal fistula=Persistance of intraembryonic portion of the allantois ( urine drains through umblicus). Urachal cyst= Persistance of local area of allantois. Urachal sinus= Persistance of the upper part of allantois.

Exstrophy of the bladder is a ventral body wall defect in which the bladder mucosa is exposed (occurs 2/100,000). Epispadias is a constant feature. Exstrophy of the bladder is due to lack of mesodermal migration between the umbilicus and genital tubercle.

Exstrophy of the cloaca is a severe ventral body wall defect in which migration of mesoderm to the midline is inhibited. The defect includes exstrophy of the bladder, spinal defects,imperforate anus, and usually omphalocele.