I. Sexual Reproduction (p. 1017)

A. The Essence of Sex (p. 1017)

1. Sexual reproduction does not necessarily involve copulation or physical contact between the two parents. What it does mean is that each offspring has two parents and a combination of genes from both.

2. In sexual reproduction, the offspring are not genetically identical to their parents and usually not even to each other.

3. Genetic diversity provides the foundation for the survival and evolution of a species.

B. The Two Sexes (p. 1017)

1. To reproduce sexually means that the parents must produce gametes that can meet and combine their genes in a zygote. The gametes must provide motility and nutrition to make the combination successful.

2. The smaller, motile gamete is the spermatozoon, while the large, nutrient-laden one is the egg.

3. By definition, any individual that produces eggs is female and one that produces sperm is male.

4. Genetically, any individual with a Y sex chromosome is classified as male and any individual lacking a Y chromosome is classified as female.

5. In mammals, the female is also the parent that provides a sheltered internal environment for the development and prenatal nutrition of the embryo.

C. Overview of the Reproductive System (p. 1017)

1. The reproductive system consists of primary and secondary sex organs. The primary sex organs (gonads) are those that produce gametes. These are the testes of the male and ovaries of the female.

2. The secondary sex organs are those that are essential to reproduction, such as ducts, glands, and a penis in males, and uterine tubes, uterus, and vagina in females.

3. Secondary sex characteristics are features that are not essential for reproduction but that attract the sexes to each other.

II. Sex Determination and Development (p. 1018)

A. Role of the Sex Chromosomes (p. 1018)

1. Most of our cells have 23 pairs of chromosomes; 22 of these pairs are autosomes and 1 pair consists of sex chromosomes (X or Y).

2. The sex of the child is determined by the male, who donates either an X or a Y to the offspring. The female can only donate an X chromosome. (fig. 27.1; TR 893)

B. Hormones and Sex Differentiation (p. 1018)

1. Sex determination does not end with fertilization, but requires an interaction between genetics and the hormones produced by mother and fetus.

2. A fetus is sexually undifferentiated until about 5 to 6 weeks when gonadal ridges appear near the kidneys. (fig. 27.2; TR 894)

3. The Y chromosome has a gene for testis-determining factor (TDF), which interacts with genes on some of the other chromosomes to initiate the development of male anatomy. By 8–10 weeks, the XY fetus begins to secrete testosterone and mullerian-inhibiting factor (MIF).

4. The female results from the absence of androgens rather than the presence of estrogens.

C. Development of the External Genitalia (p. 1020; fig. 27.3; TR 895)

1. Most people regard the external genitals as the most definitive characteristics of a male or female.

2. In the fetus, the external genitals begin from identical structures in both sexes (at 8 weeks): a phallus, urogenital folds, and labioscrotal folds.

3. By 9 weeks, the fetus begins to show sexual differentiation, and male or female genitalia are formed by the end of week 12.

4. Male and female organs that develop from the same embryonic structure are said to be homologous. Thus the penis is homologous to the clitoris and the scrotum is homologous to the labia majora.

D. Descent of the Testes (p. 1022; fig. 27.4; TR 896)

1. In the embryo, a cord of muscle and connective tissue called the gubernaculum extends from the floor of the scrotum to the gonad. It shortens as the fetus grows and guides the testis through a passageway in the groin called the inguinal canal. This descent of the testes begins in weeks 6 to 10; by 28 weeks, the testes enter the scrotum.

2. In the scrotum, the testes are kept 2°C cooler than in the pelvic cavity. This is essential for sperm production.

a. About 3% of boys are born with one or both testes undescended, a condition known as cryptorchidism. If spontaneous descent does not occur, hormone treatment or surgery is used to move the testes into the scrotum.

III. Male Reproductive Anatomy (p. 1022; figs. 27.5, 27.6, 27.7; TR 897–901)

A. Testes (p. 1022)

1. The testis itself has a white fibrous capsule called the tunica albuginea. Connective tissue septa divide the organ into 250–300 wedge-shaped lobules. (fig. 27.8)

2. Each lobule contains one to three seminiferous tubules in which sperm are produced.

a. Between the seminiferous tubules are clusters of interstitial (Leydig) cells, the source of testosterone.

b. A seminiferous tubule has a narrow lumen lined with germinal epithelium consisting of several layers of germ cells in the process of becoming sperm or sustentacular (Sertoli) cells. Sustentacular cells protect the germ cells and promote their development.

c. Tight junctions between adjacent sustentacular cells form a blood-testis barrier (BTB), which prevents proteins and other large molecules in the blood and intercellular fluid from getting to the germ cells.

3. The seminiferous tubules lead into a network called the rete testis, embedded in the capsule on the posterior side. Sperm partially mature in the rete.

4. Each testis is supplied with blood by a testicular artery and drained by a testicular vein.

B. Scrotum (p. 1026; fig. 27.9; TR 902)

1. The testes are contained in a pendulous sac, the scrotum. The scrotum is divided into right and left compartments by an internal median septum, which protects each testis from an infection in the other one. The location of the septum is externally marked by a seam called the perineal raphe.

2. A structure called the spermatic cord passes up the back of the scrotum, then anterior to the pubis, and through the inguinal ring into the inguinal canal. It contains a sperm duct called the ductus deferens and other structures that descended with the testis as it passed through the inguinal canal.

3. The scrotum has three mechanisms for regulating the temperature of the testes.

a. The cremaster muscle contains strips of the internal abdominal oblique muscle around the spermatic cord. The cremaster muscle can elevate or lower the testes.

b. The dartos muscle is a subcutaneous layer of smooth muscle that causes wrinkling of the scrotum as temperature (distance from the body) is adjusted.

c. The pampiniform plexus is an extensive network of veins that surround the testicular artery in the spermatic cord, keeping the testes cooler by acting as a countercurrent heat exchanger. (fig. 27.10; TR 903)

C. Spermatic Ducts (p. 1027)

1. After leaving the testis, the sperm travel through a series of spermatic ducts to reach the urethra.

a. Efferent ductules arise from the posterior side of each testis and carry sperm to the epididymis.

b. The epididymis is a comma-shaped coiled tubule on the posterior side of the testis. It reabsorbs about 90% of the fluid secreted by the testis. Sperm remain stored here for 40–60 days and become reabsorbed if not ejaculated prior to that time.

c. The ductus (vas) deferens passes upward from the scrotum, travels through the inguinal canal, and enters the pelvic cavity. The ductus deferens has a very narrow lumen and a thick wall of smooth muscle. During orgasm, peristaltic contractions drive sperm from the epididymis through the ductus deferens to the ejaculatory duct.

d. The ejaculatory duct lies where the ductus deferens and duct of the seminal vesicle meet. The ejaculatory duct is the last of the spermatic ducts; it joins with the urethra.

e. The male urethra is shared by the reproductive and urinary systems.

D. Accessory Glands (p. 1028)

1. There are three sets of accessory glands in the male.

a. The seminal vesicles are a pair of glands posterior to the urinary bladder. The secretion of the seminal vesicles constitutes about 60% of the semen.

b. The prostate surrounds the urethra inferior to the urinary bladder. Its thin, milky secretion contributes about 30% of the semen.

c. The bulbourethral glands are located at the inner end of the penis and produce a clear, slippery fluid that lubricates the head of the penis during sexual arousal. The purpose of its secretions is to neutralize the acidity of the residual urine in the urethra, which would be harmful to sperm.

E. Penis (p. 1028)

1. The penis serves to deposit semen in the vagina. Half of it is an internal root, and half is the externally visible shaft and glans.

2. The penis consists mainly of three cylindrical bodies called erectile tissues, which fill with blood during an erection. (fig. 27.11; TR 904, 905)

a. The corpus spongiosum passes along the ventral side of the penis and encloses the penile urethra.

b. On the dorsal side of the penis, there are a pair of corpora cavernosa.

3. All three erectile tissues are spongy in appearance and contain numerous tiny blood sinuses called lacunae.

IV. Puberty and Climacteric (p. 1030)

A. Endocrine Control of Puberty (p. 1030; fig. 27.12; TR 906)

1. As the hypothalamus matures, it begins producing gonadotropin-releasing hormone (GnRH) at puberty. This hormone travels to the anterior pituitary and stimulates the production of follicle-stimulating hormone (FSH) and luteinizing hormone (LH). These two gonadotropins, as they are called, stimulate different cells in the testis.

a. LH stimulates the interstitial cells of the testis to secrete androgens (mainly testosterone).

b. FSH is needed in order for testosterone to have an effect on the testis because it stimulates the sustentacular cells to secrete androgen-binding protein (ABP).

2. The androgens stimulate spermatogenesis (in the presence of ABP), suppress the secretion of GnRH so that FSH and LH are held in check, and stimulate the development of secondary sex characteristics and other somatic changes of puberty.

a. Testosterone stimulates the secretion of growth hormone, resulting in a growth spurt, increased muscle mass, higher metabolic rate, and larger larynx.

b. Testosterone also stimulates the brain and awakens the libido.

c. Inhibin, a hormone from the sustentacular cells, selectively suppresses FSH output from the pituitary that effectively slows sperm production without inhibiting testosterone production.

B. Aging and Sexual Function (p. 1031)

1. Testosterone production peaks at about 7 mg/day at age 20 then declines steadily to as little as one-fifth of this level by age 80.

2. As testosterone and inhibin levels decline, so does feedback inhibition of the pituitary. Consequently, FSH and LH levels rise significantly in the late 40s to early 50s, producing changes called male climacteric.

3. About 20% of men in their 60s and 50% of men in their 80s experience impotence (erectile dysfunction) for a variety of reasons.

V. Spermatogenesis, Spermatozoa, and Semen (p. 1031)

A. Meiosis (p. 1032)

1. The most significant event of puberty is the onset of spermatogenesis, a process in which germ cells undergo two divisions called meiosis and the four daughter cells differentiate into spermatozoa.

2. Meiosis is fundamentally the same in both sexes. Briefly, it consists of two divisions and the following phases: prophase I, metaphase I, anaphase I, telophase I, interkinesis, prophase II, metaphase II, anaphase II, and telophase II. (fig. 27.13; TR 907)

3. At the end of meiosis, there are four haploid cells, each containing 23 single-stranded chromosomes. Fertilization combines 23 chromosomes from the father with 23 chromosomes from the mother, reestablishing the diploid number of 46 in the zygote.

B. Spermatogenesis (p. 1032; fig. 27.14; TR 908, 909)

1. The first cells destined to become sperm cells are the primordial germ cells. These differentiate into spermatogonia, which lie along the periphery of the seminiferous tubule, outside the blood-testis barrier (BTB).

2. Spermatogonia multiply by mitosis, producing two types of daughter cells, called type A and type B spermatogonia. Type A cells remain outside the BTB and continue to multiply from puberty until death.

3. Type B spermatogonia migrate closer to the tubule lumen and differentiate into slightly larger cells called primary spermatocytes. Sustentacular cells transfer these cells through the BTB toward the inside of the tubule. (fig. 27.15; TR 910)

4. After the BTB closes behind it, the primary spermatocyte undergoes meiosis I, giving rise to two haploid secondary spermatocytes. Each of these undergoes meiosis II, dividing into two spermatids. Each stage is a little closer to the lumen of the tubule.

5. The rest of spermatogenesis is called spermiogenesis and involves no further cell division but maturation of the sperm cells. Eventually, sperm cells are washed out of the seminiferous tubule by fluid from the sustentacular cells. (fig. 27.16; TR 911)

C. The Spermatozoon (p. 1034; fig. 27.17; TR 912)

1. The spermatozoon has a pear-shaped head and a long tail.

a. The head contains the haploid nucleus, an acrosome bearing enzymes used to dissolve a path to penetrate the egg, and a flagellar basal body.

b. The tail is divided into the midpiece, principal piece, and endpiece. The midpiece contains large mitochondria that produce ATP for sperm motility. The principal piece constitutes most of the tail and consists of the axoneme surrounded by a sheath of fibers. The endpiece consists of the axoneme only and is the narrowest part of the sperm.

D. Semen (p. 1036)

1. The fluid expelled during orgasm is called semen or seminal fluid. Its major constituents are shown in table 27.1.

VI. Male Sexual Response (p. 1037)

A. Anatomical Foundations (p. 1037)

1. Masters and Johnson divided intercourse into four recognizable phases called excitement, plateau, orgasm, and resolution.

2. The penis is richly supplied with blood from an internal pudendal artery that divides into a dorsal artery and a deep artery within the root of the penis. During arousal, blood enters lacunae from the deep artery. When the penis is flaccid, most of its blood supply comes from the dorsal arteries.

3. The penis is richly innervated by sensory and motor nerve fibers. Impulses from motor fibers can bring about an erection from thoughts and input to special senses. (fig. 27.18; TR 913, 914)

B. Excitement and Plateau (p. 1038)

1. The excitement phase is characterized by vasocongestion, myotonia, and increases in heart rate, blood pressure, and pulmonary ventilation. The bulbourethral glands secrete their fluid during this phase.

2. The most obvious manifestation of male sexual arousal is erection of the penis.

3. In the plateau phase, variables such as respiratory rate, heart rate, and blood pressure are sustained at a high level, or rise slightly, for a few seconds to a few minutes before orgasm.

C. Orgasm and Ejaculation (p. 1040)

1. The orgasm, or climax, is a short but intense reaction usually marked by the discharge of semen (ejaculation). This occurs in two stages.

a. In emission, the sympathetic nervous system causes peristaltic contractions of the ductus deferens, which propels sperm from the tail of the epididymis and into the ampulla of the ductus deferens. Further contractions propel the sperm into the prostatic urethra along with prostatic fluid and secretions from the seminal vesicles.

b. Semen in the urethra activates reflexes that produce expulsion of the semen. The bulbospongiosus, which envelops the root of the corpus spongiosum, undergoes five or six strong, spasmodic contractions that compress the urethra and forcibly expel the semen.

2. Some sperm may seep from the penis prior to ejaculation.

D. Resolution (p. 1041)

1. Immediately following orgasm comes the resolution phase. Discharge of the sympathetic nervous system reduces blood flow to the penis, and gradually the penis undergoes detumescence.

2. In men, resolution is followed by a refractory period, lasting anywhere from 10 minutes to a few hours, in which it is usually impossible to attain another erection and orgasm.

VII. Male Reproductive Disorders (p. 1041; table 27.2)