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ANATOMY: URETHRA

Gross Structure

• Normal diameter: 8-9mm
• Categorized into (2): posterior vs. anterior urethra; separated by perineal membrane
  • Posterior urethra
    • Begins distal to the bladder neck and transitions to the anterior urethra at the perineal membrane
    • Comprised of (3):
      1. Bladder neck
      2. Prostatic urethra
         • Spans the length of the prostate; extends from the distal bladder neck to the distal end of the verumontanum
         • In greater proximity to the anterior surface of the prostate
         • The midpoint turns ≈35° (may range from 0-90° depending on variable anatomy) anteriorly and separates the prostatic urethra into (2):
           1. Pre-prostatic (proximal) segment
              • Periurethral glands drain into the pre-prostatic urethra.§ These glands can contribute significantly to prostate volume and can be one of the sites of origin of BPH, in addition to the transition zone
           2. Prostatic (distal) segment
              • After forming at the juncture of the vas deferens and seminal vesicles, the ejaculatory ducts travel ≈2 cm through the prostate surrounded by circular smooth muscle, until they finally open into the distal prostatic urethra. The two small openings of the ejaculatory ducts are located on either side of the utricular orifice.
         • The verumontanum is formed by the widening and protrusion of the urethral crest from the posterior wall
         • The prostatic utricle orifice (Müllerian remnant) appears like a slit at the apex of the verumontanum

            

Prostatic Urethral Anatomy

Source: Wikipedia

 

3. Membranous urethra
   • Spans between the prostatic apex and the perineal membrane
   • Normally ≈2-2.5 cm in length
   • The external (striated) sphincter
     • Partially surrounds the membranous urethra
     • Comprised of both intrinsic smooth muscle and rhabdosphincter (striated muscle)§
     • Innervated by the pudendal nerve
     • Shaped like a signet ring
• Anterior urethra: begins at the perineal membrane and continues distally to the urethral meatus
  • Comprised of (3):
    1. Bulbar urethra
       • Spans between the perineal membrane and the suspensory ligament of the penis
       • The lumen widens to form the urethral bulb
       • The bulbourethral glands, also known as Cowper glands, empty into this region at the 3 and 9 o’clock positions. The bulbourethral glands themselves are located more proximally on either side of the membranous urethra (see figure above).
       • The bulbar urethra is invested by the thickest portion of the corpora spongiosum.
    2. Penile urethra
       • Spans between the suspensory ligament of the penis to the fossa navicularis
         • Measures ≈15 cm in length in its entirety from the suspensory ligament to the meatus (includes fossa navicularis)

 

Corpus spongiosum surrounds penils urethra
Source: Wikipedia

 

• The submucosal glands of Littre are composed of small mucus-secreting cells that lubricate the urethra before ejaculation; they empty into orifices on the posterior wall of the penile urethra
  • Note that some sources refer to submucosal glands of Littre as periurethral glands but these are distinct than those described in the pre-prostatic urethra§
3. Fossa navicularis
• Located entirely within the glans penis
• Surrounded by corpus spongiosum; the penile urethra is positioned more dorsally than ventrally within the corpus spongiosum
  • In the bulbar urethra, the corpus spongiosum is surrounded by the bulbocavernosus muscle

Microanatomic structure

• Posterior urethra lined with transitional epithelium
  • The transitional urothelium may extend into prostatic ducts. These ducts need to be sampled in patients with persistently positive cytology.
• Anterior urethra lined with pseudostratified columnar epithelium that changes to stratified squamous epithelium in the fossa navicularis

Arterial supply

• Bulbourethral artery supplies the urethra, corpus spongiosum, and glans penis
  • Anterior branch of internal iliac » internal pudendal » the common penile artery » bulbourethral artery

Venous drainage

• Drains to the pudendal plexus, which drains into the internal pudendal vein.

Lymphatic drainage

• Drains to the internal iliac and common iliac nodes

Questions

1. Describe the anatomical subdivisions of the urethra.
2. What seperates the anterior and posterior urethra?
3. Which part of the urethra do the periurethral glands drain into? Ejaculatory ducts? Cowper’s glands? Submucosal glands of Littre?
4. Which part of the urethra is surrounded by corpus spongiosum?

Answers

1. Describe the anatomical subdivisions of the urethra.
   1. Posterior urethra
      1. Bladder neck
      2. Prostatic urethra
      3. Membranous urethra
   2. Anterior urethra
      1. Bulbar urethra
      2. Penile urethra
      3. Fossa navicularis
2. What seperates the anterior and posterior urethra?
• Perineal membrane
3. Which part of the urethra do the ejaculatory ducts drain into? Cowper’s gland? Submucosal glands of Littre?
   • Periurethral glands: proximal prostatic urethra
   • Ejaculatory ducts: distal prostatic urethra
   • Cowper's glands: bulbar urethra
   • Submucosal glands of Littre: penile urethra
4. Which part of the urethra is surrounded by corpus spongiosum?
   • Anterior urethra

References

• Wein AJ, Kavoussi LR, Partin AW, Peters CA (eds): CAMPBELL-WALSH UROLOGY, ed 11. Philadelphia, Elsevier, 2015, chap 21
• Wessells, Hunter, et al. "Male urethral stricture: American urological association guideline." The Journal of urology 197.1 (2017): 182-190.