Prostate

The prostate is a small, walnut-sized gland in the male reproductive system. It is located just below the urinary bladder and surrounds the prostatic urethra, the portion of the urethra that runs through the prostate. The prostate is positioned in the pelvic cavity, between the bladder and the rectum, and is supported by the pelvic floor muscles. The gland is also anterior to the rectum, making it accessible for examination through a digital rectal exam (DRE). The seminal vesicles are located just above and behind the prostate, while the ejaculatory ducts pass through it, joining the urethra inside the prostate.

Structure and Anatomy

The prostate is a vital gland in the male reproductive system, and it has a complex anatomical structure. Below is a detailed description of the prostate’s anatomy, broken down into specific sections for clarity.

Location

The prostate is located in the pelvic cavity, directly below the urinary bladder. It surrounds the prostatic urethra, which runs through the center of the gland. Key anatomical landmarks include:

• Superior (Above the Prostate): The prostate is located just beneath the bladder. The seminal vesicles are positioned above and behind the prostate.
• Anterior (In Front of the Prostate): The prostate lies behind the pubic symphysis (the front of the pelvis).
• Posterior (Behind the Prostate): The prostate is anterior to the rectum, making it accessible for palpation through a digital rectal exam (DRE).
• Inferior (Below the Prostate): The urogenital diaphragm and the external urethral sphincter are located below the prostate.

Size and Shape

The prostate is typically described as having a walnut-like shape. Its average size in an adult male is about 4 cm wide, 3 cm tall, and 2 cm thick, with a mass of around 20 grams. The prostate has a conical structure, with its base facing upward toward the bladder and its apex pointing downward toward the urogenital diaphragm.

• Base: The broad base of the prostate is in contact with the inferior surface of the bladder, where it surrounds the bladder neck.
• Apex: The pointed apex of the prostate is directed downward and lies in contact with the urogenital diaphragm.

Zones of the Prostate

The prostate can be divided into different anatomical zones, each with distinct structures and functions. These zones are based on the glandular and non-glandular tissues within the prostate.

• Peripheral Zone (PZ)
  • The peripheral zone forms the largest part of the prostate, occupying about 70% of the gland’s total volume. It is located posteriorly and laterally, surrounding the distal portion of the prostatic urethra.
  • The peripheral zone contains most of the glandular tissue and is the area where most prostate cancers and inflammation (prostatitis) develop.
• Central Zone (CZ)
  • The central zone accounts for about 25% of the prostate volume. It surrounds the ejaculatory ducts as they pass through the prostate and is situated deep within the gland, just behind the transitional zone.
  • The central zone is relatively resistant to common prostate disorders, such as cancer and hyperplasia.
• Transitional Zone (TZ)
  • The transitional zone is a small region that surrounds the proximal prostatic urethra near the bladder. It makes up about 5% of the prostate’s volume in young men but tends to enlarge with age, especially in cases of benign prostatic hyperplasia (BPH).
  • The transitional zone is responsible for the growth of the prostate during aging, leading to urinary symptoms in BPH.
• Anterior Fibromuscular Stroma
• • This non-glandular part of the prostate is composed mainly of fibrous tissue and smooth muscle fibers. It lies anteriorly to the urethra and covers the front of the prostate. The fibromuscular stroma provides structural support but does not contain glandular tissue.

Glandular Structure

The prostate is an exocrine gland, meaning it produces and releases secretions into ducts. The prostate’s internal structure consists of numerous glandular acini and ducts that are embedded within connective tissue and smooth muscle.

• Glandular Acini
• • The prostate contains multiple tubuloalveolar glands that secrete fluids into small ducts. These acini are lined by a layer of simple columnar epithelium and a surrounding layer of basal cells, which help maintain the gland’s structural integrity.
  • These glands are organized into lobules separated by fibromuscular tissue, allowing the gland to function as a cohesive unit.
• Ducts:Each glandular acinus empties into small ducts that converge to form larger prostatic ducts. These ducts open into the prostatic urethra, where the prostatic fluid mixes with sperm and other fluids to form semen.

Prostatic Urethra

The prostatic urethra is a key structure that passes through the center of the prostate, allowing both urine and semen to flow from the bladder and reproductive organs, respectively.

• Length: The prostatic urethra is about 3 cm long.
• Urethral Crest: A raised ridge along the posterior wall of the prostatic urethra, known as the urethral crest, contains the seminal colliculus, an elevation that serves as a landmark for the openings of the ejaculatory ducts.
• Prostatic Sinuses: The lateral walls of the prostatic urethra contain small depressions known as prostatic sinuses, where the prostatic ducts open and release their secretions into the urethra.

Ejaculatory Ducts

The ejaculatory ducts are paired ducts formed by the union of the seminal vesicles and the vas deferens. These ducts pass through the central zone of the prostate and open into the prostatic urethra at the seminal colliculus.

• Function: The ejaculatory ducts are responsible for carrying sperm from the vas deferens and secretions from the seminal vesicles into the urethra during ejaculation.
• Location: They are located within the central zone of the prostate, running from the posterior surface of the prostate to the prostatic urethra.

Capsule of the Prostate

The prostate is surrounded by a fibrous capsule that helps maintain its shape and provides protection to the glandular tissue within. The capsule is composed of collagen fibers, smooth muscle fibers, and elastin, and it is continuous with the fibromuscular stroma of the gland.

• Outer Layer: The outer layer of the capsule merges with the surrounding connective tissue and is closely associated with the pelvic fascia.
• Neurovascular Bundles: Located just outside the capsule, the neurovascular bundles are critical for erectile function, as they contain the nerves and blood vessels that innervate the penis.

Blood Supply

The prostate receives its blood supply primarily from branches of the internal iliac artery.

• Prostatic Arteries
  • The primary blood supply to the prostate comes from the prostatic arteries, which are branches of the inferior vesical artery, itself a branch of the internal iliac artery. Additional blood supply may come from branches of the middle rectal artery and internal pudendal artery.
• Venous Drainage
• • Venous drainage from the prostate occurs through the prostatic venous plexus, a network of veins that drain into the internal iliac veins. This plexus also communicates with the vesical venous plexus surrounding the bladder and the vertebral venous plexus, providing a potential route for the spread of prostate cancer to the spine.

Nerve Supply

The prostate is richly innervated by both sympathetic and parasympathetic fibers from the inferior hypogastric plexus (also called the pelvic plexus). These nerves play a role in regulating the contraction of smooth muscle within the prostate and the release of prostatic secretions.

• Sympathetic Innervation:Sympathetic fibers from the pelvic plexus stimulate the contraction of smooth muscle within the prostate, aiding in the release of prostatic fluid into the urethra during ejaculation.
• Parasympathetic Innervation:Parasympathetic fibers contribute to the gland’s secretory activity and support the function of the prostate during reproductive processes.

Lymphatic Drainage

The lymphatic drainage of the prostate is an important pathway for the spread of disease, particularly prostate cancer. Lymph from the prostate drains into the internal iliac lymph nodes, sacral lymph nodes, and, in some cases, the external iliac lymph nodes.

Microscopic Anatomy

At the microscopic level, the prostate is made up of glandular tissue, smooth muscle, and connective tissue.

• Epithelial Cells:The secretory acini are lined with simple columnar epithelial cells that produce the prostatic fluid. These cells are regulated by hormones, particularly testosterone.
• Basal Cells:A layer of basal cells lies between the secretory epithelium and the basement membrane. These basal cells play a role in maintaining the structure and function of the secretory cells.
• Smooth Muscle:Surrounding the glandular tissue are bands of smooth muscle that contract during ejaculation to expel prostatic fluid into the urethra.

Function

The prostate plays a crucial role in male reproductive health, primarily by producing and secreting fluid that makes up a significant portion of semen. The prostate also contributes to sperm motility, ejaculation, and the regulation of urine flow. Below is a detailed description of the key functions of the prostate.

Production and Secretion of Prostatic Fluid

One of the primary functions of the prostate is the production of prostatic fluid, which is a key component of semen.

• Prostatic Fluid Composition
  • The fluid produced by the prostate is slightly alkaline and contains various substances that support the survival and function of sperm. Prostatic fluid makes up about 20-30% of the total volume of semen and helps protect sperm once they enter the female reproductive tract.
  • The fluid contains enzymes such as prostate-specific antigen (PSA), which helps liquefy semen after ejaculation, allowing sperm to swim more freely. Other components include citric acid, zinc, and proteolytic enzymes, which help nourish and protect sperm.
• Alkalinity and Sperm Protection
• • The slightly alkaline nature of prostatic fluid helps neutralize the acidic environment of the vagina, which could otherwise be harmful to sperm. This alkalinity increases the chances of sperm survival and helps ensure successful fertilization.
  • Prostatic fluid also helps buffer any residual acidity in the male urethra (due to urine), protecting sperm from damage as they pass through the urethra during ejaculation.

Contribution to Semen Formation

The prostate contributes to the overall volume and function of semen.

• Mixing with Seminal Vesicle Fluid:The fluid from the prostate mixes with the secretions from the seminal vesicles (which contribute about 60-70% of semen volume) in the ejaculatory ducts. This combination of fluids enhances sperm motility and overall reproductive success.
• Prostate-Specific Antigen (PSA):PSA is a protease enzyme produced by the prostate that plays a crucial role in the liquefaction of semen. After ejaculation, PSA breaks down proteins in semen, making it less viscous and allowing sperm to swim freely. Without this liquefaction, sperm would have difficulty moving through the semen to reach the egg.
• Nourishment for Sperm:The prostate secretes nutrients such as citric acid, which helps nourish sperm during their journey toward the egg. This nourishment ensures that the sperm maintain their energy levels and motility, which is essential for successful fertilization.

Role in Ejaculation

The prostate plays an active role in the process of ejaculation, helping to expel semen from the male reproductive tract.

• Smooth Muscle Contraction
• • The prostate contains smooth muscle fibers that contract during ejaculation. These contractions are triggered by sympathetic nervous system signals, which cause the release of prostatic fluid into the prostatic urethra.
  • During ejaculation, the contraction of the prostate’s smooth muscle fibers helps push the prostatic fluid, along with sperm from the vas deferens and fluid from the seminal vesicles, into the urethra, where it is expelled as semen.
• Closure of the Bladder Neck:During ejaculation, the prostate also helps ensure that urine does not mix with semen. The internal urethral sphincter (a muscle located near the bladder neck) contracts to close off the bladder. This prevents the backflow of semen into the bladder (a condition called retrograde ejaculation) and ensures that only semen passes through the urethra during ejaculation.

Regulation of Urinary Flow

The prostate plays a secondary but important role in the regulation of urine flow by surrounding part of the urethra.

• Surrounding the Urethra:The prostatic urethra, which passes through the center of the prostate, allows urine to flow from the bladder to the outside of the body. The prostate’s position around the urethra means that it has an impact on the passage of urine.
• Interaction with the External Urethral Sphincter:Below the prostate lies the external urethral sphincter, a muscle that controls voluntary urination. The prostate helps support this mechanism by contributing to the structural integrity of the urethral region.
• Impact of Prostate Enlargement on Urination:As men age, the prostate often enlarges due to a condition called benign prostatic hyperplasia (BPH). This enlargement can compress the urethra, leading to urinary symptoms such as difficulty starting urination, a weak urine stream, or incomplete bladder emptying. Although this is a pathological condition, it highlights the prostate’s role in regulating urinary flow.

Hormonal Regulation and Growth

The prostate is sensitive to hormonal signals, particularly testosterone and its derivative dihydrotestosterone (DHT), which regulate its growth and function.

• Androgen Receptors in the Prostate
• • The epithelial cells and stromal cells of the prostate contain androgen receptors, which respond to androgens (male sex hormones like testosterone and DHT). These hormones are essential for the normal growth and function of the prostate.
  • During puberty, rising levels of testosterone stimulate the growth and development of the prostate, leading to its maturation.
• Conversion of Testosterone to DHT:Within the prostate, testosterone is converted to dihydrotestosterone (DHT) by the enzyme 5-alpha reductase. DHT is a more potent form of testosterone and plays a critical role in maintaining prostate function. It promotes the secretion of prostatic fluid and influences the prostate’s ability to contract during ejaculation.

Immunological Barrier

The prostate also contributes to the body’s immune defense by producing various antimicrobial proteins and enzymes.

• Antimicrobial Proteins:The prostatic fluid contains antimicrobial agents, such as zinc and proteins, that help protect the male reproductive tract from infections. These proteins can inhibit the growth of bacteria, thus reducing the risk of urinary tract infections (UTIs) and prostatitis (inflammation of the prostate).
• Barrier Against Pathogens:The prostate’s secretions, combined with the physical structure of the gland and surrounding tissues, form a barrier against pathogens entering the urethra. This protective function is particularly important during ejaculation, when the urethra is exposed to potential contaminants.

Role in Fertility

The prostate contributes directly to male fertility by producing secretions that enhance the ability of sperm to reach and fertilize an egg.

• Enhancing Sperm Motility:The fluid produced by the prostate contains enzymes and nutrients that help improve sperm motility. Zinc, an element present in high concentrations in prostatic fluid, is known to play a role in stabilizing the genetic material (DNA) in sperm, ensuring the integrity and viability of sperm cells.
• Supporting Sperm Viability:The prostatic fluid protects sperm from the acidic environment of the urethra and the female reproductive tract, ensuring that they remain viable and motile for longer periods after ejaculation. This support is critical for successful fertilization.
• Liquefaction of Semen:The prostate’s production of prostate-specific antigen (PSA) is essential for the liquefaction of semen after ejaculation. Liquefaction allows sperm to move more freely through the semen and toward the egg, increasing the chances of fertilization.

Clinical Significance

The prostate is crucial for male reproductive and urinary health, and its dysfunction can lead to several significant clinical conditions:

• Benign Prostatic Hyperplasia (BPH): BPH is the non-cancerous enlargement of the prostate that commonly occurs in older men. As the prostate enlarges, it can compress the urethra, causing urinary symptoms such as difficulty starting urination, weak stream, frequent urination, and incomplete bladder emptying.
• Prostate Cancer: One of the most common cancers in men, prostate cancer often develops in the peripheral zone of the prostate. Early detection through screening methods like prostate-specific antigen (PSA) tests and digital rectal exams (DRE) is critical for successful treatment.
• Prostatitis: This is inflammation of the prostate, which can result from infection or other causes. Prostatitis can lead to painful urination, pelvic pain, and discomfort during ejaculation.
• Prostate-Specific Antigen (PSA): PSA is a protein produced by the prostate, and elevated PSA levels can be an indicator of prostate cancer, BPH, or prostatitis. PSA tests are commonly used to screen for prostate conditions.