Facts About The Prostate Gland
The prostate gland is a small walnut sized gland, a bout 20 gm in weight, which lies below the bladder and surrounds the urethra.
The prostate gland consists of four distinct zones.
- The peripheral zone comprises the outer section of the gland mostly at the back and sides, technically called the posterior and lateral aspects. The peripheral zone contains a high proportion of the glandular cells that make the prostatic secretions. The peripheral zone can be palpated by a physician during a digital rectal examination (DRE). Seventy percent of all prostate cancers arise in this region. This is why a DRE is so important. People generally do not like this type of examination but the omission of this simple test can mean that many cancers will be missed, possibly to the detriment of the patient.
- The transitional zone, normally a small region occurs in the middle of the gland. It cannot be felt during a DRE but it can be imaged on rectal ultrasound, (TRUS). The transition zone is where most of the growth of the prostate gland occurs as men age. In large prostate glands the transition zone predominates and the regions become squashed. Enlargement or benign hyperplasia is not to be confused with cancer. They are separate diseases. About twenty percent of prostate cancers develop in the transitional zone.
- The central zone lies between the transitional zone and the peripheral zone towards the base of the prostate. Because the prostate gland is a bit like an inverted pyramid, the base of the gland is actually the top of prostate and lies next to the bladder. Only about ten percent of cancers originate in this zone.
- The fibromuscular zone lies in front of the gland. It has very few glandular elements and so consequently cancer does not commence in this region. A tumour can of course spread to this part of the gland from another region as it grows.
The seminal vesicles are extensions of the central zone. They are two glands that arise from the base of the prostate gland and lie behind the bladder. Cancer almost never starts in seminal vesicles but because of their proximity to the prostate gland, cancer can spread into these glands from the prostate. Prostate cancer in the seminal vesicles implies and advanced cancer and is not a good prognostic sign.
The prostate gland serves as the junction between the urinary and sexual systems. Urine passes through it during micturition, (the act of voiding urine), and semen is expelled through it into the urethra during ejaculation. Problems arising in the gland can affect both of these functions.
The prostate gland is integrated into the sphincter mechanism that controls urinary continence. Consequently disease in the prostate gland and or procedures performed on the prostate gland can potentially lead to incontinence or retention of urine. Urinary incontinence is a common complication of radical prostatectomy. There are two valves, one at the neck of the bladder extending halfway down into the prostatic urethra and another valve in the membranous urethra extending up through the apex of the prostate gland. The valve at the bladder neck called the internal sphincter functions automatically. It will relax when the bladder contracts to empty and then close when the bladder relaxes. The second valve at the prostatic apex can be controlled consciously. This is the valve that one will tighten up when one needs to void urine but there is no convenient toilet nearby. Usually both valves have to be damaged before incontinence will occur.
Seminal fluid is stored in two glands called seminal vesicles. During ejaculation semen passes out of these glands through ducts in the prostate gland. The semen then enters the urethra. The internal sphincter closes tightly during ejaculation forcing the semen to pass down the urethra and out of the body. If this does not happen then semen rather flows backwards into the bladder. This is called retrograde ejaculation. This problem often occurs after operations to unblock obstructing prostates or when taking medication to relieve prostatic obstruction.
As the semen flows into the prostatic urethra, the prostate adds further fluid to the mixture. This fluid alkalinises the seminal fluid and adds substances to enhance sperm motility and survival in the hostile environment of the female genital tract. Prostate specific antigen helps to prevent sperm cells clumping together. For more on PSA see the page devoted to PSA Issues.