Overview
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.
Male urethra and its segments.
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.
Gross Anatomy
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]
Embryologic development of pendulous urethra.
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).
Posterior wall of urethra.
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 45 º (range, 0-90 º) 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]
Microscopic Anatomy
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]
Pathophysiologic Variants
Although the male urethra is subject to
varying length and angulation, no common natural variants in urethral anatomy
exist. 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
15th 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.
References
- Park JM. Normal Development of the Urogenital System. In: Wein et al. Campbell-Walsh Urology. 4. 9. 2007:3121-48.
- Brooks JD. Anatomy of the Lower Urinary Tract and Male Genitalia. In: Wein et al. Campbell-Walsh Urology. 1. 9. 2007:38-77.
- Mescher AL. The Male Reproductive System. In: Mescher AL. Junqueira's Basic Histology: Text and Atlas. 12. 2010:Ch. 21.
- McAninch JW. Disorders of the Penis & Male Urethra. In: Tanagho EA and McAninch JW. Smith's General Urology. 17. 2008:625-37.
- Gearhart JP and Mathews R. Exstrophy-Epispadias Complex. In: Wein et al. Campbell-Walsh Urology. 4. 9. 2007:3497-553.
- Casale AJ. Posterior Urethral Valves and Other Urethral Anomalies. In: Wein et al. Campbell-Walsh Urology. 4. 9. 2007:3583-603.
- Jordan GH and Schlossberg SM. Surgery of the Penis and Urethra. In: Wein et al. Campbell-Walsh Urology. 1. 9. 2007:1023-97.
- Moore KL and Agur AMR. Pelvis and perineum. In: Moore KL and Agur AMR. Essential Clinical Anatomy. 2. 2002:209-73.
- Tanagho EA. Anatomy of the Genitourinary Tract. In: Tanagho EA and McAninch JW. Smith's General Urology. 17. 2008:1-16.
