Biology of the Male Reproductive System ( Introduction· Effects of Aging· Function· Puberty·Structure)

Introduction The external structures of the male reproductive system include the penis and scrotum. The internal structures include the vas deferens, testes (testicles), urethra, prostate gland, and seminal vesicles. Sperm, which carries the man's genes, is made in the testes and stored in the seminal vesicles. During ejaculation, sperm is transported along with a fluid called semen through the urethra. Male Reproductive Organs

Effects of Aging It is not clear whether aging itself or the disorders associated with aging cause the gradual changes that occur in men's sexual functioning. The frequency, duration, and rigidity of erections gradually decline throughout adulthood. Levels of the male sex hormone (testosterone) tend to decrease, reducing sex drive (libido). Blood flow to the penis decreases. Other changes include decreases in penile sensitivity and ejaculatory volume, reduced forewarning of ejaculation, orgasm without ejaculation, more rapid detumescence, and a longer refractory period. Breast Disorders in Men Breast disorders, which include breast enlargement and breast cancer, occur infrequently in men. Breast Enlargement Breast enlargement in males (gynecomastia) sometimes occurs during puberty. The enlargement is usually normal and transient, lasting a few months to a few years. Breast enlargement also commonly takes place after age 50. Male breast enlargement may be caused by certain disorders (particularly liver disorders), certain drug therapies (including the use of female sex hormones and anabolic steroids), or heavy use of marijuana, beer, or heroin. Less commonly, male breast enlargement results from a hormonal imbalance, which can be caused by rare estrogen-producing tumors in the testes or adrenal glands. One or both breasts may become enlarged. An enlarged breast may be tender. If tenderness is present, cancer is probably not the cause. Breast pain in men, as in women, is not usually a sign of cancer. Generally, no specific treatment is needed. Breast enlargement often disappears on its own or after its cause is identified and treated. Surgical removal of excess breast tissue is effective but rarely necessary. Liposuction, a surgical technique that removes tissue through a suction tube inserted through a small incision, is the preferred surgical option and sometimes is followed by additional cosmetic surgery. Breast Cancer Men can develop breast cancer, although 99% of all breast cancers develop in women. Because male breast cancer is uncommon, it may not be suspected as a cause of symptoms. As a result, male breast cancer often progresses to an advanced stage before it is diagnosed. The prognosis is the same as that for a woman whose cancer is at the same stage. Treatment options are generally the same as those used for women (surgery, radiation therapy, and chemotherapy), except that breast-conserving surgery is rarely used. Estrogen makes some breast cancers grow. Estrogen is the main female sex hormone, but it is present in males in low amounts. If an examination of tissue samples shows that estrogen is making the cancer grow, estrogen is suppressed with drugs such as tamoxifen. Function The penis becomes erect through a complex interaction of physiologic and psychologic factors. Contractions during ejaculation impel semen into the urethra and out of the penis. During sexual activity, the penis becomes erect, enabling penetration during sexual intercourse. An erection results from a complex interaction of neurologic, vascular, hormonal, and psychologic actions. Pleasurable stimuli cause the brain to send nerve signals through the spinal cord to the penis. The arteries supplying blood to the corpora cavernosa and corpus spongiosum respond by dilating. The widened arteries dramatically increase blood flow to these erectile areas. At the same time, muscles around the veins that normally drain blood from the penis tighten, slowing the outflow of blood and elevating blood pressure in the penis. This combination of increased inflow and decreased outflow is what causes the penis to become engorged with blood and increase in length, diameter, and stiffness. At the climax of sexual excitement (orgasm), ejaculation usually occurs, caused when friction on the glans penis and other stimuli send signals to the brain and spinal cord. Nerves stimulate muscle contractions along the seminal vesicles, prostate, and the ducts of the epididymis and vas deferens. These contractions force semen into the urethra. Contraction of the muscles around the urethra further propels the semen through and out of the penis. The neck (base) of the bladder also constricts to keep semen from flowing backward into the bladder. Once ejaculation takes place—or the stimulation stops—the arteries constrict and the veins relax, reducing blood inflow, increasing blood outflow, and causing the penis to become limp (detumescence). After detumescence, erection cannot be obtained for a period of time (refractory period), commonly about 20 minutes in young men. Puberty Puberty may begin as early as age 9 and continue until age 16. At puberty, the testes start to produce testosterone. Testosterone causes reproductive organs to mature, facial and pubic hair to appear, and the voice to deepen. Puberty is the stage during which a person reaches full reproductive ability and develops the adult features of their gender. In boys, puberty usually occurs between the ages of 10 and 14 years. However, it is not unusual for puberty to begin as early as age 9 or to continue until age 16. The pituitary gland, which is located in the brain, initiates puberty. The pituitary gland secretes luteinizing hormone and follicle-stimulating hormone, which stimulate the testes to produce testosterone. Testosterone is responsible for the development of secondary sex characteristics, features that distinguish the sexes but are not part of the reproductive system, such as facial hair growth and voice change. Testosterone also produces many changes in the male reproductive organs, including Elongation and thickening of the penis Enlargement of the scrotum, testes, epididymis, and prostate Darkening of the skin of the scrotum Growth of pubic hair Sperm usually develops by age 14. Ejaculation first occurs during late puberty. Structure The penis and the urethra are part of the urinary and reproductive systems. The scrotum, testes, vas deferens, and prostate gland comprise the rest of the reproductive system. The penis consists of the root (which is attached to the abdominal wall), the body (the middle portion), and the glans penis (the cone-shaped end). The opening of the urethra (the channel that transports semen and urine) is located at the tip of the glans penis. The base of the glans penis is called the corona. In uncircumcised males, the foreskin (prepuce) extends from the corona to cover the glans penis. The body of the penis consists primarily of three cylindrical spaces (sinuses) of erectile tissue. The two larger ones, the corpora cavernosa, occur side by side. The third sinus, the corpus spongiosum, surrounds the urethra and ends as the glans penis. When these spaces fill with blood, the penis becomes large and rigid (erect). The scrotum is the thick-skinned sac that surrounds and protects the testes. The scrotum also acts as a climate-control system for the testes, because they need to be slightly cooler than body temperature for normal sperm development. The cremaster muscles in the wall of the scrotum relax or contract to allow the testes to hang farther from the body to cool or to be pulled closer to the body for warmth or protection. The testes are oval bodies that average about 1.5 to 3 inches (4 to 7 centimeters) in length and 2 to 3 teaspoons (20 to 25 milliliters) in volume. Usually the left testis hangs slightly lower than the right one. The testes have two primary functions: producing sperm and producing testosterone (the primary male sex hormone). The epididymis is a coiled tube almost 20 feet (6 meters) long. It collects sperm from the testis and provides the space and environment for sperm to mature. One epididymis lies against each testis. The vas deferens is a firm duct that transports sperm from the epididymis. One such duct travels from each epididymis to the back of the prostate and joins with the seminal vesicle. In the scrotum, other structures, such as blood vessels and nerves, also travel along with each vas deferens and together form an intertwined structure, the spermatic cord. The urethra serves a dual function in males. This channel is the part of the urinary tract that transports urine from the bladder and the part of the reproductive system through which semen is ejaculated. The prostate lies just under the bladder and surrounds the urethra. Walnut-sized in young men, the prostate enlarges with age. When the prostate enlarges too much, it can block urine flow through the urethra. The seminal vesicles, located above the prostate, join with the vas deferens to form the ejaculatory ducts. The prostate and the seminal vesicles produce fluid that nourishes the sperm. This fluid provides most of the volume of semen, the secretion in which the sperm is expelled during ejaculation. Other fluid that makes up a very small amount of the semen comes from the vas deferens and from mucous glands. Testosterone Replacement Therapy Beginning at about age 30, the production of testosterone (the main male sex hormone) in men usually decreases an average of 1 to 2% per year. This decline is sometimes referred to as male menopause or andropause. However, the hormone decline in men differs greatly from what women experience in menopause, during which female hormones almost always decline rapidly over just a few years. The rate of testosterone decline varies greatly among men. Some men in their 70s have testosterone levels that match those of the average man in his 30s. Whether young or old, men with low testosterone levels may develop certain characteristics associated with aging, including decreased libido, decreased muscle mass, increased abdominal fat, thin bones that easily fracture (osteoporosis), decreased energy level, slowed mathematical and spatial thinking, and a low blood count (anemia). It is not clear whether low testosterone levels increase the risk of coronary artery disease. Many men are interested in taking testosterone to slow or reverse development of these characteristics, but currently testosterone replacement therapy is only recommended for men with abnormally low levels of testosterone. The most worrisome side effect of testosterone replacement therapy is worsening of prostate disorders. Without knowing it, many men have small prostate cancers that would likely never produce symptoms or be lethal. The body's own testosterone can make prostate cancers grow, so testosterone replacement therapy, at least theoretically, could cause an unnoticed prostate cancer to produce symptoms or become lethal. Testosterone replacement also can worsen benign prostatic hyperplasia, a noncancerous enlargement of the prostate. Testosterone replacement therapy is recommended only for men whose blood tests show low testosterone levels and who have no prostate disorders. Men taking testosterone need to be checked frequently for prostate cancer. Such testing may detect cancers early, when they are more likely to be curable.