Male Reproductive System
Apart from man, few mammals have a prostate gland and only the dog can also develop prostate cancer. The word ‘prostate’ comes from Ambroise Pare, a 16th century French surgeon who believed that it was the ‘doorkeeper’ to the bladder. A role it serves well, protecting the male from the minor stress leakage common in women.
Originally the anatomy of the gland was categorised into individual lobes, but now it is split into anatomical zones. For the radiologist the zonal anatomy has advantages as the zones are distinguishable on imaging and the various prostate disorders follow a zonal propensity.
We will first discuss the gross anatomy and relationships after an introductory assessment.
The prostate is an extraperitoneal pelvic structure and an inverted cone in shape. Paradoxically, the most superior part of the gland is termed the base of the gland and the apex is most inferior (i.e. it is an inverted cone).
- The rectumis posterior to the gland
- Superior to the gland lies the bladder
- The levator aniand obturator internus muscles are on the sides
- External urethral sphincter(and urogenital diaphragm) are located inferiorly
- Also posteriorly, between the prostate gland and the rectum, lies the Denonvillier’s fascia– an obliterated peritoneal plane
- The seminal vesiclesand vas deferens lie superoposteriorly and the neurovascular bundles (which supply the gland) skirt the gland posterolaterally from the base down to the apex of the gland (not shown on the image)
In the post-pubescent male the gland volume is <25 ml (sizes being about 3.5 cm long or high, 4.0 cm wide and 2.5 cm anterior to posterior depth). For unknown, possibly hormonal reasons, with the onset of benign prostatic hyperplasia, the gland enlarges above 40 years of age. BPH is present in 50% of 60-year-olds and 90% of 90-year-olds.
The formula for volume measurement of the gland is:
- Length x width x height x 0.52 ml
The gland is surrounded by the ‘prostate capsule’, which isn’t a true fibro-elastic capsule but a loose compression of surrounding fat and stroma, through which pass the neurovascular bundles and the ejaculatory ducts. The capsule is deficient where the two urinary sphincters – the bladder neck and the external sphincter (just inferior to the gland apex) – are located.
Local infiltration of prostate cancer is common because of the lack of a true prostate capsule and occurs along lines of ‘weakness’. In some series, up to 40% of men have had extra-prostatic cancer at radical prostatectomy.
Careful anatomical dissection studies have revealed distinct glandular zones. Three zones have been defined with a further separate anterior portion:
- The peripheral zone (PZ) – lies posteriorly
- The central zone (CZ) – between PZ and TZ
- The transition zones (TZ) – centrally located on either side of the urethra
- The anterior fibro-muscular stroma – most anterior portion, non-glandular
Sometimes a further, more pragmatic zonal terminology is required:
The peripheral (or outer) gland and the central (or inner) gland; and the anterior gland.
Inner or outer gland
A separate ultrasound terminology divides the gland into the peripheral and central glands, or the outer and inner glands, as it is sometimes impossible to identify the central zone as a separate structure. Thus the peripheral and central zones are terminologically incorporated into a single unit – the peripheral or outer gland. During imaging, either with TRUS or MRI, abnormalities may be better referred to as within the inner or outer gland.
The term anterior gland is less well defined, but often used by clinicians. It refers to that portion anterior to the urethra. Biopsies of the ‘anterior gland’ may be requested in some cases.
The prostate arteries and veins travel within the postero-lateral neurovascular bundles, and are important anatomical landmarks during cancer staging by imaging.
The gland has a rich arterial supply from branches of the iliac arteries, accounting for the potential ‘bloodiness’ of prostate surgery.
- The prostate arteryarises from the inferior vesical artery, which itself is a branch of the anterior division of the internal iliac artery. The prostate artery divides into capsular and urethral arteries; the former supply two thirds of the gland and the latter the rest of the gland
- Separate branches of the inferior vesical artery supply the seminal vesicles, and occasionally the base of the gland
Multiple perforating veins drain the gland into a venous plexus (venous plexus of Santorini), which eventually drains into the internal iliac system. There are some further venous connections that directly connect with the lumbar veins (also called the Batson’s plexus).
They are a route for haematogenous spread of prostate cancer, and explains the frequency of prostate cancer deposits in the lumbar spine.
Changes with Age
The gland is poorly developed in children and matures after puberty, reaching its full size of 12-25 ml. In the young man, the peripheral zone accounts for 75% of the volume, the CZ about 20% and the rest is the TZ.
With age (>40 years), the TZ enlarges due to BPH whilst the central zone atrophies and the peripheral zone stays static. Thus, by the time of presentation for imaging, the PZ may be seen as a thin peripheral rind (Fig 1).
The deep pelvis is a restricted space and is insufficient to accommodate significant gland enlargement. Beyond a certain size, the gland will preferentially enlarge superiorly and protrude into the bladder base – so called median lobe enlargement, and the prostatic urethra will be distorted.
Changes in the shape of the gland has a bearing on prostate biopsy as these are directed to preferentially sample the peripheral zone. A thin, stretched PZ requires angulated biopsies.
The Prostatic Urethra
The prostatic urethra runs through the gland from the bladder base down to the urogenital diaphragm. It’s normally midline, except in the presence of asymmetric glandular hypertrophy.
At rest it has no lumen, but during micturition, its cross-section varies from round to triangular. The triangulated portion has a mound of posteriorly placed tissue (termed the verumontanum) and the ejaculatory ducts drain here.
There is a variable amount of smooth muscle around the urethra and this, together with the urothelial margin, accounts for its sono-visibility in the collapsed state. However, the lumen is well seen only during micturition.
At the bladder end lies the internal sphincter or the bladder neck. It has a dual function. It:
- Maintains urinary continence
- Stops retrograde ejaculation during ejaculation
The distal or external sphincter lies just inferior to the apex of the gland, is composed of striated muscle and is the main continent sphincter, but during ejaculation, it relaxes to allow antegrade semen flow.
Seminal Vesicles and Ejaculatory Ducts
The seminal vesicles are paired storage sacs just posterior and superior to the prostate base. They are of variable size and are often asymmetric. Embryologically, they are outpouchings of the vas deferens and merge with the vas deferens to form the ejaculatory ducts (ED).
The ejaculatory ducts pierce the base of the gland and run through the central zone in a close midline position. The normal undilated ducts are difficult to separate from each other and are commonly referred to as the ejaculatory duct complex. They drain into the posterior urethra at the verumontanum.
Normal EDs measure <0.1 mm. ED and seminal vesicle (SV) dilatation is seen with ejaculatory duct obstruction, due to a stone or stricture, which may result in obstructive infertility.
The EDs are also a route of spread of prostate cancer directly into the seminal vesicles. This is of importance, as seminal vesicle invasion is a poor prognostic feature.