Mm M

Figure 22.7

Crossing-over mixes up traits. (a) Pairing of homologous chromosomes, (b) chromatids crossing over, (c) results of crossing-over. The different colors represent the fact that one homologous chromosome comes from the individual's father and one from the mother.

2. Metaphase I. During the first metaphase, chromosome pairs line up about midway between the poles of the developing spindle, and they are held under great tension, like two groups of people playing tug-of-war. Each chromosome pair consists of two chromosomes, which equals four chromatids. Each chromosome attaches to spindle fibers from one pole. The chromosome alignment is random with respect to maternal and paternal origin of the chromosomes. That is, each of the 23 chromosomes contributed from the mother may be on the left or the right, and the same is true for the paternal chromosomes—it is similar to the

, Chromatids

, Chromatids

Centrosome

Prophase II

Spindle fiber

Metaphase II

Centrosome

Prophase II

Spindle fiber

Anaphase II

Metaphase II

Anaphase II

Chromosome

Chromosome

Telophase II

Telophase II completed

Telophase II

Figure 22.8

Stages in the second meiotic division.

Telophase II completed number of ways that 23 pairs of children could line up, while maintaining the pairs. In figure 22.6, for example, if the upper chromosome pair has its maternal chromosome on the left (red) and its paternal chromosome on the right (green), then the second pair of chromosomes, depicted below the first pair, has the reverse configuration. Chromosomes can line up with respect to each other in many, many combinations.

3. Anaphase I. Homologous chromosome pairs separate, and each replicated member moves to one end of the spindle. Thus, each new cell receives only one replicated member of a homologous pair of chromosomes, overall reducing the chromosome number by one-half.

4. Telophase I. The original cell divides in two. Nuclear membranes form around the chromosomes, nucleoli reappear, and the spindle fibers disassemble into their constituent microtubules. Then the second meiotic division begins.

Meiosis II (fig 22.8) is very similar to a mitotic division. During prophase II, chromosomes condense so that they reappear, still replicated. They move into positions midway between the poles of the developing spindle. In metaphase II, the replicated chromosomes attach to spindle fibers. In anaphase II, centromeres separate, freeing the chromatids to move to opposite poles of the spindles. The former chromatids are now considered to be chromosomes. In telophase II, each of the two cells resulting from meiosis I divides to form two cells. Therefore, each cell undergoing meiosis has the potential to produce four gametes. In males, the gametes mature into four sperm cells. In females, three of the products of meiosis are "cast aside" as polar bodies, and one cell becomes the egg.

Meiosis generates astounding genetic variety. Any one of a person's more than 8 million possible combinations of 23 chromosomes can combine with any one of the more than 6 million combinations of his or her mate, raising the potential variability to more than 70 trillion genetically unique individuals! Crossing-over contributes even more genetic variability. Figure 22.9 illustrates in a simplified manner how maternal and paternal traits reassort during meiosis.

During spermatogenesis (sper"mah-to-jen'e-sis), the primary spermatocytes each divides to form two secondary spermatocytes. Each of these cells, in turn, divides to form two spermatids, which mature into sperm cells. Meiosis reduces the number of chromosomes in each cell by one-half. Consequently, for each primary spermatocyte that undergoes meiosis, four sperm cells with 23 chromosomes in each of their nuclei are formed. Because the chromosome number is halved, when a sperm and egg join in a process called fertilization (fer"ti-li-za'shun) the new individual has a complete set of 23 pairs of chromosomes.

Parent cell

Maternal chromatids

Paternal chromatids

Gene for blood type

Parent cell

Paternal chromatids

Gene for blood type

Maternal chromatids

Figure 22.9

As a result of crossing-over, the genetic information in sperm cells and egg cells varies from cell to cell. Colors represent parent of origin.

Figure 22.9

As a result of crossing-over, the genetic information in sperm cells and egg cells varies from cell to cell. Colors represent parent of origin.

The spermatogonia are located near the base of the germinal epithelium. As spermatogenesis occurs, cells in more advanced stages are pushed along the sides of sus-tentacular cells toward the lumen of the seminiferous tubule.

Near the base of the epithelium, membranous processes from adjacent sustentacular cells fuse by specialized junctions (occluding junctions) into complexes that divide the tissue into two layers. The spermatogo-nia are on one side of this barrier, and the cells in more advanced stages are on the other side. This membranous complex helps maintain a favorable environment for development of sperm cells by preventing certain large molecules from moving from the interstitial fluid of the basal epithelium into the region of the differentiating cells.

Spermatogenesis occurs continually in a male, starting at puberty. The resulting sperm cells collect in the lumen of each seminiferous tubule, then pass through the rete testis to the epididymis, where they accumulate and mature.

Sperm have fascinated biologists for centuries. Anton van Leeuwenhoek was the first to view human sperm under a microscope in 1678, concluding that they were parasites in semen. By 1685, he had modified his view, writing that sperm contain a preformed human being and are seeds requiring nurturing in a female to start a new life.

Structure of a Sperm Cell

A mature sperm cell is a tiny, tadpole-shaped structure about 0.06 millimeters long. It consists of a flattened head, a cylindrical body, and an elongated tail.

The oval head of a sperm cell is primarily composed of a nucleus and contains highly compacted chro-matin consisting of twenty-three chromosomes. A small protrusion at its anterior end, called the acrosome, contains enzymes, including hyaluronidase, that aid the sperm cell in penetrating an egg cell (fig. 22.10).

The midpiece of a sperm contains a central, filamentous core and many mitochondria organized in a spiral. The tail (flagellum) consists of several microtubules enclosed in an extension of the cell membrane. The mitochondria provide ATP for the lashing movement of the tail that propels the sperm cell through fluid. The scanning electron micrograph in figure 22.11 shows a few mature sperm cells.

Many toxic chemicals that affect sperm hamper their ability to swim, so the cells cannot transmit the toxin to an egg. One notable exception is cocaine, which attaches to thousands of binding sites on human sperm cells, without apparently harming the cells or impeding their movements. Sperm can ferry cocaine to an egg, but it is not known what harm, if any, the drug causes. We do know that fetuses exposed to cocaine in the uterus may suffer a stroke, or, as infants, be unable to react normally to their surroundings.

Explain the function of the sustentacular cells in the seminiferous tubules.

Describe the major events that occur during meiosis. How does meiosis provide genetic variability? Describe spermatogenesis. Describe the structure of a sperm cell.

Mitochondria

Golgi apparatus

Mitochondria

Excess cytoplasm

Golgi apparatus

Acrosome

Figure 22.10

(a) The head of the sperm develops largely from the nucleus of the formative cell. (b) Parts of a mature sperm cell.

Excess cytoplasm

Acrosome

-Head

-Head

Prostate Ejacu

Figure 22.10

(a) The head of the sperm develops largely from the nucleus of the formative cell. (b) Parts of a mature sperm cell.

Figure

Scanning electron micrograph of human sperm cells (1,400x).

Figure

Scanning electron micrograph of human sperm cells (1,400x).

Male Internal Accessory Organs

The internal accessory organs of the male reproductive system include the epididymides, vasa deferentia, ejacu-latory ducts, and urethra, as well as the seminal vesicles, prostate gland, and bulbourethral glands.

Epididymis

Each epididymis (ep"i-did'i-mis) (pl., epididymides) is a tightly coiled, threadlike tube about 6 meters long (see figs. 22.1, 22.12, and reference plate 52). The epididymis is connected to ducts within a testis. It emerges from the top of the testis, descends along its posterior surface, and then courses upward to become the vas deferens.

The inner lining of the epididymis is composed of pseudostratified columnar cells that bear nonmotile cilia. These cells secrete glycogen and other substances that support stored sperm cells and promote their maturation.

When immature sperm cells reach the epididymis, they are nonmotile. However, as they travel through the epididymis as a result of rhythmic peristaltic contractions, they mature. Following this aging process, the sperm cells can move independently and fertilize egg

Epithelial cells

Nonmotile cilia Sperm cells

Figure

Cross section of a human epididymis (50x micrograph enlarged to 145x).

Epithelial cells

Nonmotile cilia Sperm cells ure 22.12

Figure

Cross section of a human epididymis (50x micrograph enlarged to 145x).

cells (ova). However, they usually do not "swim" until after ejaculation.

Vas Deferens

Each vas deferens (vas def'er-enz) (pl., vasa deferentia), also called ductus deferens, is a muscular tube about 45 centimeters long that is lined with pseudostratified columnar epithelium (fig. 22.13). It begins at the lower end of the epididymis and passes upward along the medial side of a testis to become part of the spermatic cord. Each vas deferens passes through the inguinal canal, enters the abdominal cavity outside the parietal peritoneum, and courses over the pelvic brim. From there, it extends backward and medially into the pelvic cavity, where it ends behind the urinary bladder.

Near its termination, the vas deferens dilates into a portion called the ampulla. Just outside the prostate gland, the tube becomes slender again and unites with the duct of a seminal vesicle. The fusion of these two ducts forms an ejaculatory duct, which passes through the prostate gland and empties into the urethra through a slitlike opening (see fig. 22.1).

Seminal Vesicle

A seminal vesicle (see fig. 22.1) is a convoluted, saclike structure about 5 centimeters long that is attached to the vas deferens near the base of the urinary bladder. The glandular tissue lining the inner wall of the seminal vesicle secretes a slightly alkaline fluid. This fluid helps regulate the pH of the tubular contents as sperm cells travel to the outside. The secretion of the seminal vesicle also con-

Lumen Epithelium

Smooth muscle

Lumen Epithelium

Smooth muscle

Sperm cell in lumen of vas deferens

Pseudostratified columnar epithelium

- Smooth muscle layer

Figure 22.13

(a) Scanning electron micrograph of a cross section of the vas deferens (85x). (b) Light micrograph of the wall of the vas deferens (250x micrograph enlarged to 700x).

Sperm cell in lumen of vas deferens

Pseudostratified columnar epithelium

- Smooth muscle layer

Figure 22.13

(a) Scanning electron micrograph of a cross section of the vas deferens (85x). (b) Light micrograph of the wall of the vas deferens (250x micrograph enlarged to 700x).

Tissues and Organs: A Text-Atlas of Scanning Electron Microscopy, by R. G. Kessel and R. H. Kardon. © 1979 W. H. Freeman and Company.

tains fructose, a monosaccharide that provides energy to the sperm cells, and prostaglandins, which stimulate muscular contractions within the female reproductive organs, aiding the movement of sperm cells toward the egg cell.

At emission, the contents of the seminal vesicles empty into the ejaculatory ducts. This greatly increases the volume of the fluid discharged from the vas deferens.

H Describe the structure of the epididymis. ^9 Trace the path of the vas deferens. ^9 What is the function of a seminal vesicle?

Seminal Vessels Prostate

Secretory cells of the prostate gland

Smooth muscle

Urethra

Figure 22.14

Light micrograph of the prostate gland (10x).

Secretory cells of the prostate gland

Smooth muscle

Urethra

Figure 22.14

Light micrograph of the prostate gland (10x).

Prostate Gland

The prostate gland (see figs. 22.1 and 22.14) is a chestnut-shaped structure about 4 centimeters across and 3 centimeters thick that surrounds the beginning of the urethra, just inferior to the urinary bladder. It is composed of many branched tubular glands enclosed in connective tissue. Septa of connective tissue and smooth muscle extend inward from the capsule, separating the tubular glands. The ducts of these glands open into the urethra.

The prostate gland secretes a thin, milky fluid. This alkaline secretion neutralizes the sperm cell-containing fluid, which is acidic from accumulation of metabolic wastes from stored sperm cells. Prostatic fluid also enhances the motility of sperm cells, which remain relatively nonmotile in the acidic contents of the epididymis. In addition, the prostatic fluid helps neutralize the acidic secretions of the vagina, thus helping to sustain sperm cells that enter the female reproductive tract.

The prostate gland releases its secretions into the urethra as smooth muscles contract in its capsular wall. As this release occurs, the contents of the vas deferens and the seminal vesicles enter the urethra, which increases the volume of the fluid. Clinical Application 22.1 discusses the effects of prostate enlargement.

Bulbourethral Glands

The bulbourethral glands (Cowper's glands) are two small structures, each about a centimeter in diameter. They are located inferior to the prostate gland lateral to the membranous urethra and are enclosed by fibers of the external urethral sphincter muscle (see fig. 22.1).

The bulbourethral glands are composed of many tubes whose epithelial linings secrete a mucouslike fluid.

This fluid is released in response to sexual stimulation and lubricates the end of the penis in preparation for sexual intercourse (coitus). Females secrete most of the lubricating fluid for intercourse, however.

Semen

The fluid the urethra conveys to the outside during ejaculation is called semen (se'men). It consists of sperm cells from the testes and secretions of the seminal vesicles, prostate gland, and bulbourethral glands. Semen is slightly alkaline (pH about 7.5), and it includes prostaglandins and nutrients.

The volume of semen released at one time varies from 2 to 5 milliliters. The average number of sperm cells in the fluid is about 120 million per milliliter.

Sperm cells remain nonmotile while they are in the ducts of the testis and epididymis, but begin to swim as they mix with the secretions of accessory glands. However, sperm cells cannot fertilize an egg cell until they enter the female reproductive tract. Development of this ability, called capacitation, entails changes that weaken the acrosomal membranes of the sperm cells. When sperm cells are placed with egg cells in a laboratory dish to achieve fertilization—a technique called in vitro fertilization, discussed in Clinical Application 22.3— chemicals are added to simulate capacitation.

Although sperm cells can live for many weeks in the ducts of the male reproductive tract, they usually survive only a day or two after being expelled to the outside, even when they are maintained at body temperature. On the other hand, sperm cells can be stored and kept viable for years if they are frozen at a temperature below —100°C. Clinical Application 22.2 describes some causes of male infertility.

99 Where is the prostate gland located? ^9 What is the function of the prostate gland's secretion? ^9 What is the function of the bulbourethral glands? □ What are the components of semen?

MALE EXTERNAL Reproductive Organs

The male external reproductive organs are the scrotum, which encloses the testes, and the penis. The urethra passes through the penis.

Scrotum

The scrotum is a pouch of skin and subcutaneous tissue that hangs from the lower abdominal region posterior to the penis. The subcutaneous tissue of the scrotal wall lacks fat but contains a layer of smooth muscle fibers that constitute the dartos muscle. Exposure to cold stimulates these muscles to contract, the scrotal skin to wrinkle, and

Prostate Enlargement

The prostate gland is small in boys, begins to grow in early adolescence, and reaches adult size several years later. An adult's prostate gland is about the size of a walnut. Usually, the gland does not grow again until age fifty, when in half of all men it enlarges enough to press on the urethra. This condition is called benign prostatic hypertrophy (BPH). As many as 90 percent of men over age 70 may have BPH. It produces a feeling of pressure on the bladder because it cannot empty completely, and the man feels the urge to urinate frequently. An early sign may be dribbling after urination. Retained urine can lead to infection and inflammation, bladder stones, or kidney disease.

Medical researchers do not know what causes prostate enlargement. Risk factors include a fatty diet, having had a vasectomy, and possibly occupational exposure to batteries or the metal cadmium. The enlargement may be benign or cancerous. Because prostate cancer is highly treatable if detected early, men should have their prostates examined regularly. Four out of five men who have prostate cancer are over age 65.

Diagnostic tests for prostate cancer include a rectal exam; visualization of the prostate, urethra, and bladder with a device that is inserted through the penis, called a cytoscope; as well as a blood test to detect elevated prostate specific antigen (PSA), a cell surface protein normally found on prostate cells. Elevated PSA levels indicate an enlarged prostate, possibly from a benign or cancerous growth. Ultrasound may provide further information.

Table 22A summarizes treatments for an enlarged prostate. Many men opt for a "watchful waiting" approach, continuing to have frequent checkups to monitor the enlargement, but not taking action until symptoms arise. Surgery to treat prostate cancer is highly effective. It once commonly left a man incontinent and with erectile dysfunction. However, control of urination often returns within a few weeks, and newer surgical methods preserve the nerves that are necessary for erection to occur. ■

Medical Treatments for an Enlarged Prostate Gland

Surgical removal of some prostate tissue or entire gland Radiation

Drug (Proscar, or finasteride) to block testosterone's growth-stimulating effect on the prostate

Alpha blocker drugs, which relax muscles near the prostate, relieving pressure

Microwave energy delivered through a probe inserted into the urethra or rectum

Balloon inserted into the urethra and inflated with liquid

Liquid nitrogen delivered by a probe through the skin to freeze the tumor

Device (stent) inserted between lobes of prostate to relieve pressure on the urethra the testes to move closer to the pelvic cavity, where they can absorb heat. Exposure to warmth stimulates the fibers to relax and the scrotum to hang loosely, and provides an environment 3°C (about 5°F) below body temperature, which is more conducive to sperm production and survival.

A medial septum divides the scrotum into two chambers, each of which encloses a testis. Each chamber also contains a serous membrane, which covers the front and sides of the testis and the epididymis, helping to en sure that the testis and epididymis move smoothly within the scrotum (see fig. 22.1).

Penis

The penis is a cylindrical organ that conveys urine and semen through the urethra to the outside. It is also specialized to enlarge and stiffen by a process called erection, which enables it to be inserted into the vagina during sexual intercourse.

Subcutaneous — tissue

Subcutaneous — tissue

Figure 22.15

(a) Interior structure of the penis. (b) Cross section of the penis.

Connective tissue

External urethral orifice (a)

Figure 22.15

(a) Interior structure of the penis. (b) Cross section of the penis.

Dorsal vein

Dorsal nerve Dorsal artery Deep artery

Corpora cavernosa Tunica albuginea

Urethra -Corpus spongiosum Prepuce Glans penis

Dorsal vein

Dorsal nerve Dorsal artery Deep artery

Corpora cavernosa Tunica albuginea

Urethra -Corpus spongiosum Prepuce Glans penis

Subcutaneous tissue

Connective tissue

The body, or shaft, of the penis is composed of three columns of erectile tissue, which include a pair of dorsally located corpora cavernosa and a single, ventral corpus spongiosum. A tough capsule of white dense connective tissue called a tunica albuginea surrounds each column. Skin, a thin layer of subcutaneous tissue, and a layer of connective tissue enclose the penis (fig. 22.15).

The corpus spongiosum, through which the urethra extends, enlarges at its distal end to form a sensitive, cone-shaped glans penis. This structure covers the ends of the corpora cavernosa and bears the urethral opening— the external urethral orifice. The skin of the glans is very thin, hairless, and contains sensory receptors for sexual stimulation. A loose fold of skin called the prepuce (foreskin) begins just posterior to the glans and extends anteriorly to cover it as a sheath. A surgical procedure called circumcision is used to remove the prepuce.

At the root of the penis, the columns of erectile tissue separate. The corpora cavernosa diverge laterally in the perineum and are firmly attached to the inferior surface of the pubic arch by connective tissue. These diverging parts form the crura (sing., crus) of the penis. The single corpus spongiosum is enlarged between the crura as the bulb of the penis, which is attached to membranes of the perineum.

99 Describe the structure of the penis. ^9 What is circumcision?

^9 How is the penis attached to the perineum?

Erection, Orgasm, and Ejaculation

During sexual stimulation, parasympathetic nerve impulses from the sacral portion of the spinal cord release the vasodilator nitric oxide, which causes the arteries leading into the penis to dilate, increasing blood flow

Figure 22.16

Mechanism of penile erection.

Figure 22.16

Mechanism of penile erection.

into erectile tissues. At the same time, the increasing pressure of arterial blood entering the vascular spaces of the erectile tissue compresses the veins of the penis, reducing flow of venous blood away from the penis. Consequently, blood accumulates in the erectile tissues, and the penis swells and elongates, producing an erection (fig. 22.16).

The culmination of sexual stimulation is orgasm (or'gazm), a pleasurable feeling of physiological and psychological release. Orgasm in the male is accompanied by emission and ejaculation.

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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