Male Urethra Anatomy

The male urethra is a narrow fibromuscular tube that conducts urine and semen from the bladder and ejaculatory ducts, respectively, to the exterior of the body (see the image below). Although the male urethra is a single structure, it is composed of a heterogeneous series of segments: prostatic, membranous, and spongy.

Most proximally, the prostatic urethra is responsible for involuntary continence, transmission of semen into the common genitourinary tract, and the most common site of bladder outlet obstruction in the Western world. The membranous urethra is critical to voluntary continence and, because of its rigid attachments, is highly susceptible to injury in pelvic trauma. The spongy urethra is surrounded by the corpus spongiosum and forms the terminal conduit communicating with the outside of the body.

Knowledge of male urethral anatomy is essential for all health professionals because urethral catheterization is one of the most commonly performed procedures in health care. The male urethra is susceptible to a variety of pathologic conditions, ranging from traumatic to infectious to neoplastic. Pathophysiologic variants of the urethra may have devastating consequences, such as renal failure and infertility.

The posterior male urethra forms from the urogenital sinus (see the image below). This sinus derives from the endoderm-derived cloaca, which is separated from the anorectal canal by the growth of the urorectal septum in the fourth week of gestation. The spongy urethra is formed after the seventh week by tubularization of the urethral folds along the urethral groove under the influence of dihydrotestosterone. The most distal portion of the urethra is likely formed by invagination of an epithelial tag at the distal end of the genital tubercle. ^[1]

The male urethra originates at the bladder neck and terminates at the urethral meatus on the glans penis. It is roughly 15-25 cm long in the adult and forms an "S" curve when viewed from a median sagittal plane in an upright, flaccid position (see the image below). The male urethra is often divided into 3 segments on the basis of its investing structures: prostatic urethra, membranous urethra, and spongy (or penile) urethra.

Other systems for naming the parts of the urethra have been described. The urethra can be separated simply into anterior and posterior urethra. The spongy urethra can be subdivided into fossa navicularis, pendulous urethra, and bulbous (bulbar) urethra. Finally, the bladder neck, or preprostatic urethra, can be classified as a distinct part of the urethra (see the image below).

Prostatic urethra

The prostatic urethra is the portion of the urethra that traverses the prostate. It originates in the region of the bladder neck, courses roughly 2.5 cm inferiorly, and terminates at the membranous urethra. It lies in a retropubic location and is bordered superiorly by the bladder and supported inferiorly by the sphincter urethrae externus muscle and the perineal membrane (formerly called the urogenital diaphragm). It is invested in the prostate, a glandular and fibrostromal organ that secretes seminal fluids and has clinical relevance.

The urethra runs through the prostate eccentrically, with most of the prostatic tissue in a posterior and inferior location. The prostatic urethra is surrounded by an inner circular layer and an outer longitudinal layer of smooth muscle. The urethra forms an angle of roughly 45º (range, 0-90º) at the midpoint of the prostatic urethra. The segment proximal to this location is surrounded by the involuntary internal sphincter. It is also the area most commonly affected by benign prostatic hyperplasia (BPH).

The posterior wall of the prostatic urethra contains the urethral crest, which is bordered laterally by prostatic sinuses, into which the prostatic glands drain. The most prominent aspect of this crest is the seminal colliculus, or verumontanum, where the paired ejaculatory ducts and the opening of the prostatic utricle (a small midline paramesonephric duct remnant) meet the lumen of the urethra. The seminal colliculus has no functional significance but is a crucial landmark in urethroscopy and transurethral surgery.

Membranous urethra

The shortest and least distensible portion of the urethra is the membranous urethra. This region spans from the apex of the prostate to the bulb of the penis. It is invested in the external urethral sphincter muscle and the perineal membrane.

The external sphincter is related anteriorly to the dorsal venous complex and is connected to the puboprostatic ligaments and the suspensory ligament of the penis. The external urethral sphincter muscle and the perineal membrane fix the urethra firmly to the ischial rami and inferior pubic rami, rendering this portion of the urethra susceptible to disruption with pelvic fracture.

Spongy urethra

The spongy urethra is the region that spans the corpus spongiosum of the penis. It is divided into the pendulous urethra and the bulbous (or bulbar) urethra. The pendulous urethra is invested in the corpus spongiosum of the penis in the pendulous portion of the penis. The urethra is located concentrically within the corpus spongiosum.

In the distal urethra lies the fossa navicularis, a small dilation of the urethra just proximal to the urethral meatus. The meatus is a slitlike orifice with its long axis in a midline sagittal plane. The urethral meatus is slightly ventral to the tip of the penis. The bulbous urethra is invested in the bulb of the penis, the portion of corpus spongiosum that lies between the split corpora cavernosa in the superficial perineal space.

Bulbourethral (Cowper) glands, a male homologue of the greater vestibular (Bartholin) glands, originate in the external urethral sphincter muscle but terminate in ducts that empty into the bulbous urethra. The spongy urethra lies closer to the dorsum of the penis in the bulb.

Vasculature and lymphatic drainage

The prostatic urethra is supplied by the inferior vesical artery, which branches to penetrate the prostate and the bladder neck in superolateral positions. The bulbourethral artery supplies the membranous and bulbar urethra, whereas the pendulous urethra is supplied by the deep penile artery, a branch of the internal pudendal artery. In general, venous drainage mirrors the arterial supply. The prostatic and membranous urethra drain to the obturator and internal iliac nodes.

Lymphatic drainage from the spongy urethra drains to the deep and superficial inguinal nodes. ^[2]

The male urethra is a fibromuscular tube. It has distinct longitudinal folds that protrude into its lumen and make it readily identifiable on cross-section. The lining of the urethra varies from segment to segment but transitions from the urothelium of the bladder to the keratinized stratified squamous epithelium of the glans. The prostatic urethra is lined with transitional cell epithelium (urothelium). The membranous urethra is lined with stratified columnar and pseudostratified epithelium. Also, a rich vascular submucosa exists in the membranous urethra.

Finally, the penile urethra is enclosed by the corpus spongiosum and lined with stratified columnar and pseudostratified epithelium with stratified squamous epithelium distally. The entire posterior urethra is lined with a submucosa and a series of muscular sphincters. The urethra is lined on the dorsal surface by the glands of Littre, which are concentrated more distally. Additionally, small diverticula, called lacunae of Morgagni, and a larger lacuna magna can be found at the fossa navicularis. ^[3]

Although the male urethra varies in length and angulation, no common natural variants in urethral anatomy are recognized. Pathophysiologic variants include duplication, urethrorectal fistulae, congenital strictures, hypospadias, epispadias, and posterior or anterior urethral valves.

Duplication

Urethral duplication is a rare anomaly that typically occurs in the sagittal plane. Incontinence and infection are presenting symptoms; these anomalies can usually be picked up on newborn examination. A dorsal duplication is associated with a normal urethral meatus at the tip of the glans and an epispadiac urethra and dorsal chordee. The dorsal urethra may be blind-ended or associated with bladder exstrophy.

Ventral duplication and duplication in the same horizontal plane are other variants of urethral duplication. Treatment is necessary if incontinence or infections are an issue. Fulguration or excision of the abnormal urethra is the standard therapy.

Urethrorectal fistulae

Urethrorectal fistulae are rare and are usually associated with imperforate anus. The signs of this are passage of stool and air through the meatus. Alternatively, in cases with a patent anus, urine may pass via the anus. Management of urethrorectal fistula in the presence of an imperforate anus involves either opening the anus and closing the fistula or fecal diversion if the distance between the blind-ended rectum and the perineum is too far for immediate reconstruction.

Congenital strictures

Congenital urethral strictures are rare but most commonly occur at the membranous urethra and fossa navicularis. They can be diagnosed with excretory urography, retrograde urethrography, or urethroscopy. Treatment involves direct-vision internal urethrotomy for membranous or fossa navicularis strictures or dilation for membranous strictures. Failure of endoscopic therapy warrants reconstruction.

Hypospadias

Hypospadias is the most common urethral anomaly in males, occurring in 1 in 300 live births. In addition to a ventral ectopic urethral meatus, the typical physical findings include an incomplete dorsal hood appearance to the foreskin and a ventral chordee. Between the eighth and 15^th weeks of gestation, under influence of dihydrotestosterone, the urethral folds fuse, and the glans canalizes to form the urethra.

Failure of this fusion may occur with in utero exposure to estrogens or progestins. Hypospadias is classified in severity according to the location of the failed fusion. Most cases of hypospadias are distal (ie, glanular or coronal). More proximal cases (ie, penile shaft, penoscrotal, or perineal hypospadias) may necessitate extensive and staged reconstructive efforts. The goal of treatment is to provide a functional penis that allows the boy to void while standing and deposit semen in the vagina.

It is critical that boys with hypospadias are not circumcised in the newborn nursery; the extra foreskin may be used for reconstruction. Penoscrotal and perineal hypospadias are evidence of feminization and may indicate a need an evaluation for disorder of sexual differentiation. ^[4]

Epispadias

Epispadias is rare, occurring in 1 in 120,000 males. This pathologic variant results from failure of the genital tubercle to migrate appropriately in the fifth week of gestation. As a result, the urethral meatus is on the dorsum of the penis or at the penopubic junction. Proximal epispadias is often associated with incontinence and dorsal chordee. Severe epispadias may be associated with bladder exstrophy. Cosmesis is good with urethroplasty and correction of chordee, but continence is difficult to achieve surgically. ^[5]

Posterior urethral valves

Posterior urethral valves are thin membranes of mucosa in the distal prostatic urethra that cause varying degrees of obstruction when the child voids. They occur in 1 in 8,000 to 25,000 live births and are responsible for 10% of in utero diagnoses of obstructive uropathy. Antenatal ultrasonography is usually helpful in identifying clinically significant cases of posterior urethral valves, revealing a constellation of oligohydramnios, thickened bladder wall, full bladder, dilated posterior urethra, and bilateral hydroureteronephrosis.

Management involves early drainage of the bladder, ablation of valves, and long-term management of bladder dysfunction and renal failure. Roughly 30% of boys progress to end-stage renal disease. ^[6]

Anterior urethral valves

Anterior urethral valves are far less common than posterior urethral valves. Unlike posterior urethral valves, they tend to manifest later in childhood, often with voiding symptoms or infection, and they have fewer long-term sequelae of hydronephrosis and renal failure. Typically, an anterior urethral valve is not a true valve but, rather, a diverticulum in the spongy urethra that balloons under the pressure of micturition; the distal edge of this diverticulum causes obstruction of the urethral lumen.

Anterior urethral valves can be managed with transurethral ablation, but if they are associated with a large diverticulum, open excision may be necessary.