Genitals, Male

The male genitals (or genitalia) are those parts of the body or organs involved in reproduction. The genitals may include only those external, visible parts (primary genitalia) such as the penis or the scrotum, or they may also include the internal reproductive structures and organs (secondary genitalia) such as the testes, prostate, and urethra. Male genitalia consist of those organs and tissues that function to produce sperm (the male sex cell) and to transport it so that it can come in contact with the female egg (ovum). Their successful union (or fertilization) is what allows humans to reproduce.

PHYSIOLOGY AND MORPHOLOGY OF MALE GENITALIA

The scrotum is the sac that contains the testes (an essential organ in the production of sex hormones and sperm—the male sex cell). It is divided into two compartments—each of which holds one of the pair of testes—by a connective tissue septum. The scrotum is visible externally and consists of skin covering a layer of smooth muscle (the dartos muscle which contracts in cold temperature and causes the skin of the scrotum to become firm and wrinkled). The scrotum functions to help maintain the testes at an optimum temperature for sperm production. Abdominal cremaster muscles cause the scrotum to contract nearer to the body raising the temperature of the testes. During relaxation or warm weather, these muscles relax and allow the scrotum to relax as well and become loose and thin which lets the testes descend further from the body and thus lowers their internal temperature. This is significant because spermatogenesis (the production of sperm) may not occur if the temperature of the organ is either too warm or too cold.

The testes (singular testis) or testicles are two ovoid glands about four-five centimeters (cm) long (also known as gonads) found in males. The left testicle is usually located about one cm lower than the right one. They are both suspended external to the body in the scrotal sac and supported by the spermatic cords (a collection of testicular blood vessels and nerves and ducts). Testes are analogous to the ovaries in females in that both are involved in the production of the reproductive cells (sperm in males) and the secretion of sex hormones (testosterone in males) which stimulate primary sexual characteristics (e.g., sex organs) and secondary sexual characteristics (including distribution of hair growth and body physique). The testes are formed during embryonic development and descend into the scrotal sac near the end of pregnancy or shortly after birth. Though present in a newborn, they are small and nonfunctional until puberty (around ten to fourteen years of age). Until then, secondary sexual characteristics are absent.

At puberty, the hypothalamus (in the brain) secretes a hormone which in turn stimulates the anterior pituitary gland to begin producing hormones that stimulate the production of testosterone (the primary masculinizing hormone) in the testes. This hormone is secreted from the interstitial cells (or cells of Leydig) which are present just after birth but absent during childhood. During puberty, these interstitial cells begin producing large quantities of testosterone which stimulates the growth of the primary and secondary sexual organs and tissue. During this time, the penis, testes, scrotal sac, and other internal and external genitalia begin to grow and function.

The mature testes are enclosed by a white fibrous capsule (the tunica albuginea) which radiates into the testes dividing it into 200 to 250 cone-shaped lobules which contain the seminiferous tubules, the site of spermatogenesis. At puberty, hormonal signals initiate the growth of the seminiferous tubules and the subsequent production of sperm. Microscopically each of the tubules is surrounded by clusters of the interstitial cells. Germ cells (precursors to sperm) and Sertoli cells (which nourish the germ cells and probably produce hormones as well) are located within the seminiferous tubules. It is at this site that the spermatogonia (an early stage of sperm development) undergo division to become primary spermatocytes which continue to divide to produce secondary spermatocytes. Further divisions produce spermatids which eventually become the sperm cells (spermatozoa).

The sperm cells then travel from the seminiferous tubules through a series of ducts that allow the sex cells to mature and ultimately exit the body during ejaculation. The sperm first travels through the rete testes (a tubular network connected to the seminiferous tubules), which then exits the testes by way of the efferent ductules. From there, the sperm enters the epididymis (a tightly coiled and narrow tube posterior to the testes) where it continues to mature. The vas deferens (or ductus deferens) is a duct that is a continuation of the epididymis and travels from the scrotal sac (where it can be palpated as a movable cord—one for each testes) to the pelvic cavity. Smooth muscle in the walls of the duct propels the sperm from the epididymis through the vas deferens. Sperm may remain in the vas deferens (depending on the frequency of ejaculation) over a month with no loss of viability or fertility (a vasectomy—cutting the vas defer-ens in the scrotal sac—prevents conception by interrupting the path that sperm must follow to reach the outside of the body).

As it continues along its path, the vas deferens becomes associated with the spermatic cord (the testicular artery and veins, lymph vessels, testicular nerves, and cremaster muscle). The cord travels through the inguinal canal to the prostate gland. There, the end of the vas deferens increases in diameter and becomes the ampulla (named for the flask-like shape) of the ductus deferens. Next to each ampulla and connected by a short duct is the seminal vesicle. These sac-shaped glands were erroneously thought to store the sperm until ejaculation (and hence the name), but later it was discovered that these structures' function is to secrete large quantities of fructose, prostaglandins, and fibrinogen (to provide nourishment for the sperm and to facilitate fertilization) which contribute to the volume of the ejaculated semen (about 30%). The two ducts come together to form the ejaculatory ducts which are two short tubes that that pass through the prostate, a doughnut-shaped gland about the size of a walnut that lies directly beneath the bladder and in front of the rectum. The gland secretes an alkaline fluid into the urethra that protects the sperm from the acid environments of the male urethra and the female vagina (whose acidic environment would otherwise kill the sperm). Additional alkaline fluid is secreted by the Cowper (or bulbourethral) glands, which are located just below the prostate. Sperm leaves the vas deferens with the newly accumulated seminal fluids and continues through the urethra where it ultimately is expelled from the body.

The male urethra is a small tube about eight inches long that extends from the base of the bladder, through the prostate, and through the shaft of the penis. The male urethra is made up of three sections (the prostatic, the membranous, and the cavernous) and contains glands that secrete a mucus substance. The urethral tube exits the body at the urinary meatus at the distal end of the penis.

The male urethra serves dual purposes. It provides a pathway for both urine and seminal fluid to exit the body, though it does not allow the fluids to exit at the same time. When seminal fluid passes into the urethra, the urinary sphincter muscles automatically contract and prevent urine from passing into the tube.

The penis is the external male reproductive organ. Internally the penis contains three columns of erectile tissue which, upon sexual stimulation, engorge with blood causing the penis to grow in length and firmness resulting in an erection. The two columns on the sides and underside of the (non-aroused) penis are the corpora cavernosa, and a third and narrower column travels along the front and central aspect of the penis and is called the corpus spongiosum and includes the end of the penis (the glans penis). The urethra passes through this corpus spongiosum, through the glans, and opens at the external urethral orifice.

The penis is covered with skin that is loosely attached along its shaft and more tightly just below the glans. A well-developed network of sensory and nerve receptors is located just below the skin. A thinner layer of skin covers the glans making it particularly sensitive to stimulation. The foreskin (or prepuce), a loose fold of skin, covers the glans penis. The surgical removal of the foreskin (circumcision) is commonly performed in many cultures for religious and cultural reasons.

SEXUAL FUNCTION

Male genitalia functions ultimately for reproduction. For that to occur, the male sperm must be exposed to and fertilize the ovum. To this end, coitus (sexual intercourse or copulation) must occur. The penis must become erect. This is accomplished by a parasympathetic reflex that is initiated by sexual stimulation (tactile, visual, and/or mental). When aroused, the arteries in the penis dilate and flood the corpus cavernosum and spongiosum. This compresses the veins and prevents the blood from leaving the penis causing it to enlarge and become firm.

Emission is the reflex movement of the sperm and other secretions (from the prostate and other glands) from the vas deferens to the prostatic urethra (the portion nearest to the prostate). Ejaculation, another reflex, follows as a result of continued sexual arousal. This is associated with increased heart rate and blood pressure, hyperventilation, dilation of skin blood vessels, and intense sexual excitement. Ejaculation marks the climax or orgasm in males.

DISEASES OF THE MALE GENITALS

Cryptorchidism (or undescended testicles) is a condition where one or both testes fail to descend into the scrotum. Usually the testicles drop down at the end of pregnancy or shortly after birth. If they do not, the affected testis is unable to produce sperm and is functionally sterile. This is because it remains exposed to a higher internal body temperature compared to that of the scrotal sac. The condition is easily diagnosed at birth by palpating the scrotum and is treated by the administration of testosterone (or other hormones), which may allow the testis to descend. Alternatively surgery may be performed.

Another condition that may develop is an inguinal hernia. The inguinal canal narrows after the testes descend but creates a weak spot in the abdominal wall. If the inguinal canal ruptures (or enlarges), a hernia (a loop of intestine that protrudes through the abdominal wall) may develop causing pain and, in severe cases, a cutting off of the intestine. The condition can be corrected surgically.

Elective (or nontherapeutic) circumcision is a contested practice whereby the foreskin (or prepuce) is sur-gically removed on newborn males. It is done for religious purposes (as in Jewish or Islamic faith) or for sociocultural and/or aesthetic reasons. At debate is whether the risks of routine circumcision are worth any potential benefits of the procedure. The most common risks of circumcision include bleeding or infection. Though these effects are usually mild, in rare cases they may result in excessive blood loss or sepsis (systemic infection). Less common risks include adhesions between the remaining foreskin and the glans or urinary retention. Removal of too much tissue may result in phimosis (a tightening of the foreskin that prevents it from being drawn back from the glans). Alternatively too little of the prepuce may be removed and thereby fail to achieve the aesthetic appearance intended. Serious side effects from the procedure are extremely rare, but may include partial amputation of the glans penis, fistulas, meningitis, and (rarer yet) death. Benefits include a reduced incidence of urinary tract infections in male infants, a decrease in the incidence of penile cancer in adults, and possibly a decrease in the susceptibility to certain sexually transmitted diseases including human immunodeficiency virus (HIV). Despite the benefits, the American Medical Association (AMA) does not recommend routine elective circumcisions.

Hypogonadism may result when there is an absence of testosterone (or other male hormones) due to the failure of the testes to develop or a genetic lack of androgen (male hormone) receptor sites. The result is that male organs and tissues fail to develop and instead, normal female sex organs and tissues grow in their place. In rare cases, hypogonadism may be caused by the genetic lack of gonadotropin-releasing hormone (GnRH) secreted by the hypothalamus and may result in hypothalamic eunuchism (or Fröhlich's syndrome). This is frequently associated with a hypothalamic disorder causing a person to overeat leading to obesity.

Impotence (the failure to achieve or sustain an erection) may be caused by multiple factors including physiological (such as low testosterone levels or diabetes which damages blood vessels necessary to elicit a penile response) or psychological. The condition may be treated by administration of erectile dysfunction drugs (such as Viagra©, Levitra©, or Cialis©) or male hormones. Medical grade vacuum pumps improve erectile function in some men. Surgical penile implants may be indicated in severe cases.

Prostate enlargement commonly develops as men grow older. If it grows large enough, the prostate closes off the urethra (which passes through the prostate) making urination difficult or impossible. Urine retention may result. If other treatments fail, prostatectomy (or the surgical removal of the prostate) can cure the condition. The prostate can enlarge for many reasons including infections and tumors, both benign and cancerous. Prostate cancer is the most common form of cancer found in American males and the third highest cause of cancer death in men. Though one in six American men will develop the disease, only one in thirty-four will die of it (American Cancer Society, "All about Prostate Cancer"). Depending on the aggressiveness of the cancer, treatment can range from keeping a watchful eye on the tumor's progression to drug therapy, radiation, and/or surgery.

Sexually transmitted diseases (such as syphilis) may lead to the loss of fertility by damaging the tissues necessary for the production of sperm or male hormones. Further the presence of a sexually transmitted infection makes one more susceptible to acquiring HIV/AIDS, if exposed. Men with HIV have a slight increased risk of developing testicular cancer. Condylomata (or genital warts) varying in size between microscopic and as much as an inch in diameter may appear on the external genitals. Untreated, these benign growths may be readily passed between sexual partners.

Steroids (or synthetic androgens) use by athletes may result in a negative-feedback effect on the hypothalamus and anterior pituitary. This in turn decreases the hormones that normally stimulate the testes. As a result, the testes may atrophy and cause sterility.

Tumors, both cancerous and benign, may grow in the male reproductive tissue and organs such as the prostate, and more rarely the testicles and the penis. Testicular cancer predominately affects young men and is one of the most curable cancers (with only 1 in 5,000 males dying of this form of carcinoma in their lifetime according the American Cancer Society, "All about Testicular Cancer." Undescended testis (cryptorchidism) is one of the leading risk factors for developing testicular cancer. The condition can be treated with chemotherapy, radiation, and/or surgery. Despite the lack of conclusive studies that suggest that regular self-examination of the testes offers any defense against the outcome of the disease, many physicians recommend men perform manual self-checks monthly. Penile cancers are extremely rare. They may range in severity from very slow growing tumors to aggressive melanomas which may spread to other parts of the body. Treatment is through pharmacologic agents, radiation therapy, and/or surgery (often microscopic or laser surgery which is less disfiguring cosmetically and functionally than removal of the penis). Precancerous lesions and tumors may (rarely) appear on the foreskin. They are generally slow-growing and treatment will usually prevent their spread into the deeper tissues of the penis.

Tumors may rarely occur in the interstitial cells in the testes causing the organ to produce up to 100 times the normal amount of testosterone. When this condition presents in young males, the excess hormone causes the sex organs and tissues to develop prematurely as well as the bones and muscles leading to short stature due to the concurrent early fusing of the epiphysis of the long bones.

see also Penis.

BIBLIOGRAPHY

American Cancer Society, a. "All About Penile Cancer." Available from http://www.cancer.org.

American Cancer Society, b. "All About Prostate Cancer." Available from http://www.cancer.org.

American Cancer Society, c. "All About Testicular Cancer." http://www.cancer.org.

American Medical Association. 1999. "Report 10 of the Council on Scientific Affairs (I-99)." Available from http://www.ama-assn.org/ama/pub/category/13585.html.

Guyton, Arthur C. 1991. Textbook of Medical Physiology. 8th edition. Philadelphia: Saunders.

Seeley, Rod R.; Trent D. Stephens; and Philip Tate. 1991. Essentials of Anatomy and Physiology. St. Louis, MO: Mosby Year Book.

Thibodeau, Gary A. Anatomy and Physiology. 1987. St. Louis, MO: Times Mirror/Mosby College Publishing.

                                      Diane Sue Saylor