To say that the continuation of the human species is heavily dependent on procreation is a gross understatement. This natural wonder is made possible as a result of primary and secondary sexual characteristics that develop at birth and during puberty, respectively. In males, there are a group of accessory reproductive organs that facilitate the process of reproduction.
The prostate gland will be the accessory reproductive organ of focus for this article (for completion, the other two are the bulbourethral glands and the seminal vesicles). The gross and histological anatomy of this structure will be reviewed in addition to its neurovascular supply and some clinically relevant points.
The prostate gland is a six-sided amalgamation of glandular and fibromuscular tissue that resides in the pelvic cavity. The typical dimensions of a healthy prostate gland are 4 x 3 x 2 cm (its width being the greatest), while weighing about 20 grams.
The gland is encapsulated by a true internal connective tissue capsule and a false external capsule, which is a continuation of the pelvic fascia. Its base sits at the neck of the urinary bladder, surrounding the proximal portion of the urethra. The urethra courses through the gland (known here as the prostatic urethra) and exits inferiorly at the apex.
The prostate gland was previously described as being a lobular organ. Subsequent exploration of its anatomy has resulted in it now being divided into specific anatomical zones, rather than lobes. There are three zones of the prostate gland, namely the
- peripheral zone
- transition zon
- central zones
and in addition to that the anterior fibromuscular stroma.
The transition zone is the most central part of the gland that circumscribes the distal end of the preprostatic urethra (proximal to the verumontanum or seminal colliculus; where the ejaculatory and prostatic ducts pierce the posterior wall of the prostatic urethra) to a point just proximal to the ejaculatory ducts and the apex of the central zone. The transition zone is encircled by the conical central zone. It extends from the base of the prostate gland to the verumontanum. This region also encompasses the ejaculatory ducts posterior to the preprostatic urethra.
The peripheral zone is the outermost region of the prostate gland. It encircles the central zone posteroanteriorly and most of the transition zone. With the exception of the anterior portion of the prostatic urethra, the peripheral zone contains most of the tube (the preprostatic part and the remainder of the tube).
The zone does not extend superiorly to the base of the gland; however, it continues inferiorly to form its distal part. The part of the prostatic urethra that is not enclosed within the peripheral zone is covered by the anterior fibromuscular stroma. The inferior muscle fibers of the anterior fibromuscular stroma are supported by striated muscles from the urethral sphincter below and superior fibers by detrusor muscle fibers from the urinary bladder above, all of which are mixed with connective tissue.
The hallmark histological feature of the prostate is the fibromuscular stroma in which there are clusters of smooth muscles mixed with elastic fibers. Surrounded by this mixture of tissue are prostatic glands, which are responsible for the production of approximately 27% of seminal fluid. These glands, under the directives of 5-α-dihydrotestosterone (DHT), secrete a watery mixture of prostate-specific antigen (PSA), prostatic acid phosphatase, fibrinolysin, and amylase into the prostatic sinuses (grooves lateral to the luminal aspect of the verumontanum).
Prostatic glands vary widely in size and have lumens that are lined by connective tissue folds. The connective tissue foldings result in the acini appearing highly irregular. Generally, they are lined by simple columnar or pseudostratified epithelium. Prostatic concretions (precipitations of prostatic glandular secretions) can also be found in the lumen of prostatic glands and are indicative of the age of the patient, as their frequency increases with age.
The presence of the prostatic urethra is also a key histological feature of the prostate. It has a horseshoe appearance (courtesy of the verumontanum), and epithelial projections thanks to the highly folded transitional epithelial layer. Posterior to the concavity of the prostatic urethra, the ejaculatory ducts and the prostatic utricle (blind ended duct along the midline of the verumontanum) can also be appreciated within the stroma of the gland.
The prostate gland receives arterial blood by way of three major vessels:
- internal pudendal artery
- inferior vesical artery
- middle rectal arteries.
The middle rectal and inferior vesical arteries arise from a common branch of the internal iliac artery. While the middle rectal artery only gives a few unnamed branches to the prostate, the inferior vesical artery gives a main branch to the urinary bladder and one to the prostate. The branch to the prostate then subdivides into two groups of arteries; one group primarily supplies the urethra and deep parts of the gland (urethral branches) while another group supplies the capsule and peripheral parts of the gland (capsular branches). The pudendal artery travels in the pudendal canal to supply the gland and other pelvic and reproductive structures.
Initially, a prostatic venous plexus lies between the true and false capsules of the gland. They subsequently drain to the prostatic venous plexus receives at the anterolateral part of the prostate gland. This plexus also receives deoxygenated blood from the deep dorsal vein of the penis and subsequently drains to the inferior vesical vein, which in turn drains to the internal iliac vein.
The glands are innervated by parasympathetic fibers of the pelvic splanchnic nerves by way of the prostatic plexus, which receives its fibers from the inferior hypogastric plexus (for erection and secretomotor of acini). The inferior hypogastric plexus also receives preganglionic sympathetic fibers from the superior hypogastric plexus to provide motor innervation to the smooth muscles of the stroma of the gland (for ejaculation and smooth muscle contraction).
The posterior lobe of the prostate is drained via three primary pathways:
A lateral pathway which drains to the external iliac lymph nodes . These lymph vessels also drain the terminal portion of the ductus deferens, and seminal glands.
A laterodorsal pathway, which drains into the internal iliac nodes, via a course following the prostatic artery
And finally a dorsal pathway, which drains to the sacral lymph nodes as well as the promontorial common iliac lymph nodes.
Lymph drained from the anterior lobe of the prostate can be traced via two routes:
The majority of lymph drained from the anterior surface proceeds to the external iliac lymph nodes, via the paravesical space.
Alternatively, some vessels from the anterior lobe leave the prostate from the posterior surface, draining into the a group of nodes known as inferior gluteal lymph nodes, which are part of the internal iliac lymph nodes.
(Note: Both the external and internal iliac nodes drain to the common iliac nodes, which subsequently drain into the right & left lumbar nodes)
Important to note is that many of the lymph vessels of the prostate anastomose with their counterparts of several neighbouring organs, for example of the urinary bladder and rectum. This can have important clinical implications, especially in cases of prostate cancer.
As was previously mentioned, there are six sides to the prostate gland, each having distinct anatomical landmarks related to them.
- The base, which is the most superior aspect, is immediately related to the urinary bladder and the internal urethral sphincter muscles. Posterosuperiorly, the base is related to the seminal vesicles and the inferior part of the retrovesical portion of the rectovesical space (this is different from the rectovesical pouch within the pelvic peritoneal cavity).
- The gland’s apex is located at the inferior aspect of the organ. Its inferior relations include the bulbourethral (Cowper’s) glands, the external urethral sphincter muscles, and the retroprostatic part of the rectovesical space.
- The anterior part of the prostate gland is adjacent to the retropubic space of Retzius (which is bordered by the posterior part of the pubic symphysis).
- Posteriorly, the rectovesical fascia of Denonvilier courses from the apex of the rectovesical pouch (in the pelvic peritoneum) to the apex of the prostate gland. It divides the rectovesical space into its three parts, the third of which is the prerectal space. The distal rectum is also posteriorly related to the prostate gland.
- Finally, the pubourethralis portions of the levator ani muscles lie adjacent to both inferolateral surfaces of the gland.
A digital rectal examination allows the clinician to feel any nodular growth on the posterior surface of the gland. This is typically where prostatic adenocarcinomas can be palpated as these tumors most often arise in the peripheral zone.
Benign prostatic hyperplasia, however, are more prominent in the transition zone. Subsequently, this will result in urinary retention which could result in bladder and kidney infection if not adequately treated.