Prostate
The prostate is an accessory gland of the male reproductive system and a muscle-driven mechanical switch between urination and ejaculation. It is found in all male mammals. It differs between species anatomically, chemically, and physiologically. Anatomically, the prostate is found below the bladder, with the urethra passing through it. It is described in gross anatomy as consisting of lobes and in microanatomy by zone. It is surrounded by an elastic, fibromuscular capsule and contains glandular and connective tissue.
The prostate produces and contains fluid that forms part of semen, the substance emitted during ejaculation as part of the male sexual response. This prostatic fluid is slightly alkaline, and milky or white in appearance. 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, because of the action of smooth muscle tissue within the prostate. 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. The word prostate is derived from Ancient Greek , meaning "one who stands before", "protector", "guardian", with the term originally used to describe the seminal vesicles.
Structure
The prostate is a exocrine gland of the male reproductive system. In adults, it is about the size of a walnut, and has an average weight of about, usually ranging between. The prostate is located in the pelvis. It sits below the urinary bladder and surrounds the urethra. The part of the urethra passing through it is called the prostatic urethra, which joins with the two ejaculatory ducts. The prostate is covered in a surface called the prostatic capsule or prostatic fascia.The internal structure of the prostate has been described using both lobes and zones. Because of the variation in descriptions and definitions of lobes, the zone classification is used more predominantly.
The prostate has been described as consisting of three or four zones. Zones are more typically able to be seen on histology, or in medical imaging, such as ultrasound or MRI.
| Name | Fraction of adult gland | Description |
| Peripheral zone | 70% | The back of the gland that surrounds the distal urethra and lies beneath the capsule. About 70–80% of prostatic cancers originate from this zone of the gland. |
| Central zone | 20% | 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 | 5% | The transition zone surrounds the proximal urethra. ~10–20% of prostate cancers originate in this zone. It is the region of the prostate gland that grows throughout life and causes the disease of benign prostatic enlargement. |
| Anterior fibro-muscular zone | This area, not always considered a zone, is usually devoid of glandular components and composed only, as its name suggests, of muscle and fibrous tissue. |
The "lobe" classification describes lobes that, while originally defined in the fetus, are also visible in gross anatomy, including dissection and when viewed endoscopically. The five lobes are the anterior lobe or isthmus, the posterior lobe, the right and left lateral lobes, and the middle or median lobe.
Inside of the prostate, adjacent and parallel to the prostatic urethra, there are two longitudinal muscle systems. On the front side runs the urethral dilator, on the backside runs the muscle switching the urethra into the ejaculatory state.
Blood and lymphatic vessels
The prostate receives blood through the inferior vesical artery, internal pudendal artery, and middle rectal arteries. These vessels enter the prostate on its outer surface where it meets the bladder, and travel forward to the apex of the prostate. Both the inferior vesical and the middle rectal arteries often arise together directly from the internal iliac arteries. On entering the bladder, the inferior vesical artery splits into a urethral branch, supplying the urethral prostate; and a capsular branch, which travels around the capsule and has smaller branches, which perforate into the prostate.The veins of the prostate form a network – the prostatic venous plexus, primarily around its front and outer surface. This network also receives blood from the deep dorsal vein of the penis, and is connected via branches to the vesical plexus and internal pudendal veins. Veins drain into the vesical and then internal iliac veins.
The lymphatic drainage of the prostate depends on the positioning of the area. Vessels surrounding the vas deferens, some of the vessels in the seminal vesicle, and a vessel from the posterior surface of the prostate drain into the external iliac lymph nodes. Some of the seminal vesicle vessels, prostatic vessels, and vessels from the anterior prostate drain into internal iliac lymph nodes. Vessels of the prostate itself also drain into the obturator and sacral lymph nodes.
Microanatomy
The prostate consists of glandular and connective tissue. Tall column-shaped cells form the lining of the glands. These form one layer or may be pseudostratified. The epithelium is highly variable and areas of low cuboidal or flat cells can also be present, with transitional epithelium in the outer regions of the longer ducts. Basal cells surround the luminal epithelial cells in benign glands. The glands are formed as many follicles, which drain into canals and subsequently 12–20 main ducts. These in turn drain into the urethra as it passes through the prostate. There are also a small amount of flat cells, which sit next to the basement membranes of glands, and act as stem cells.The connective tissue of the prostate is made up of fibrous tissue and smooth muscle. The fibrous tissue separates the gland into lobules. It also sits between the glands and is composed of randomly orientated smooth-muscle bundles that are continuous with the bladder.
Over time, thickened secretions called corpora amylacea accumulate in the gland.
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, including prostate-specific proteins, such as the prostate specific antigen, and the prostatic acid phosphatase.Development
In the developing embryo, at the hind end lies an inpouching called the cloaca. This, over the fourth to the seventh week, divides into a urogenital sinus and the beginnings of the anal canal, with a wall forming between these two inpouchings called the urorectal septum. The urogenital sinus divides into three parts, with the middle part forming the urethra; the upper part is largest and becomes the urinary bladder, and the lower part then changes depending on the biological sex of the embryo.The prostatic part of the urethra develops from the middle, pelvic, part of the urogenital sinus, which is of endodermal origin. Around the end of the third month of embryonic life, outgrowths arise from the prostatic part of the urethra and grow into the surrounding mesenchyme. The cells lining this part of the urethra differentiate into the glandular epithelium of the prostate. The associated mesenchyme differentiates into the dense connective tissue and the smooth muscle of the prostate.
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. To function properly, the prostate needs male hormones, which are responsible for male sex characteristics. The main male hormone is testosterone, which is produced mainly by the testicles. It is dihydrotestosterone, a metabolite of testosterone, that predominantly regulates the prostate. The prostate gland enlarges over time, until the fourth decade of life.
Function
In ejaculation
The prostate secretes fluid, which becomes part of the semen. Its secretion forms up to 30% of the semen. Semen is the fluid emitted through the male urethra during the sexual response. Sperm are emitted from the vas deferens into the male urethra via the ejaculatory duct, which lies within the prostate gland. Semen is moved into the urethra following contractions of the smooth muscle of the vas deferens and seminal vesicles, following stimulation, primarily of the glans penis. Stimulation sends nerve signals via the internal pudendal nerves to the upper lumbar spine; the nerve signals causing contraction act via the hypogastric nerves. After traveling into the urethra, the seminal fluid is ejaculated by contraction of the bulbocavernosus muscle. The secretions of the prostate include proteolytic enzymes, prostatic acid phosphatase, fibrinolysin, zinc, and prostate-specific antigen. Together with the secretions from the seminal vesicles, these form the major fluid part of semen. The prostate contains various metals, including zinc, and is known to be the primary source of most metals found in semen, which are released during ejaculation.In urination
The prostate's changes of shape, which facilitate the mechanical switch between urination and ejaculation, are mainly driven by the two longitudinal muscle systems running along the prostatic urethra. These are the urethral dilator on the urethra's front side, which contracts during urination and thereby shortens and tilts the prostate in its vertical dimension thus widening the prostatic section of the urethral tube, and the muscle switching the urethra into the ejaculatory state on its backside.In case of an operation, e.g. because of benign prostatic hyperplasia, damaging or sparing of these two muscle systems varies considerably depending on the choice of operation type and details of the procedure of the chosen technique. The effects on postoperational urination and ejaculation vary correspondingly.