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Embryology: Development of the Lower Urinary Tract

Embryology of the Prostate and Urethra

The prostate develops from the endoderm-derived epithelium of the urogenital sinus near the entry of the Wolffian (mesonephric) ducts. Regional urogenital mesenchyme mediates androgen signaling—particularly through dihydrotestosterone—onto the epithelium, which induces epithelial buds and glandular tubules. The mesenchyme gives rise to the fibromuscular stroma, including connective tissue and smooth muscle.

Caudal to the prostate, the urogenital sinus subdivides into a pelvic part (pars pelvina) and a phallic part (pars phallica). In males, the pelvic part forms the membranous urethra, whereas the spongy (penile) urethra arises predominantly from the phallic part. The penile urethra forms through ventral tubularization of the urethral plate along the phallus. The distal (glandular) segment forms by canalization of an epithelial cord; sources differ regarding the degree of ectodermal contribution to this distal portion.

In females, the urethra forms predominantly from the urogenital sinus, with the pelvic part contributing the largest portion. The phallic part contributes to the vaginal vestibule and the region of the external urethral meatus.

Embryology of the Urinary Bladder

The cloaca is an endoderm-lined common cavity for the hindgut and the allantoic stalk and is closed externally by the cloacal membrane. The allantois, an endodermal outpouching associated with the yolk sac region, enters the cloaca and extends into the connecting stalk (future umbilical cord). The cranially advancing urorectal septum partitions the cloaca into the anorectal canal and the urogenital sinus (approximately by the middle of the second month). The urinary bladder develops from the cranial (vesical) part of the urogenital sinus. Cranially, the bladder initially remains connected to the umbilical region via the allantoic stalk (urachus); this tract normally obliterates before birth and persists as the median umbilical ligament (plica umbilicalis mediana). Depending on the failure of the regression, various urachal remnants may persist.

The ureter develops from the ureteric bud, which sprouts from the distal Wolffian (mesonephric) duct. Remodeling and incorporation of distal Wolffian duct segments into the bladder wall separate the ureter functionally from the Wolffian duct and shift the ureteral orifices cranially and laterally within the bladder wall. The incorporated segment contributes to formation of the bladder trigone. The remaining Wolffian duct in males forms the ejaculatory duct in the region of the prostatic urethra.

Embryology of the Male External Genitalia

Indifferent Development of the External Genitalia

Until approximately week 8 (crown–rump length of about 25 mm), the external genitalia develop in an indifferent pattern [Fig. indifferent development of the external genitalia]. In the region of the cloacal membrane, the genital tubercle and the urogenital and anal primordia form. After cloacal partitioning, the anterior component becomes the urogenital sinus. Lateral to the urogenital groove, the urethral folds develop, and farther laterally the labioscrotal swellings form. Endoderm-derived tissue from the urogenital sinus extends into the urogenital plate.

Embryology of the Male External Genitalia

The genital tubercle elongates to form the phallus, and the glans penis differentiates at its distal end. The urogenital groove closes through fusion of the urethral folds over the sinus region, thereby forming the spongy (penile) urethra. The line of fusion remains visible in the midline as the perineal, scrotal, and penile raphe. Canalization of the distal urethral Anlage begins with an ectodermal invagination, followed by canalization, formation of the fossa navicularis, and connection to the spongy urethra.

The prepuce forms through an epithelial lamella that grows inward from the distal aspect of the penis. Complete foreskin retraction is often not possible after birth and gradually increases during childhood. The bulbourethral glands (Cowper glands) arise from endoderm-derived tissue of the urogenital sinus.

Hypospadias is a common congenital disease of the penis with an abnormal ventral opening of the meatus of the urethra, a deficient ventral prepuce (dorsal hood) and an uretral plate reaching from meatus to the tip of the glans (chordee).

References

Benninghoff, A. (1993): Makroskopische Anatomie, Embryologie und Histologie des Menschen. 15. Auflage. München; Wien; Baltimore: Urban & Schwarzenberg.

  Deutsche Version: Embryologie: Entwicklung des unteren Harntrakts

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