Male urethra

The male urethra is a tubular structure that serves as a passage for urine and, in most species, also for semen during ejaculation. It extends from the urinary bladder to the external urethral orifice at the tip of the penis.

1. General Structure and Divisions

The male urethra is divided into two main parts:

• The pelvic part (pelvic urethra) lies within the pelvic cavity, passing through the prostate in some species (e.g., dogs). Subdivided into:

  • Preprostatic part (very short; between the bladder and prostate, not distinct in most animals)

  • Prostatic part (surrounded by the prostate gland)

• The penile part (penile urethra) runs through the penis and is surrounded by the corpus spongiosum.

2. Histological Layers

• Mucosa: Transitional epithelium in the proximal regions, gradually becoming stratified columnar or squamous distally.

• Submucosa: Contains urethral glands (especially in the pelvic urethra) which contribute to seminal plasma.

• Muscularis: Two smooth muscle layers—inner longitudinal and outer circular. In distal segments, skeletal muscle may contribute to the external urethral sphincter (urethralis muscle).

• Adventitia or Serosa: Outer connective tissue covering.

3. Species Variations

Dog

• Urethra runs through the corpus spongiosum, along the ventral aspect of the penis.

• Prostate surrounds the entire proximal urethra.

• The urethra has a sigmoid flexure only in the fetus.

Cat

• Similar to dog but with smaller prostate and additional bulbourethral glands.

• Urethra is narrow and vulnerable to obstruction by uroliths, especially at the penile tip.

Horse

• Urethra opens at the tip of the glans penis through the urethral process.

• Surrounded by well-developed corpus spongiosum.

• Urethral sinus and fossa present at the glans tip (common site for smegma accumulation).

Ruminants (e.g., bull, ram)

• Long urethral process extends beyond the glans.

• S-shaped sigmoid flexure in both the penis and the urethra.

• Prone to urolithiasis and obstruction at the sigmoid flexure and urethral process.

Pig

• Corkscrew-shaped penis; urethra follows this helical path.

• Long, narrow urethra; accessory sex glands are highly developed.

4. Functional Aspects

• Urinary Function: Conducts urine from the bladder to the outside.

• Reproductive Function: During ejaculation, semen passes through the urethra. The urethral crest and colliculus seminalis (elevated area in the prostatic urethra) serve as sites of ductal openings for the ejaculatory ducts, prostate, and seminal vesicles.

5. Clinical Relevance

• Urethral Obstruction: Common in small ruminants and cats; often due to uroliths.

• Catheterization: Varies by species due to anatomical constraints (e.g., prepuce in pigs, urethral recess in bulls).

• Surgical Access: Knowledge of urethral segments is essential for urethrostomy procedures (e.g., perineal urethrostomy in cats).