Structure
The prostate is a fibroglandular organ which is involved in the production of semen. It is covered by a thin fibrous capsule, and is pierced by the urethra and ejaculatory ducts. It is widest superiorly and posteriorly, and narrowed anteriorly and inferiorly, and is often described as an upturned pyramid.
- The anterior surface is narrow and is connected with the puboprostatic ligament.
- The paired inferolateral surfaces, which are separated from the levatores ani by the prostatic venous plexus, curve inwards both anteriorly and inferiorly.
- The posterior surface is broad, narrows inferiorly, and may be defined by a shallow longitudinal depression into right and left sides. The ejaculatory ducts enter the prostate near the superior extent of this ridge.
- The base of the prostate lies just posterior to the neck of the bladder and is continuous with it. The urethra exits the bladder and enters the prostate in the anterior part of the base.
- The apex of the prostate, directed inferiorly, is small and in contact with the urogenital diaphragm. The urethra exits through the apex.
Oblique Sagittal Section through the Prostate
An important landmark within the prostate is the prostatic portion of the urethra, which takes a curved path from the base to the apex of the gland. The posterior surface of the prostatic urethra is distinguished by a raised ridge, the urethral, which runs from the base to the apex. The deeper portions on each side of this ridge are known as the prostatic sulci. In the central part of this ridge, there is a large prominence known as the seminal colliculus or verumontanum. Related to this structure are:
- The prostatic utricle, a 4-6 mm sinus which opens into the centre of the colliculus and proceeds posteriorly. This is the embryological remnant of the uterovaginal canal.
- The openings of the ejaculatory ducts, which lie inferolaterally to the utricle. The ejaculatory ducts enter the superior part of prostate, near the midline on the posterior surface, and travel inferiorly and medially to enter the urethra.
Relations
The relations of the prostate are:
- Above, contiguous with the neck of the bladder
- Anteriorly, with the puboprostatic ligaments, prostatic venous plexus, and symphysis pubis
- Posteriorly, separated from the rectum by the prostatic fascia and the mesorectum.
- Posterosuperiorly, with the seminal vesicles, ejaculatory ducts and ductus deferens.
- Laterally, with the levator ani, separated by the prostatic plexus of veins.
- Inferiorly, with the deep transverse perineal muscle and sphincter urethrae, bulbourethral glands, and the bulb of the penis.
Sagittal Relations
Coronal Relations
Anatomical 'Lobes'
Historically, there has been a great deal of debate as to the anatomical structure of the prostate. There are a number of differing divisions, but they usually include:
- The ventral or anterior lobe, which lies anterior to the urethra.
- Two lateral lobes lateral to the ejaculatory ducts.
- A single posterior lobe between the ejaculatory ducts.
- A middle lobe located above the ejaculatory ducts.
This has been largely done away with by the concept of functional prostate zones.
Functional Zones
Functional Zonal Anatomy of the Prostate
Although embryologically derived, the anatomical lobar concepts were found to be anatomically and functionally indistinct. A zonal approach to prostate anatomy was developed, based on the histological and functional properties of the gland:
- The central zone, which lies posterior to the urethra and surrounds the two ejaculatory ducts.
- The transitional zone, which lies mainly anterior and superior to the prostatic colliculus.
- The preprostatic sphincter, which surrounds the proximal prostatic urethra and is surrounded by the transitional zone.
- The peripheral zone, which extends over the posterior and lateral surfaces of the prostate.
- The anterior fibromuscular zone, which is located in the anterior part of the prostate.
Microscopic Structure
The prostate is formed by tubuloacinar glands and connective tissue containing smooth muscle, in varying amounts depending on the zone. Glands have a bilayered epithelium; a superficial, secretory columnar layer and a deep cuboidal layer.
This deep layer is an important feature, as it is lost in malignancy.
Anterior Fibromuscular Zone / Preprostatic Sphincter
These zones contains virtually no glandular tissue, and assists in continence.
Transitional Zone
The transitional zone is very small in the healthy state and contains less than 5% of glandular tissue. Its ducts empty on to the anterior and lateral aspect of the urethra, marking it as separate from the other two glandular zones.
Central Zone
The central zone is smaller than the peripheral zone, but the stroma is denser and the acini are larger. Overall the central zone contains 50% of the glandular epithelium. Acini drain into the prostatic sulci, with ducts travelling posteriorly and superiorly from the urethra.
Peripheral Zone
The peripheral zone is characterized by large numbers of acini and ducts which all drain into the posterior part of the prostatic urethra. It typically has smaller acini and less complex ductal branching than the central zone.
Physiological changes
The prostate is a relatively mobile organ in the pelvis, mostly due to altered filling of the bladder and pelvis. Movement can vary up to 7.5 mm during a course of radiotherapy treatment.
Pathological changes
Two significant pathologies affect the prostate:
- Prostate carcinoma is a relatively common malignancy of men. Despite possessing an equivalent amount of epithelial tissue, the peripheral zone is far more likely to develop carcinoma than the central zone.
- Benign prostatic hypertrophy involves enlargement of the transitional zone. This leads to obstruction of the urethra and symptoms of bladder outflow obstruction (nocturia, poor stream, difficulty starting/stopping). The enlarged transitional zone may bulge superiorly into the neck of the bladder.
Neurovascular Supply
Arterial Supply
Arterial supply to the prostate is derived from branches of the internal iliac artery:
- The inferior vesical artery, which passes to the inferior part of the bladder, also gives off several branches to the prostate and is usually the largest contributor of arterial blood
- Prostatic branches of the middle rectal artery pass anteriorly to reach the prostate.
- Prostatic branches of the pudendal artery pass through the perineal muscles and ascend to the prostate gland.
Venous Drainage
Venous blood collects in the periprostatic venous plexus, where it is returned to the internal iliac vein by inferior vesical veins. Some blood is returned via the vertebral plexus of veins, which some believe to explain the prevelance of vertebral metastases in prostate cancer.
Lymphatics
Lymphatics from the prostate typically travel to internal iliac nodes, including the more anterior group of obturator nodes. Lymph fluid may also pass to the presacral nodes or external iliac nodes, but this is less common.
Innervation
The prostate receives autonomic supply from the inferior hypogastric plexus, which lies along the internal iliac artery.
- Sympathetic supply is derived from lumbar splanchnic nerves, which form the superior hypogastric plexus. This continues as the inferior hypogastric plexus, which gives off prostatic branches.
- Pre-ganglionic parasympathetic fibres arise from S2 to S4 as pelvic splanchnic nerves. They synapse with post-ganglionic fibres in the inferior hypogastric plexus, which are then directed to the prostate.
Routes of Cancer Spread
Local Invasion
Prostate cancers typically involve the entire prostate diffusely. The first step in more advanced local invasion is through the prostatic capsule (T3a disease), and they commonly invade the seminal glands (T3b disease). Invasion into the bladder, rectum or penile bulb is less common (T4 disease).
Lymphatic Spread
As per the lymphatic drainage described above, prostate cancer typically spreads to internal iliac nodes (including the obturator group). Less commonly it may spread to presacral or internal iliac nodes. More advanced cases will spread to common iliac and lumbar nodes.
Haematogenous Spread
Prostate cancer is very likely to spread to the bones of the axial skeleton if it becomes metastatic beyond nodes. Vertebral metastases are a common presentation (although less so with the advent of PSA testing).
