Reproductive System, Male

Definition

The male reproductive system is composed of organs that work together to produce sperm and deliver them to the female reproductive tract for fertilization of the ovum.

Description

The normal male reproductive system is composed of numerous anatomical structures, including the testis, the excretory ducts, the auxiliary glands, the penis, and the various hormones that control reproductive functions.

Testis

The testis is responsible for the production and maturation of sperm in a process called spermatogenesis. It is also the site of synthesis and secretion of androgens (male sex hormones). The testes (plural) develop in the abdomen and descend into the scrotum in the normal male. The scrotum is a muscular sac in which the testes hang from the spermatic cord.

The testis is subdivided into the tubular compartment and the interstitial compartment. The tubular compartment is composed of up to 900 seminiferous tubules, which are populated by three main types of cells: germ cells, peritubular cells, and Sertoli cells. Germ cells become mature sperm in the spermatogenic process. Peritubular cells produce various factors that aid in the transportation of mature sperm to the epididymis. Sertoli cells secrete various factors that determine the sperm production and testis size of an adult male.

Androgens are produced in the interstitial compartment of the testis. Leydig cells are responsible for the production and secretion of testosterone. Immune cells such as macrophages and lymphocytes are also found in the interstitial compartment, and aid in the proliferation and hormone production of Leydig cells.

Sperm cells are composed of a head (containing the nucleus and acrosome), the body (containing the mitochondria, or energy-producing organelles), and the tail. The nucleus contains the cell's genetic material (chromatin) while the acrosome contains enzymes that are capable of penetrating the protective layers around the egg. The mitochondria provide energy for tail motility; this is essential for movement of the sperm through the female reproductive tract.

Excretory ducts

The excretory ducts are responsible for the transfer of sperm from the seminiferous tubules of the testis to the urethra and include the epididymis, the vas deferens, and intratesticular ducts. The epididymis is a tubular structure through which sperm exiting the seminiferous tubules pass. Testicular sperm are not fully mature and would not be able to fertilize an ovum (egg). Complete maturation occurs in the epididymis in the two to twelve days that sperm are typically stored before being passed to the vas deferens. The vas deferens functions to carry mature sperm from the epididymis to the urethra; it is also called the ductus deferens. Secretions from the auxiliary glands are mixed with sperm in the vas deferens to form semen.

Auxiliary glands

The auxiliary glands include two bulbourethral glands, one prostate, and two seminal vesicles. These glands contribute the secretions that compose semen. The bulbourethral glands (also called the glands of Cowper) secrete a fluid that lubricates the urethra prior to ejaculation. The prostate secretes a fluid rich in zinc, citric acid, choline, and various proteins. The secretions of the seminal vesicle are high in fructose (an energy source for sperm) and prostaglandins (fatty acid derivatives).

Penis

The penis is the male organ of sexual reproduction and consists of three elongated bodies that cause erection, the two corpora cavernosa and the corpus spongiosum. These tissues become engorged with blood when stimulated by the nervous system during arousal. Blood is supplied by the superficial and deep arterial systems (which carry blood to the penile skin and erectile tissue, respectively). The urethra runs through the corpus spongiosum to the glans penis (distal end of the penis). The organ is covered with loose skin that forms the prepuce (foreskin) over the glans penis.

Function

Endocrine control

Normal reproductive function is dependent on complex interactions between various hormones. A portion of the brain called the hypothalamus secretes releasing hormones that travel to the pituitary gland, located at the base of the brain. The secretion of gonadotropin-releasing hormone (GnRH) from the hypothalamus triggers the release of luteinizing hormone (LH) and follicle-stimulating hormone (FSH) from the pituitary gland. LH stimulates testosterone production by Leydig cells in the testis, and FSH promotes spermatogenesis.

Male sexual act

The male sexual act can be divided into three main steps: erection, emission, and ejaculation. Erection is the result of increased blood flow to the erectile tissues of the penis; stimulation of the nervous system during arousal causes a release of acetylcholine (a neurotransmitter) that in turn causes vasodilation (increase in the diameter of blood vessels ). Emission is the passage of sperm and secretions into the urethra mediated by release of the hormone adrenaline. Ejaculation occurs when the sperm are forced from the urethra by contraction of the bulbocavernous muscles. A release of noradrenaline causes the blood vessels in the penis to contract, decreasing blood flow and resulting in detumescence (loss of erection).

Fertilization

In order to fertilize the ovum, ejaculated sperm must move into the vaginal tract, pass through the cervix, survive in the uterus, and enter the fallopian tubes. Usually only healthy, motile sperm will reach the ovum and have the opportunity to fertilize it. Numerous protective layers (including the oolemma, the zona pellucida, and the zona radiata) surround the ovum, and sperm cells must penetrate each of these layers for fertilization to occur. Binding of a sperm cell to the zona pellucida induces the acrosome reaction, which permits the sperm to penetrate the zona pellucida and reach the egg membrane. The sperm and egg membranes fuse to form a zygote, and subsequent reactions prevent the binding of additional sperm cells to the egg membrane.

Common diseases and disorders

Diseases of the male reproductive system are classified based on the localization (e.g., testis, pituitary gland, etc.) and cause (e.g., congential malformation, cancerous tumor, etc.) of the disorder. Some common examples of andrological disorders include:

• Infertility: Male infertility may be the symptom of multiple disorders. A blockage in both of the vas deferentia or a testicular disorder may result in the complete absence of sperm (azoospermia). Low sperm counts might result from a prolonged increase in scrotal temperature—as in the case of a varicocele, a disturbance in testicular blood circulation. Retrograde ejaculation is another cause of male infertility; semen travels in the wrong direction, up the urethra to the bladder instead of down toward the penis.
• Hypogonadism: This describes a condition in which there is decreased sexual development and growth of the testes. Hypogonadism may result from tumors, hormone imbalances, or chromosomal abnormalities. Its symptoms (after puberty ) include voice alteration, decreased size of testes, gynecomastia (enlargement of mammary glands), an infantile penis, or osteoporosis.
• Erectile dysfunction: It is estimated that the incidence of erectile dysfunction (ED) is twice as high as that of coronary heart disease. ED may result from reduced penile blood flow, low serum levels of testosterone, use of psychotropic drugs, alcohol abuse, metabolic disorders such as diabetes mellitus, or muscle cell impairment.
• Prostate cancer: The prostate surrounds the urethra and secretes seminal fluids. Prostate cancer is the second most common cause of cancer death of men in the United States, and the second most commonly diagnosed form of cancer (after skin cancer).

KEY TERMS

Acrosome— A compartment in the head of the sperm that contains enzymes that allow the sperm to penetrate the protective layers of an egg.

Androgen— A male sex hormone.

Azoospermia— The complete absence of sperm in ejaculate.

Gynecomastia— Enlargement of the male mammary gland; a symptom of hypogonadism.

Hypogonadism— A condition in which there is decreased sexual development and growth of the testes.

Leydig cells— Found in the interstitial compartment of a testis; responsible for the production and secretion of testosterone.

Sertoli cells— Found in the tubular compartment of a testis; aids in the process of spermatogenesis.

Spermatogenesis— The process of the formation of sperm.

Varicocele— An abnormal swelling of veins in the scrotum.

Zygote— The cell resulting from the fusion of the male and female gametes.

Resources

BOOKS

Kirby, Roger S., Michael G. Kirby, and Riad N. Farah, eds. Men's Health. Oxford, UK: Isis Medical Media, Ltd., 1999.

Nieschlag, E. and H. M. Behre, eds. Andrology: Male Reproductive Health and Dysfunction. Berlin: Springer-Verlag, 2001.

PERIODICALS

Lue, Tom F. "Erectile Dysfunction." New England Journal of Medicine (June 2000): 1802-1813.

ORGANIZATIONS

American Society of Andrology. 74 New Montgomery, Suite 230, San Francisco, CA 94105. (415) 764-4823. 〈http://www.andrologysociety.com〉.

OTHER

Berkow, Robert, Mark H. Beers, Andrew J. Fletcher, and Robert M. Bogin, eds. "Infertility." The Merck Manual of Medical Information: Home Edition. 2001.