Male reproductive system

Penis

The penis consists of a root and a free body enveloped in skin that is thin, elastic and largely devoid of hair or glandular elements ( Fig. 72.1 ). The root is attached to the inferior surface of the perineal membrane and consists of a central bulb and a laterally placed crus on either side, angled towards the ischiopubic ramus. The expanded terminal part of the penis is the glans, containing a rounded base, the corona, which separates it from the shaft. The glans is covered by the prepuce (foreskin), a loose fold of retractable skin attached to the ventral surface of the glans under the corona at the frenulum. The urethra opens at the apex of the glans: this is its narrowest point and is a common entry point for endo-urological procedures such as cysto-urethroscopy and ureteroscopy.

The penile shaft contains erectile columns. The crus continues forwards to become the paired corpora cavernosa and the bulb becomes the corpus spongiosum ( Fig. 72.2 ). The corpora cavernosa lie side by side, separated by the spongiosum in the midline on the ventral surface of the penis. The corpus spongiosum contains the penile urethra throughout its entire length. It expands distally to form the glans penis, which sits over the tips of the corpora cavernosa over Buck's fascia. Appreciation of this anatomical plane led to the development of glansectomy rather than partial penectomy for penile cancers confined to the glans.

The deep dorsal vein lies in the dorsal groove between the two corpora cavernosa, flanked laterally by a pair of dorsal arteries. The branches of the dorsal nerves lie lateral to the arteries and fan out over the rest of the dorsal surface of the penis, extending laterally round to the ventral side. At its base, the penis is suspended by two ligaments (the fundiform and suspensory ligaments of the penis), which are continuous with Buck's fascia of the penis and anchor it to the pubic symphysis. Rupture of the suspensory ligament can mimic a penile fracture but usually presents with more limited bruising and tenderness at the dorsal root of the penis, with a loss of stability of the penis in full erection and development of a ‘wandering’ penis.

The deep (Buck's) fascia of the penis surrounds the three corpora. It is an extension of the deep perineal fascia, blending distally with the tunica and proximally becoming continuous with dartos and the deep perineal fascia, helping to fix the spongiosum and crura to the pelvic bones and perineal fascia. It is important to remember that Buck's fascia lies immediately superficial to the dorsal neurovascular bundle, which directly overlies the tunica albuginea. The superficial dartos fascia, a continuation of Colles’ fascia of the perineum, lies more superficially. This layer contains all the superficial veins (including the superficial dorsal vein), arteries, nerves and lymphatics, and must be appreciated during penile surgery. Exposure of the entire penile shaft – for penile fracture repair, for example – is best achieved by degloving the penile shaft skin with the dartos fascia, by creating a plane under this fascia. It can be stripped back very readily over Buck's fascia, paying careful attention to the few communicating vessels running between the superficial and deep fascial layers. Mobilization of the penile skin with its dartos fascia ensures maintenance of the integrity and viability of the overlying skin and is also the basis for penile skin flaps used in some hypospadias repairs and other penile reconstructions. To achieve the best cosmetic and functional results, closure of any penile wound in two layers to re-establish a continuous dartos fascia as well as skin layer is advisable, especially in cases of revision surgery.

An understanding of the layers of the penis is also useful clinically in penile trauma. If Buck's fascia remains intact, urine or blood that is extravasated will be contained within the penile shaft (leading to the development of an ‘aubergine or eggplant penis’). However, disruption of Buck's fascia leads to extravasation into the superficial perineal pouch, causing a perineal haematoma in a butterfly distribution because the extent of spread is limited by an intact Colles’ fascia. If Colles’ fascia is disrupted, urine and blood can spread into the abdominal wall and scrotum. When the penis is being operated on, closure of Buck's fascia, if opened, is good practice to assist with both haemo­stasis of the deeper structures and prevention of haematoma into the more superficial layers. While this should be readily achieved with elective operations, such as plication or plaque incision and graft operations for Peyronie's disease, it may not be possible in cases of penile trauma. Additional mobilization of Buck's fascia for a centimetre on either side of the incision will aid subsequent closure of this layer.

Within Buck's fascia, the erectile columns are enveloped by a tough fibrous membrane, the tunica albuginea of the corpus ( Fig. 72.3 ). This consists of an outer longitudinal layer and an inner circular layer enveloping the two corpora cavernosa. Fibres extend from the circular layer to form a vertical septum separating the two cavernosa. Deposition of plaques of collagen and irregular fibrin can occur within the tunica albuginea, leading to the curvature seen in Peyronie's disease.

Expansion of the sinusoidal spaces within the corpora during erection compresses subtunical and emissary veins against the tough tunica albuginea, which results in entrapment of blood in the corporal space and the rigidity of the erection. As the penis becomes erect, the tunica albuginea stretches and thins, increasing its susceptibility to injury/rupture in this state (penile fracture). The thinnest part of the tunica lies ventrally, beneath and adjacent to the corpus spongiosum, and is the area most vulnerable to rupture with a penile fracture. Direct closure and repair of the tunica can effectively restore its integrity to allow a return of normal erectile function. However, more extensive operations on the tunica, such as plaque incision and grafting operations in more significant cases of Peyronie's disease, can lead to erectile dysfunction as a result of impairment and disruption of the venous occlusive mechanism of the penis. The tunica albuginea surrounding the corpus spongiosum is considerably thinner and more elastic than that surrounding the cavernosa. This allows expansion of the urethra during ejaculation and also means that the urethra and surrounding spongiosum are less prone to rupture during penile fracture. Urethral/spongiosal injury should always be suspected in cases where there is any frank haematuria/meatal bleeding after penile fracture, or when both corporal bodies have tunical disruption, because the tear often extends behind and through the corpus spongiosum.

Vascular supply

The arterial supply of the penis is derived from the internal pudendal artery, a branch of the internal iliac artery. The common penile artery supplies the deep structures of the penis via the bulbourethral artery, the dorsal artery of the penis and the deep cavernosal artery of the penis ( Fig. 72.4 ). The bulbourethral artery enters the corpus spongiosum to supply the penile bulb, urethra and glans. The dorsal artery of the penis runs along the dorsal surface of the corporal bodies, as paired branches on either side of the dorsal penile vein. These vessels give off circumferential branches that supply the spongiosum and the urethra. The deep cavernosal arteries are paired and run along the centre of the corpora cavernosa towards the glans. The skin of the penis receives additional arterial supply via the external pudendal branches of the femoral arteries. It is important to be aware that the course, anastom­oses and branching patterns of the penile arteries are highly variable. A single cavernosal artery may supply both corporal bodies or may be absent; an accessory pudendal artery may supplement or replace the branches of the common penile artery.

Some of the venous return from the corpora arises from deep veins accompanying the arteries, which go on to join the internal pudendal veins. Most of the venous blood is drained via the deep dorsal vein, which lies in the midline groove between the two corpora cavernosa ( Fig. 72.5 ). It pierces the suspensory ligament, passing inferior to the pubic symphysis, and drains into the prostatic plexus. The superficial venous system on the dorsal and dorsolateral aspect of the penis drains the penile shaft and prepuce, forming a single superficial dorsal vein that drains into the great saphenous vein via the superficial external pudendal veins. The deep and superficial dorsal veins are the target of venous ligation surgery for erectile dysfunction due to ‘venous leak’; this surgery has very poor long-term outcomes, due to the highly developed collateral venous circulation of the penis.

Innervation

Paired dorsal nerves, branches of the pudendal nerve, provide a rich sensory innervation predominantly to the glans ( Fig. 72.6 ). These nerves run with the dorsal vein and arteries, and give off ventral branches as they run along the shaft of the penis. Parasympathetic and sympathetic supply is through the cavernous nerves from the pelvic plexus, which enter the corpora cavernosa at the crus. Parasympathetic supply (S1–4) travels via the nervi erigentes in the pelvic splanchnic nerves to reach the pelvic plexus. Stimulation of these nerves causes vasodilation and leads to the development of an erection. Sympathetic supply (T11–L1) travels in the sympathetic trunk before reaching the pelvic plexus and is responsible for contraction of the seminal vesicles and prostate, and hence ejaculation, and also leads to detumescence. Bulbospongiosus and ischiocavernosus, two superficial perineal muscles that contract spasmodically during ejaculation, are supplied by the perineal nerve, which also arises from the pudendal nerve.

Scrotum

The scrotum is a sac of skin that contains the two testes. The skin is thin, pigmented and rich in sebaceous and sweat glands. A midline raphe extends from below the urethral meatus, down the ventral shaft of the penis and scrotum to the anus. It represents the line of fusion of the genital tubercles and, being relatively avascular, is a common site of surgical incisions in scrotal surgery. Deep to the raphe, the scrotum is separated into two compartments by a septum composed of all the layers of the scrotal wall except the skin. The testes are suspended by the spermatic cords within these compartments.

Beneath the skin lie the dartos muscle and fascia, which are continuous with Colles’, Scarpa's and dartos fasciae of the penis ( Fig. 72.7 ). Deeper still lie the external spermatic fascia, the cremasteric fascia and the internal spermatic fascia, which are continuous with the equivalent layers covering the spermatic cord and hence are derived from the aponeuroses of the external oblique and internal oblique and the transversalis fascia, respectively. The internal spermatic fascia is loosely attached to the parietal layer of the tunica vaginalis before reaching the visceral layer and subsequently the tunica albuginea of the testis. The parietal and visceral layers of the tunica vaginalis are derived from the peritoneum and become continuous with the posterolateral border of the testis at the point where it is fixed to the scrotal wall. The gubernaculum in the inferior pole of the testis also attaches the scrotal wall to the testis. A high insertion of the tunica vaginalis on to the spermatic cord allows a longer length of cord with the freedom to move and twist; when coupled with deficiencies in the gubernaculum, this anatomical variation can predispose to testicular torsion (a bell-clapper deformity).