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. View Media Gallery

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.

Male urethra and its segments. View Media Gallery

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]