The prostate (from Ancient Greek προστάτης, prostátēs, literally "one who stands before", "protector", "guardian") is an exocrine gland of the male reproductive system in most mammals. It differs considerably among species anatomically, chemically, and physiologically.
Anatomically, the prostate can be subdivided in two ways: by zone or by lobe. It does not have a capsule; rather an integral fibromuscular band surrounds it. It is sheathed in the muscles of the pelvic floor, which contract during the ejaculatory process. The prostate also contains some smooth muscles that also help expel semen during ejaculation.
The function of the prostate is to secrete a fluid which contributes to the volume of the semen. This prostatic fluid is slightly alkaline, milky or white in appearance, and in humans usually constitutes roughly 30% of the volume of semen, the other 70% being spermatozoa and seminal vesicle fluid. The alkalinity of semen helps neutralize the acidity of the vaginal tract, prolonging the lifespan of sperm. The prostatic fluid is expelled in the first part of ejaculate, together with most of the sperm. In comparison with the few spermatozoa expelled together with mainly seminal vesicular fluid, those in prostatic fluid have better motility, longer survival, and better protection of genetic material.
Disorders of the prostate include enlargement, inflammation, infection, and cancer.
- 1 Structure
- 2 Function
- 3 Clinical significance
- 4 Other animals
- 5 Additional images
- 6 See also
- 7 References
- 8 External links
The classical description of a healthy human male prostate portrays it as slightly larger than a walnut. The mean weight of the normal prostate in adult males is about 11 grams, usually ranging between 7 and 16 grams. The volume of the prostate can be estimated by the formula 0.52 × length × width × height. A volume of over 30 cm3 is regarded as prostatomegaly (enlarged prostate). A study stated that prostate volume among patients with negative biopsy is related significantly with weight and height (body mass index), so it is necessary to control for weight. The prostate surrounds the urethra just below the urinary bladder and can be felt during a rectal exam.
One can sub-divide the prostate in two ways: by zone or by lobe. It does not have a capsule; rather an integral fibromuscular band surrounds it. It is sheathed in the muscles of the pelvic floor, which contract during the ejaculatory process.
The "lobe" classification is more often used in anatomy. The prostate is incompletely divided into five lobes:
|Anterior lobe (or isthmus)||roughly corresponds to part of transitional zone|
|Posterior lobe||roughly corresponds to peripheral zone|
|Right & left Lateral lobes||span all zones|
|Median lobe (or middle lobe)||roughly corresponds to part of central zone|
The "zone" classification is more often used in pathology. John E. McNeal first proposed the idea of "zones" in 1968. McNeal found that the relatively homogeneous cut surface of an adult prostate in no way resembled "lobes" and thus led to the description of "zones".
The prostate gland has four distinct glandular regions, two of which arise from different segments of the prostatic urethra:
|Name||Fraction of gland||Description|
|Peripheral zone (PZ)||Up to 70% in young men||The sub-capsular portion of the posterior aspect of the prostate gland that surrounds the distal urethra. ~70–80% of prostatic cancers originate from this portion of the gland.|
|Central zone (CZ)||Approximately 25% normally||This zone surrounds the ejaculatory ducts. The central zone accounts for roughly 2.5% of prostate cancers; these cancers tend to be more aggressive and more likely to invade the seminal vesicles.|
|Transition zone (TZ)||5% at puberty||~10–20% of prostate cancers originate in this zone. The transition zone surrounds the proximal urethra and is the region of the prostate gland that grows throughout life and causes the disease of benign prostatic enlargement. (2)|
|Anterior fibro-muscular zone (or stroma)||Approximately 5%||This zone is usually devoid of glandular components and composed only, as its name suggests, of muscle and fibrous tissue.|
The secretory epithelium is mainly pseudostratified, comprising tall columnar cells and basal cells, which are supported by a fibroelastic stroma — containing randomly oriented smooth-muscle bundles — that is continuous with the bladder. The epithelium is highly variable and areas of low cuboidal or squamous epithelium are also present, with transitional epithelium in the distal regions of the longer ducts. Within the prostate, the urethra coming from the bladder is called the prostatic urethra and merges with the two ejaculatory ducts.
Three histological types of cells are present in the prostate gland: glandular cells, myoepithelial cells, and subepithelial interstitial cells.
Gene and protein expression
About 20,000 protein coding genes are expressed in human cells and almost 75% of these genes are expressed in the normal prostate. About 150 of these genes are more specifically expressed in the prostate with about 20 genes being highly prostate specific. The corresponding specific proteins are expressed in the glandular and secretory cells of the prostatic gland and have functions that are important for the characteristics of semen. Some of the prostate specific proteins are enzymes, such as the prostate specific antigen (PSA), and the ACPP protein.
The prostatic part of the urethra develops from the pelvic (middle) part of the urogenital sinus (endodermal origin). Endodermal outgrowths arise from the prostatic part of the urethra and grow into the surrounding mesenchyme. The glandular epithelium of the prostate differentiates from these endodermal cells, and the associated mesenchyme differentiates into the dense stroma and the smooth muscle of the prostate. The prostate glands represent the modified wall of the proximal portion of the male urethra and arise by the 9th week of embryonic life in the development of the reproductive system. Condensation of mesenchyme, urethra, and Wolffian ducts gives rise to the adult prostate gland, a composite organ made up of several tightly fused glandular and non-glandular components.
Male sexual response
During male seminal emission, sperm is transmitted from the vas deferens into the male urethra via the ejaculatory ducts, which lie within the prostate gland. Ejaculation is the expulsion of semen from the urethra. It is possible for some men to achieve orgasm solely through stimulation of the prostate gland, such as prostate massage or anal intercourse.
Prostatic secretions vary among species. They are generally composed of simple sugars and are often slightly alkaline. In human prostatic secretions, the protein content is less than 1% and includes proteolytic enzymes, prostatic acid phosphatase, beta-microseminoprotein, and prostate-specific antigen. The secretions also contain zinc with a concentration 500–1,000 times the concentration in blood.
To function properly, the prostate needs male hormones (androgens), which are responsible for male sex characteristics. The main male hormone is testosterone, which is produced mainly by the testicles. It is dihydrotestosterone (DHT), a metabolite of testosterone, that predominantly regulates the prostate.
Prostatitis is inflammation of the prostate gland. It can be caused by infection with bacteria, or other noninfective causes. Inflammation of the prostate can cause painful urination or ejaculation, groin pain, difficulty passing urine, or constitutional symptoms. The prostate is enlarged (prostatomegaly) and tender on digital rectal examination. A culprit bacteria may grow in a urine culture.
Acute prostatitis and chronic bacterial prostatitis are treated with antibiotics. Chronic non-bacterial prostatitis, or male chronic pelvic pain syndrome is treated by a large variety of modalities including alpha blockers, nonsteroidal antiinflammatories and amitriptyline. Other treatments may include physical therapy, psychotherapy, antihistamines, anxiolytics, nerve modulators, phytotherapy,[unreliable medical source?], surgery, and more. More recently, a combination of trigger point and psychological therapy has proved effective for category III prostatitis as well.
Benign prostatic hyperplasia
Benign prostatic hyperplasia refers to a non-malignant enlargement (hyperplasia) of the prostate that is very common in older men. It is often identified when the prostate has enlarged to the point where urination becomes difficult. Symptoms include needing to urinate often (frequency), or taking a while to get started (hesitancy). If the prostate grows too large, it may constrict the urethra and impede the flow of urine, making urination difficult and painful and, in extreme cases, completely impossible, causing urinary retention. Over time, chronic retention may cause the bladder to become larger and cause a backflow of urine into the kidneys (hydronephrosis).
BPH can be treated with medication, a minimally invasive procedure or, in extreme cases, surgery that removes the prostate. In general, treatment often begins with a alpha antagonist medication such as tamsulosin, which reduces the tone of the smooth muscle found in the ureter that passes through the prostate, making it easier for urine to pass through. Minimally invasive procedures include transurethral needle ablation of the prostate (TUNA) and transurethral microwave thermotherapy (TUMT). These outpatient procedures may be followed by the insertion of a temporary prostatic stent, to allow normal voluntary urination, without exacerbating irritative symptoms. In some cases, "obesity management may be an effective method to reduce prostate volume."
The surgery most often used in such cases is called transurethral resection of the prostate (TURP or TUR). In TURP, an instrument is inserted through the urethra to remove prostate tissue that is pressing against the upper part of the urethra and restricting the flow of urine. TURP results in the removal of mostly transitional zone tissue in a patient with BPH. Older men often have corpora amylacea (amyloid), dense accumulations of calcified proteinaceous material, in the ducts of their prostates. The corpora amylacea may obstruct the lumens of the prostatic ducts, and may underlie some cases of BPH.
Life-style changes to improve the quality of urination include urinating in the sitting position. This reduces the amount of residual volume in the bladder, increases the urinary flow rate and decreases the voiding time.
Prostate cancer is one of the most common cancers affecting older men in developed countries and a significant cause of death for elderly men. Screening for prostate cancer may be in the form of a physical rectal examination or measurement of prostate specific antigen (PSA) level in the blood. However, given the significant risk of overdiagnosis with widespread screening in the general population, prostate cancer screening is controversial.
The prostate is found as a male accessory gland in all placental mammals excepting edentates, martens, badgers and otters. The prostate glands of male marsupials are disseminate and proportionally larger than those of placental mammals. In some marsupial species, the size of the prostate gland changes seasonally. The structure of the prostate varies, ranging from tubuloalveolar (as in humans) to branched tubular. The gland is particularly well developed in dogs, foxes and boars, though in other mammals, such as bulls, it can be small and inconspicuous. Dogs can produce in one hour as much prostatic fluid as a human can in a day. They excrete this fluid along with their urine to mark their territory. In many rodents and bats, the prostatic fluid contains a coagulant. This mixes with and coagulates semen during copulation to form a mating plug that temporarily prevents further copulation. In cetaceans the prostate is composed of diffuse urethral glands and is surrounded by a very powerful compressor muscle.
The prostate gland originates with tissues in the urethral wall. This means the urethra, a compressible tube used for urination, runs through the middle of the prostate. This leads to an evolutionary design fault for some mammals, including human males. The prostate is prone to infection and enlargement later in life, constricting the urethra so urinating becomes slow and painful.
Skene's gland is found in both female humans and rodents. Historically it was thought to be a vestigial organ, but recently it has been discovered that it produces the same protein markers, PSA and PAB, as the male prostate. This means Skene's gland functions as a female prostate, a histologic homolog to the male prostate gland.
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