Formation Of Sperm

Males begin to produce sperm during puberty, the adolescent stage of development when changes in the body make reproduction possible. Two hormones released by the anterior pituitary regulate the functioning of the testes. Luteinizing hormone (LH) stimulates secretion of the sex hormone testosterone. Testosterone is the main androgen (male sex hormone) produced by the testes. Cells located between the seminiferous tubules secrete testosterone. Follicle-stimulating hormone (FSH), along with testosterone, stimulates sperm production in the seminiferous tubules. A male will continue to produce sperm as long as his testosterone level is high enough—usually for most of his life.

The formation of gametes in humans involves the process of meiosis. Meiosis results in a reduction of the number of chromosomes from the diploid (2n) number to the haploid (1n) number. As the cells that produce sperm within the testes undergo meiosis, their chromosome number drops from 46 to 23. Four sperm cells result from each cell that begins meiosis. These immature sperm then undergo significant changes that prepare the sperm for passage through the female reproductive system.

The structure of a mature sperm is shown in Figure 51-2. Notice that a mature sperm consists of three regions—a head, a midpiece, and a tail, or flagellum. The tip of the head region contains enzymes. During fertilization, these enzymes help the sperm penetrate the protective layers that surround an egg cell. Also located in the head region are the 23 chromosomes that will be delivered to the egg. The midpiece is packed with mitochondria. These mitochondria supply the energy that is required for sperm to reach an egg. The tail consists of a single, powerful flagellum that propels the sperm.

Path of Sperm Through the Male Body

Mature sperm move through and past several other male reproductive structures, some of which further prepare the sperm for a possible journey through the female reproductive system. The path taken by sperm as they exit the body is shown in Figure 51-3.

Sperm move from the seminiferous tubules in the testes to the epididymis (EP-uh-DID-i-mis), a long, coiled tubule that is closely attached to each testis. Within each epididymis, a sperm matures and gains the ability to swim as its flagellum completes development. Although most sperm remain stored in each epididymis, some leave the epididymis and pass through the vas deferens (vas DEF-uh-RENZ), a duct that extends from the epididymis. Smooth muscles that line each vas deferens contract to help move sperm along as they exit the body. Each vas deferens enters the abdominal cavity, where it loops around the urinary bladder and merges with the urethra. The urethra is the duct through which urine exits the urinary bladder. Thus, in a male, both urine and sperm exit the body through the urethra, but not at the same time.

figure 51-2

A mature sperm is an elongated cell with three distinct parts (a head, a midpiece, and a tail), all of which are enclosed by a cell membrane.

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Urethra Penis

In the urethra, sperm mix with fluids that are secreted by three exocrine glands—the seminal vesicles, the prostate gland, and the bulbourethral (buhl-boh-yoo-REE-thruhl) glands. Ducts that extend from these glands connect with the urethra. The glands secrete fluids that nourish and protect the sperm as they move through the female reproductive system. The seminal vesicles, which lie between the bladder and the rectum, produce a fluid rich in sugars that sperm use for energy. The prostate (PRAHS-tayt) gland, which is located just below the bladder, secretes an alkaline fluid that neutralizes the acids in the female reproductive system. Before sperm leave the body, the bulbourethral glands secrete an alkaline fluid that neutralizes traces of acidic urine in the urethra. Together, sperm and these secretions form a fluid called semen (SEE-muhn). To help sperm move through the female reproductive system, semen also contains prostaglandins that stimulate contractions of smooth muscles that line the female reproductive tract.

Delivery of Sperm

The urethra passes through the penis, the organ that deposits sperm in the female reproductive system. When a male becomes sexually aroused, the spongy tissue in the penis, which is shown in Figure 51-4, fills with blood. This causes the penis to become erect, enabling it to deposit sperm. Semen is forcefully expelled from the penis by contractions of the smooth muscles that line the urethra. This process is called ejaculation (ee-JAK-yoo-LAY-shun). Each ejaculation expels 3 to 4 mL (0.10 to 0.14 fl oz) of semen. Sperm make up only 10 percent of this volume. Although a single ejaculation can expel 300 million to 400 million sperm, very few of these sperm reach the site of fertilization. Most sperm are killed by the acidic environment of the female reproductive tract.

Urinary

Urinary

Urethra Penis

Seminal vesicle

Rectum

Epididymis

Testis Scrotum figure 51-3

Bulbourethral gland

Seminal vesicle

Rectum

Epididymis

Testis Scrotum figure 51-3

Bulbourethral gland

The male reproductive system consists of several internal and external structures. Arrows indicate the path taken by sperm as they leave the body.

figure 51-4

When the spaces in the spongy tissue of the penis fill with blood, the penis becomes erect.

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