spongy urethra

**Anatomy – the 4 parts of the male urethra (proximal to distal):** 1. Preprostatic (intramural) urethra – bladder neck; surrounded by the internal urethral sphincter (smooth muscle, sympathetic control). 2. Prostatic urethra – contains the urethral crest, seminal colliculus (verumontanum), and prostatic utricle (müllerian remnant); ejaculatory ducts and prostatic ducts open here. Widest and most dilatable part. 3. Membranous urethra – passes through the deep perineal pouch/urogenital diaphragm; surrounded by the external urethral sphincter (skeletal muscle, pudendal nerve, S2–S4). Shortest and least distensible; narrowest part except the external meatus. 4. Spongy (penile/cavernous) urethra – longest (~15 cm), traverses the corpus spongiosum.

**Subdivisions and features of the spongy urethra:** - Bulbar urethra: proximal, within the bulb of the penis. Bulbourethral (Cowper) gland ducts open here (glands themselves lie in the deep perineal pouch). It is dilated. - Penile (pendulous) urethra: runs through the penile shaft. - Fossa navicularis: dilatation in the glans just proximal to the external meatus. - Glands of Littré (urethral glands) open into the spongy urethra and mucus secretion lubricates it. - External urethral meatus: the narrowest part of the entire urethra.

**Histology:** Mostly pseudostratified or stratified columnar epithelium. The fossa navicularis and distal meatus have nonkeratinized stratified squamous epithelium. (Prostatic urethra = transitional/urothelium; membranous = pseudostratified/stratified columnar.)

**Embryology:** Proximal urethra (prostatic, membranous) derives from the endoderm of the urogenital sinus. Most of the spongy urethra forms by fusion of the urethral folds (endoderm). The distal glans portion (fossa navicularis) arises from an ectodermal glandular plate/cord that canalizes. Failure of fusion of the urethral folds causes hypospadias (ventral opening; associated with chordee). Epispadias (dorsal opening) results from faulty genital tubercle positioning and is associated with bladder exstrophy.

**Clinical significance:** - Straddle injury → rupture of the bulbar (spongy) urethra, below the urogenital diaphragm. Urine and blood extravasate into the superficial perineal pouch. They are limited by Colles fascia and can spread into the scrotum, around the penis, and up the anterior abdominal wall deep to Scarpa fascia. They are prevented from reaching the thighs by fascial attachments to the fascia lata, and from the anal region by the perineal body/posterior attachment of the urogenital diaphragm. Presentation: perineal/scrotal butterfly hematoma, blood at the meatus, inability to void. Diagnose with retrograde urethrogram before catheterization. - Compare: pelvic fracture → membranous urethra injury (above/at the urogenital diaphragm), with a high-riding prostate and extravasation into the retropubic space. - Gonococcal urethritis commonly affects the spongy urethra. Chronic inflammation leads to strictures, usually of the bulbar urethra. - Urethral stricture: obstructive voiding symptoms. Evaluate with retrograde urethrogram. - Catheterization trauma: false passage commonly occurs at the bulbomembranous junction. - Lymphatic drainage: spongy urethra → deep inguinal nodes; glans/distal urethra → deep inguinal nodes (and external iliac); prostatic and membranous urethra → internal iliac nodes. - Urethral carcinoma: squamous cell carcinoma is the most common type overall, especially in the bulbomembranous urethra. - Innervation: pudendal nerve (somatic sensory); the corpus spongiosum is supplied by the bulb artery and urethral artery from the internal pudendal artery.

**High-yield pearls:** Spongy urethra = longest segment; anterior urethra = spongy (bulbar + penile). Posterior urethra = prostatic + membranous. Anterior urethral injury → straddle injury (bulbar). Posterior urethral injury → pelvic fracture.