The male reproductive organs

Fig. 6.13 Retrograde urethrogram in the male: oblique view.

1. Balloon of catheter in navicular fossa 2. Penile urethra 3. Bulbous urethra 4. Membranous urethra 5. Impression of verumontanum in prostatic urethra 6. Filling of utricle (not usually seen) 7. Air bubbles in contrast

The female urethra

This is 4 cm long. It extends from the internal urethral sphincter at the bladder neck through the urogenital diaphragm to the external urethral meatus anterior to the vaginal opening. It curves obliquely downward and anteriorly. Multiple tiny urethral glands (of Skene) open into the urethra at the external meatus. The external urethral sphincter is at the urogenital diaphragm but is less well developed than that in the male. The involuntary internal urethral sphincter at the bladder neck appears to be more important in females.

THE MALE REPRODUCTIVE ORGANS

(see Figs 6.7, 6.12-6.18)

The prostate gland (Fig. 6.14) This gland is shaped like an upside-down truncated cone and surrounds the base of the bladder and the proximal (prostatic) urethra, extending inferiorly to the urogenital diaphragm and external sphincter. It has: • A base related to the bladder above; • An apex inferiorly sitting on the pelvic (urogenital) diaphragm; • An anterior wall which is separated from the pubic symphysis by the retropubic fatty space (of Retzius); • A posterior wall related to the rectum behind; and • Two inferolateral walls related to the muscles of the pelvic side wall and the anterior part of levator ani on either side.

The urethra enters the upper part of the prostate near its anterior surface and then assumes a somewhat more central position before curving slightly anteriorly again to exit at the apex of the gland.

Posterosuperiorly are the seminal vesicles. Fascia known as Denonvillier's fascia separates the prostate and seminal vesicles from the rectum. The puboprostatic ligament runs from the anterior inferior part of the gland to the pubic bone and provides support. The prostate gland has a fibrous (true) capsule and is further surrounded by a fibrous sheath derived from pelvic fascia and continuous w i th the puboprostatic ligaments. The prostatic venous plexus lies between the true capsule and the fibrous sheath. The prostatic capsule is thin and is frequently deficient posteriorly.

According to traditional anatomy, the gland is described as having the following lobes which are not well demarcated from one another:

• A muscular anterior lobe (or isthmus) which is anterior to the urethra and is composed mainly of fibromuscular fibres and contains little if any glandular tissue; • A posterior lobe which is posterior to the urethra and inferior to the insertion of the ejaculatory ducts; • A median lobe between the urethra and the ejaculatory ducts; and • Two lateral lobes, which form the bulk of the gland.

The five lobes can only be differentiated in the fetus up to 20 weeks' gestation; in the mature gland only three lobes two lateral and one median - can be distinguished, w i th the fibromuscular stroma anteriorly. These lobes may be palpated through the rectum.

Zonal anatomy of the prostate (Fig. 6.14) The gland may more usefully be described based on its internal architecture as having three glandular zones w i th the non-glandular isthmus anteriorly: • The peripheral zone, which contains about 70% glandular tissue; • The central zone, which contains 25% glandular tissue; and • The transition zone, which contains 5% glandular tissue in the normal prostate.

The central zone surrounds the urethra above the level of the ejaculatory ducts which pass through it, and roughly corresponds to the median lobe. The transition zone is a narrow area surrounding the urethra inside the central zone at the level of the ejaculatory ducts and

Fig. 6.14 Prostate gland: zonal anatomy.

verumontanum. The peripheral zone makes up the rest, occupying the posterior, lateral and apical parts of the gland. The peripheral zone is separated from the central and transitional zones of the gland by a fibrous layer. The prostate is contained within a sheath or false capsule derived from pelvic fascia. This sheath is composed of smooth muscle, skeletal muscle and loose connective tissue, penetrated by numerous vessels and nerves. This is a layer of concentrically orientated fibromuscular tissue that is inseparable from the prostatic stroma histologically, but which contains no glandular elements. This layer is absent at the apex.

With ageing, the transition zone hypertrophies and the central zone atrophies. For radiological purposes the central and transitional zones cannot be distinguished and the entire inner gland is usually referred to as the central gland. The relevance of the zonal anatomy is that the majority of prostate cancers occur in the peripheral zone of the gland. Thus imaging of the prostate for local staging of cancer and image-guided biopsies for diagnosis focus on this region. Benign prostatic hypertrophy affects the transition zone in the central gland and, when pronounced, occupies most of the volume of the enlarged prostate.

The seminal vesicles

(see Figs 6.12, 6.15b and 6.28b) These are paired sacculated diverticula that lie transversely behind the prostate and store seminal fluid. These convoluted tubes narrow at their lower end to fuse w i th the vas deferens to become the ejaculatory ducts.

Blood supply of prostate gland and seminal vesicles Blood supply is from the internal iliac arteries, mainly via branches from the inferior vesical artery. Venous drainage is via the prostatic venous plexus, which surrounds the sides and base of the gland, to the internal iliac veins. The prostatic venous plexus lies between the true capsule and the fibrous outer sheath of the gland. The prostatic venous plexus communicates posteriorly w i th the internal vertebral venous plexus, providing a potential route for the spread of prostatic cancers.

Nerve supply Innervation is from parasympathetic fibres from the pelvic splanchnic nerves (S2-S4).

Lymph drainage This is via lymphatic channels that accompany blood vessels to internal iliac and sacral nodes.

Radiology of the prostate gland and seminal vesicles

Ultrasound

The prostate gland is most frequently imaged by transrectal ultrasound (usually to direct biopsy). This is achieved w i th an ultrasound probe in the rectum. Inbuilt linear and sector scanners allow longitudinal and transverse imaging of the gland, which lies immediately anteriorly. The prostate gland yields uniform, low-level echoes. The seminal vesicles can be identified as sacculated structures above and lateral to the base of the gland. The urethra is seen as an anechoic central structure w i th a thin echogenic wall. It is surrounded by a relatively hypoechoic area comprised of the smooth muscle, the periurethral glands and the transition zone (in the normal non-hypertrophied gland). The central zone is that part of the gland surrounding and above the ejaculatory ducts. The ejaculatory ducts may sometimes be identified forming a triangular hypoechoic 'beak' as they project into the central zone. The posterolateral peripheral zone can generally be identified and may be separated from the lateral aspect of the gland by a hypoechoic line. The prostatic 'capsule' or outer fibromuscular layer is not well defined and prominent vessels in the periprostatic soft tissues may blend w i th the gland sono¬ graphically, making its margin difficult to define. The peripheral zone is isoechoic and the central part of the prostate is relatively hypoechoic, but may contain hyperechoic foci or areas of calcification. Because the anatomical zones cannot be truly differentiated on imaging, it has been suggested that the peripheral zone be referred to as the 'outer gland' and the inner part of the prostate be referred to as the 'inner gland'.

Cross-sectional imaging MRI is an excellent method of imaging the prostate, which is seen on routine imaging (see Fig. 6.15) but is best assessed w i th an endorectal coil.

The zonal anatomy is seen on the T2-weighted image. The peripheral zone is of uniformly high intensity and contrasts w i th the intermediate signal intensity of the transitional and central zones. As the transition zone hypertrophies and

Fig. 6.15 MRI scan of male pelvis: (a) sagittal image through midline showing urethra; (b) sagittal image to the right of midline showing seminal vesicle.

1. Bladder 2. Proximal urethra 3. Internal urethral sphincter 4. Prostate gland 5. Symphysis pubis 6. Retropubic fatty space (of Retzius) 7. Sigmoid colon 8. Rectum 9. Anal canal 10. Coccygeus muscle 11. Levator ani 12. Corpus spongiosum 13. Part of corpus cavernosum 14. Rectus muscle 15. Sacrum 16. Spinal canal containing

CSF and sacral nerve roots 17. Seminal vesicle

compresses the peripheral zone w i th ageing and w i th benign prostatic hyperplasia, a low-signal band separates them on T2-weighted images. The transitional and central zones cannot be distinguished. The urethra is of relatively high signal intensity on T2-weighted images. The periprostatic venous plexus is of high signal intensity on T2 images, as is the fat in the retropubic space of Retzius, contrasting w i th the low-intensity puboprostatic ligament and pubo¬ coccygeus muscle. The anatomical outer 'capsule' is also seen as a low-signal outer band. Fat-suppressed T2-weighted images improve contrast between the peripheral zone and the surrounding fat. Denonvillier's fascia may be identified as a low-intensity line between the posterior aspect of the prostate and the rectum.

On T1-weighted images the prostate and the venous plexus and the seminal vesicles are of relatively low signal intensity and the zonal anatomy cannot be distinguished. The neurovascular bundles may be identified postero¬ laterally, entering the gland as hypointense structures outlined by high-signal periprostatic fat. They are important to assess preoperatively for involvement by tumour. 'Nerve-sparing' prostatic surgery may be attempted in an effort to preserve potency if the neurovascular bundle is not affected.

The seminal vesicles are seen posterior to the prostate and bladder on axial and sagittal images.

On CT (see Fig. 6.28b) the gland is seen as a round structure of soft tissue density inferior to the bladder. Its margin is well seen where outlined posteriorly by fat, and the seminal vesicles are identified as convoluted structures above prostate between bladder and rectum. Periprostatic vessels are frequently identified in the surrounding fat. However, the zonal anatomy is not distinguished, and at the apex where it abuts the lower fibres of levator and is surrounded by the external urethral sphincter, the prostatic margins cannot be distinguished.

Vasography (injection of contrast into the vas), occasionally performed in the investigation of male infertility, fills the seminal vesicles and outlines the ejaculatory ducts in the normal situation.

The testis, epididymis and spermatic cord

(Fig. 6.16; see Fig. 6.17)

The testis This is an oval sperm-producing gland that is described as having upper and lower poles. It is suspended by the spermatic cord in the scrotal sac and is covered by a tightly adherent fibrous capsule called the tunica albuginea. The tunica is thickened posteriorly and forms a fibrous septum known as the mediastinum testis. Multiple fibrous septae divide the testis into lobules. The tunica albuginea, mediastinum and fibrous septae connect to form the support and internal architecture of the testis. Some 250-400 lobules contain sperm-producing cells, which are drained by highly

Fig. 6.16 Testis and epididymis: (a) internal architecture.

coiled seminiferous tubules. The seminiferous tubules then converge on the mediastinum testis, forming 20 or so larger ducts. These enter the mediastinum testis, forming a network of channels within the stroma known as the rete testis. From here, 10-15 efferent ductules from the superior mediastinum pierce the tunica near the upper pole to convey sperm to the head of the epididymis.

The upper pole of the testis has a small sessile projection called the appendix of the testis, which is situated just below the head of the epididymis. This is present in over 90% of cases and is a remnant of the mullerian duct. Other testicular appendages may also exist.

The testis and epididymis are invaginated anteriorly into a double serous covering, the tunica vaginalis (analogous to the invagination of bowel into the peritoneum). The tunica vaginalis is continuous w i th the peritoneum during development via the processus vaginalis. This is normally obliterated at birth. This loose covering has visceral and parietal layers.

The outer covering of the testis and epididymis is the thick scrotum, which has a midline septum dividing the paired scrotal contents. This has several layers from external to internal. These comprise skin, tunica dartos (a thin fibroelastic vascular layer), and external spermatic, cremasteric and internal spermatic fasciae derived from the fascial layers of the anterior abdominal wall.

Fig. 6.16 Testis and epididymis: (b) blood supply; (c) transverse section.

The epididymis This is a convoluted duct that measures about 6 m when unravelled. It is intimately related to the testis, w i th its head lying on the upper pole, its body along the posterolateral aspect of the testis and its tail lying inferiorly. From here it turns back on itself at an acute angle to become the vas deferens. The head of the epididymis may have one or more small sessile projections superiorly known as the appendix(ces) of the epididymis. The efferent ductules of the testis drain sperm to the head of the epididymis. The epididymis concentrates, stores and transports sperm to the ejaculatory ducts and has a role in its maturation.

The vas deferens

This extends from the tail of the epididymis through the scrotum, inguinal canal and pelvis to fuse w i th the duct of the seminal vesicle to form the ejaculatory duct in the prostate gland.

The spermatic cord The spermatic cord contains the vas deferens, the arterial supply, draining veins, lymphatics and nerves. It is tightly covered by a fibrous sheath comprised of internal spermatic, cremasteric and external spermatic fascial layers derived from transversalis, internal and external oblique abdominal fasciae.

The spermatic cord and epididymis are supplied by branches of the inferior vesical artery (deferential artery) and inferior epigastric artery (cremasteric artery).

The scrotum is supplied by branches from the internal pudendal artery (posterior scrotal artery), external pudendal branches of the femoral artery, and branches of the inferior epigastric artery (cremasteric).

Fluid collections in the scrotum

Fluid may be present in the sac of the tunica vaginalis in a condition known as hydrocoele. If the tunica communicates w i th the peritoneal cavity via an open processus vaginalis, the f l u id collection is called a congenital hydrocoele. Cysts related to the spermatic cord are seen when isolated areas of the processus vaginalis have failed to obliterate.

Cysts of the epididymis arise from the efferent spermatic ducts in the head of the epididymis. They are found above and behind the testis and are known as spermatocoeles.

Blood supply The testis is supplied by the testicular artery, which arises directly from the aorta at the level of the renal arteries. Numerous veins drain from the testis at the mediastinum to a complex plexus of veins called the pampiniform venous plexus, which is situated superiorly. The epididymis and scrotum are drained by the cremasteric venous plexus, which runs posterior to the testis related to the epididymis. Both plexuses have anastomosing channels and drain superiorly, running w i th the vas deferens in the spermatic cord. They anastomose to become one testicular (internal spermatic) vein at the upper end of the inguinal canal. The right testicular vein drains into the inferior vena cava and the left into the left renal vein. Lymph drainage The testis drains to para-aortic nodes via the testicular vessels. The scrotum drains to superficial inguinal nodes.

Radiology of the testis

Ultrasound (Fig. 6.17) The testes may be imaged by ultrasound as oval structures having a homogeneous granular echotexture w i th uniform medium-level echoes. The mediastinum may be identified as a linear echogenic band running superolater¬ ally. Occasionally the rete testis is prominent as multiple anechoic structures in the posterosuperior aspect of the testis. The septae may sometimes be appreciated as linear echogenic thin bands running through the testis. These can be more readily appreciated if the testis is swollen in pathological conditions. On longitudinal scans, the upper and lower poles are identified. Vessels are often identified running through the testis and hypoechoic linear structures. The appendix of the testis may be identified on highresolution images, especially if outlined by fluid.

The head of the epididymis is seen to rest on the upper pole posteriorly. The body of the epididymis is seen posterolateral to the testis, and the spermatic cord is medial to the epididymis. The various parts of the epididymis are

Fig. 6.17 Ultrasound of testis: (a) longitudinal image; (b) transverse image; (c) transverse image showing prominent rete testis - a normal variant.

1. Skin 2. Testis 3. Epididymis 4. Fluid in scrotal sac 5. Rete testis

better seen if outlined by fluid. The scrotal septum is seen between the testes. A small amount of anechoic f l u id may surround the normal testis. The appendix of the epididymis may be seen if f l u id is present.

The tortuous veins of the pampiniform plexus may be seen related to the cord above the upper pole. Slow venous flow may be demonstrated by colour Doppler techniques.

Magnetic resonance imaging Use of a surface coil improves resolution. The tunica albuginea, mediastinum testis and fibrous septae are of low signal intensity compared w i th the high signal intensity of normal testicular tissue on T2-weighted imaging. The testis is of homogeneous low signal intensity on T1-weighted imaging. The epididymis is of variable intensity.

The penis (Fig. 6.18) The anatomy of the penis is described w i th the penis lying on the anterior abdominal wall, i.e. the ventral surface is the surface that normally lies on the scrotum. The penis comprises three cylinders of endothelium-lined erectile tissue which arise from the perineum: a ventral corpus spongiosum, which surrounds the penile urethra, and paired dorsal corpora cavernosa. The corpora are each surrounded by a thick fibrous capsule, the tunica albuginea. Superficial to the tunica albuginea is Buck's fascia, which is the deep fascia of the penis. This binds the three corpora together and is a continuation of the deep perineal fascia.

The corpus spongiosus is expanded to form the glans penis inferiorly and the bulb superiorly. The enlarged posterior part of the bulb is penetrated by the bulbar part of the urethra, which then runs in the corpus spongiosum to the external meatus. The distal corpus spongiosum expands to form the conical glans penis, which projects beyond the ends of the corpora cavernosa. The corpora cavernosa are fused in the median plane, but diverge posterosuperiorly to form the crura, which attach to the inferior part of the internal surface of the corresponding ischiopubic ramus.

Blood supply of the penis

The arterial supply originates from the internal pudendal arteries. Paired deep dorsal arteries lie external to the tunica albuginea and course lateral to the deep dorsal vein, supplying the skin and the glans. Paired cavernosal arteries run in the corpora cavernosa. The corpora cavernosa are drained through emissary veins of the wall of the corpora and then to the deep dorsal vein.

Radiology of the penis

Ultrasound

The corpora and urethra may be identified by ultrasound. The corpora yield low-level echoes and the urethra is seen as a circular echogenic structure w i th a central anechoic area. The penile arteries may be imaged using colour flow Doppler.

Magnetic resonance imaging This w i ll also show the corpora, which are of high signal intensity on T2-weighted images and of intermediate signal on T1-weighted images. The fascial layers are of low signal on both T1 and T2. The corpora are very vascular on contrast-enhanced images.

Cavernosography This entails the injection of contrast into the corpora. This examination is performed for the investigation of erectile dysfunction.