The Prostate Gland
The prostate is the largest accessory gland in the male reproductive system. It secretes proteolytic enzymes into the semen, which act to break down clotting factors in the ejaculate. This allows the semen to remain in a fluid state, moving throughout the female reproductive tract for potential fertilisation.
The prostate is commonly described as being the size of a walnut. Roughly two-thirds of the prostate is glandular in structure and the remaining third is fibromuscular. The gland itself is surrounded by the fibrous capsule of the prostate.
Anatomical Position and Structure
The prostate is positioned inferiorly to the neck of the bladder and superiorly to the external urethral sphincter (see Figure 1.0), with the levator ani muscle lying inferolaterally to the gland. Most importantly, posteriorly to the prostate lies the ampulla of the rectum – this anatomical arrangement is utilised during Digital Rectal Examinations (DRE) by physicians needing to examine the gland.
The proteolytic enzymes leave the prostate via the prostatic ducts. These open into the prostatic portion of the urethra, secreting the enzymes into the semen immediately before ejaculation.
The prostate is divided into anatomical lobes (inferoposterior, inferolateral, superomedial, and anteromedial) by the urethra and the ejaculatory ducts as they pass through the organ. However, more important clinically is histological division of the prostate into zones:
- Central zone – Surrounds the ejaculatory ducts, embryologically derived from the Wolffian duct.
- Transitional zone – Located centrally and surrounds the urethra, embryologically derived from the urogenital sinus.
- Peripheral zone – Makes up the main body of the gland and located posteriorly, embryologically derived from the Urogenital Sinus. The peripheral zone is the zone felt against the rectum on DRE.
The fibromuscular stroma is situated anteriorly in the gland. It merges with the tissue of the urogenital diaphragm. The zones of the prostate are important in BPH and prostatic carcinoma (see ‘Clinical Relevance’ section).
The arterial supply to the prostate comes from the prostatic arteries, which are mainly derived from the internal iliac arteries. They also arise from the internal pudendal and middle rectal arteries.
Venous drainage of the prostate is via the prostatic venous plexus, draining into the internal iliac veins. However, the prostatic venous plexus also connects posteriorly by networks of veins, including the Batson venous plexus, to the internal vertebral venous plexus.
The prostate receives sympathetic, parasympathetic and sensory innervation from the inferior hypogastric plexus. The smooth muscle of the prostate gland is innervated by sympathetic fibres, which activate during ejaculation.
Clinical Relevance – Disorders of the Prostate
Benign Prostatic Hyperplasia (BPH)
Benign prostatic hyperplasia is the increase in size of the prostate, without the presence of malignancy. It is much more common with advancing age. The enlarging prostate compresses on both the bladder and the urethra (see Figure 1.1), resulting in men presenting with urinary frequency, urinary urgency, and difficulty in initiating micturition. BPH is usually caused by enlargement from the transitional zone of the prostate.
Prostatic carcinoma will present with similar symptoms as BPH. The malignant cells commonly originate from the peripheral zones, resulting in symptoms presenting late during advanced stages of the disease. A DRE may reveal a hard irregular prostate gland. Malignant cells will commonly spread via Batson venous plexus to the vertebral bodies and cause metastases.