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Chapter 22: Reproductive Systems

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Chapter Summary

Introduction (page 881)

Various reproductive organs produce sex cells and sex hormones, sustain these cells, or transport them from place to place.

Organs of the Male Reproductive System (page 881)

The male reproductive organs produce and maintain sperm cells, transport these cells, and produce male sex hormones. The primary male sex organs are the testes, which produce sperm cells and male sex hormones. Accessory organs include the internal and external reproductive organs.

Testes (page 881)

1. Descent of the testes a. Testes originate posterior to the parietal peritoneum near the level of the developing kidneys.

b. The gubernaculum guides the descent of the testes into the lower abdominal cavity and through the inguinal canal.

c. Undescended testes fail to produce sperm cells because of the high abdominal temperature.

2. Structure of the testes a. The testes are composed of lobules separated by connective tissue and filled with the seminiferous tubules.

b. The seminiferous tubules unite to form the rete testis that joins the epididymis.

c. The seminiferous tubules are lined with epithelium, which produces sperm cells.

d. The interstitial cells that produce male sex hormones occur between the seminiferous tubules.

3. Formation of sperm cells a. The epithelium lining the seminiferous tubules includes sustentacular cells and spermatogenic cells.

(1) The sustentacular cells support and nourish the spermatogenic cells.

(2) The spermatogenic cells give rise to spermatogonia.

b. Meiosis consists of two divisions, each progressing through prophase, metaphase, anaphase, and telophase.

(1) In the first meiotic division, homologous, replicated chromosomes (each consisting of two chromatids held together by a centromere) separate, and their number is halved.

(2) In the second meiotic division, the chromatids part, producing four haploid cells from each diploid cell undergoing meiosis.

(3) The meiotic products mature into sperm cells or oocyte and polar bodies.

(4) Meiosis leads to genetic variability because of the random alignment of maternally and paternally derived chromosomes in metaphase I and crossing-over.

c. The process of spermatogenesis produces sperm cells from spermatogonia.

(1) Meiosis reduces the number of chromosomes in sperm cells by one-half (forty-six to twenty-three).

(2) Spermatogenesis produces four sperm cells from each primary spermatocyte.

d. Membranous processes of adjacent sustentacular cells form a barrier within the epithelium.

(1) The barrier separates early and advanced stages of spermatogenesis.

(2) It helps provide a favorable environment for differentiating cells.

4. Structure of a sperm cell a. Sperm head contains a nucleus with twenty-three chromosomes.

b. Sperm body contains many mitochondria.

c. Sperm tail propels the cell.

Male Internal Accessory Organs (page 889)

1. Epididymis a. The epididymis is a tightly coiled tube on the outside of the testis that leads into the vas deferens.

b. It stores and nourishes immature sperm cells and promotes their maturation.

2. Vas deferens a. The vas deferens is a muscular tube that forms part of the spermatic cord.

b. It passes through the inguinal canal, enters the abdominal cavity, courses medially into the pelvic cavity, and ends behind the urinary bladder.

c. It fuses with the duct from the seminal vesicle to form the ejaculatory duct.

3. Seminal vesicle a. The seminal vesicle is a saclike structure attached to the vas deferens.

b. It secretes an alkaline fluid that contains nutrients, such as fructose, and prostaglandins.

c. This secretion is added to sperm cells during emission.

4. Prostate gland a. This gland surrounds the urethra just below the urinary bladder.

b. It secretes a thin, milky fluid, which enhances the motility of sperm cells and neutralizes the fluid containing the sperm cells as well as acidic secretions of the vagina.

5. Bulbourethral glands a. These glands are two small structures inferior to the prostate gland.

b. They secrete a fluid that lubricates the penis in preparation for sexual intercourse.

6. Semen a. Semen is composed of sperm cells and secretions of the seminal vesicles, prostate gland, and bulbourethral glands.

b. This fluid is slightly alkaline and contains nutrients and prostaglandins.

c. Sperm cells in semen begin to swim, but these sperm cells are unable to fertilize egg cells until they enter the female reproductive tract.

Male External Reproductive Organs (page 891)

1. Scrotum a. The scrotum is a pouch of skin and subcutaneous tissue that encloses the testes.

b. The dartos muscle in the scrotal wall causes the skin of the scrotum to be held close to the testes or to hang loosely, thus regulating the temperature for sperm production and survival.

2. Penis a. The penis conveys urine and semen.

b. It is specialized to become erect for insertion into the vagina during sexual intercourse.

c. Its body is composed of three columns of erectile tissue surrounded by connective tissue.

d. The root of the penis is attached to the pelvic arch and membranes of the perineum.

3. Erection, orgasm, and ejaculation a. During erection, the vascular spaces within the erectile tissue become engorged with blood as arteries dilate and veins are compressed.

b. Orgasm is the culmination of sexual stimulation and is accompanied by emission and ejaculation.

c. Semen moves along the reproductive tract as smooth muscle in the walls of the tubular structures contract, stimulated by a reflex.

d. Following ejaculation, the penis becomes flaccid.

Hormonal Control of Male Reproductive Functions (page 896)

1. Hypothalamic and pituitary hormones

The male body remains reproductively immature until the hypothalamus releases GnRH, which stimulates the anterior pituitary gland to release gonadotropins.

a. FSH stimulates spermatogenesis.

b. LH (ICSH) stimulates the interstitial cells to produce male sex hormones.

c. Inhibin prevents oversecretion of FSH.

2. Male sex hormones a. Male sex hormones are called androgens.

b. Testosterone is the most important androgen.

c. Testosterone is converted into dihydrotestosterone in some organs.

d. Androgens that fail to become fixed in tissues are metabolized in the liver and excreted.

e. Androgen production increases rapidly at puberty.

3. Actions of testosterone a. Testosterone stimulates the development of the male reproductive organs and causes the testes to descend.

b. It is responsible for the development and maintenance of male secondary sex characteristics.

4. Regulation of male sex hormones a. A negative feedback mechanism regulates testosterone concentration.

(1) As the concentration of testosterone rises, the hypothalamus is inhibited, and the anterior pituitary secretion of gonadotropins is reduced.

(2) As the concentration of testosterone falls, the hypothalamus signals the anterior pituitary to secrete gonadotropins.

b. The concentration of testosterone remains relatively stable from day to day.

Organs of the Female Reproductive System (page 898)

The primary female sex organs are the ovaries, which produce female sex cells and sex hormones. Accessory organs are internal and external.

Ovaries (page 898)

1. Ovary attachments a. Several ligaments hold the ovaries in position.

b. These ligaments include broad, suspensory, and ovarian ligaments.

2. Ovary descent a. The ovaries descend from posterior to the parietal peritoneum near the developing kidneys.

b. They are attached to the pelvic wall just inferior to the pelvic brim.

3. Ovary structure a. The ovaries are subdivided into a medulla and a cortex.

b. The medulla is composed of connective tissue, blood vessels, lymphatic vessels, and nerves.

c. The cortex contains ovarian follicles and is covered by cuboidal epithelium.

4. Primordial follicles a. During prenatal development, groups of cells in the ovarian cortex form millions of primordial follicles.

b. Each primordial follicle contains a primary oocyte and a layer of flattened epithelial cells.

c. The primary oocyte begins to undergo meiosis, but the process is soon halted and is not continued until puberty.

d. The number of oocytes steadily declines throughout the life of a female.

5. Oogenesis a. Beginning at puberty, some oocytes are stimulated to continue meiosis.

b. When a primary oocyte undergoes oogenesis, it gives rise to a secondary oocyte in which the original chromosome number is reduced by one-half (from forty-six to twenty-three).

c. A secondary oocyte may be fertilized to produce a zygote.

6. Follicle maturation a. At puberty, FSH initiates follicle maturation.

b. During maturation, the oocyte enlarges, the follicular cells proliferate, and a fluid-filled cavity appears and produces a secondary follicle.

c. Ovarian cells surrounding the follicle form two layers.

d. Usually only one follicle reaches full development.

7. Ovulation a. Ovulation is the release of an oocyte from an ovary.

b. The oocyte is released when its follicle ruptures.

c. After ovulation, the oocyte is drawn into the opening of the uterine tube.

Female Internal Accessory Organs (page 902)

1. Uterine tubes a. These tubes convey egg cells toward the uterus.

b. The end of each uterine tube is expanded, and its margin bears irregular extensions.

c. Ciliated cells that line the tube and peristaltic contractions in the wall of the tube move an egg cell into the tube's opening.

2. Uterus a. The uterus receives the embryo and sustains it during development.

b. The vagina partially encloses the cervix.

c. The uterine wall includes the endometrium, myometrium, and perimetrium.

3. Vagina a. The vagina connects the uterus to the vestibule.

b. It receives the erect penis, conveys uterine secretions to the outside, and provides an open channel for the fetus during birth.

c. The vaginal orifice is partially closed by a thin membrane, the hymen.

d. Its wall consists of a mucosa, muscularis, and outer fibrous coat.

Female External Reproductive Organs (page 906)

1. Labia majora a. The labia majora are rounded folds of adipose tissue and skin that enclose and protect the other external reproductive parts.

b. The upper ends form a rounded elevation over the symphysis pubis.

2. Labia minora a. The labia minora are flattened, longitudinal folds between the labia majora.

b. They are well supplied with blood vessels.

3. Clitoris a. The clitoris is a small projection at the anterior end of the vulva; it corresponds to the male penis.

b. It is composed of two columns of erectile tissue.

c. Its root is attached to the sides of the pubic arch.

4. Vestibule a. The vestibule is the space between the labia minora that encloses the vaginal and urethral openings.

b. The vestibular glands secrete mucus into the vestibule during sexual stimulation.

5. Erection, lubrication, and orgasm a. During periods of sexual stimulation, the erectile tissues of the clitoris and vestibular bulbs become engorged with blood and swollen.

b. The vestibular glands secrete mucus into the vestibule and vagina.

c. During orgasm, the muscles of the perineum, uterine wall, and uterine tubes contract rhythmically.

Hormonal Control of Female Reproductive Functions (page 908)

Hormones from the hypothalamus, anterior pituitary gland, and ovaries play important roles in the control of sex cell maturation and the development and maintenance of female secondary sex characteristics.

1. Female sex hormones a. A female body remains reproductively immature until about ten years of age when gonadotropin secretion increases.

b. The most important female sex hormones are estrogens and progesterone.

(1) Estrogens are responsible for the development and maintenance of most female secondary sex characteristics.

(2) Progesterone causes changes in the uterus.

2. Female reproductive cycle a. The menstrual cycle is characterized by regularly recurring changes in the uterine lining culminating in menstrual flow.

b. A menstrual cycle is initiated by FSH, which stimulates maturation of a follicle.

c. Granulosa cells of a maturing follicle secrete estrogens, which are responsible for maintaining the secondary sex traits and thickening the uterine lining.

d. Ovulation is triggered when the anterior pituitary gland releases a relatively large amount of LH.

e. Following ovulation, the follicular cells and thecal cells give rise to the corpus luteum.

(1) The corpus luteum secretes estrogens and progesterone, which cause the uterine lining to become more vascular and glandular.

(2) If an oocyte is not fertilized, the corpus luteum begins to degenerate.

(3) As the concentrations of estrogens and progesterone decline, the uterine lining disintegrates, causing menstrual flow.

f. During this cycle, estrogens and progesterone inhibit the release of LH and FSH; as the concentrations of these hormones decline, the anterior pituitary secretes FSH and LH again, stimulating a new menstrual cycle.

3. Menopause a. Eventually the ovaries cease responding to FSH, and cycling ceases.

b. Menopause is characterized by a low concentration of estrogens and a continuous secretion of FSH and LH.

c. The female reproductive organs undergo varying degrees of regressive changes.

Pregnancy (page 912)

1. Transport of sex cells a. Ciliary action aids movement of the egg cell to the uterine tube.

b. A sperm cell moves by its tail lashing and muscular contraction in the female reproductive tract.

2. Fertilization and implantation a. With the aid of an enzyme, a sperm cell penetrates the zona pellucida.

b. When a sperm cell penetrates an egg cell membrane, changes in the egg cell membrane and the zona pellucida prevent entry of additional sperm.

c. Fusion of the nuclei of a sperm and an egg cell complete fertilization.

d. The product of fertilization is a zygote with forty-six chromosomes.

3. Early embryonic development a. Cells undergo mitosis, giving rise to smaller and smaller cells.

b. The developing offspring (preembryo) moves down the uterine tube to the uterus, where it implants in the endometrium.

4. Hormonal changes during pregnancy a. Embryonic cells produce hCG that maintains the corpus luteum.

b. Placental tissue produces high concentrations of estrogens and progesterone.

(1) Estrogens and progesterone maintain the uterine wall and inhibit secretion of FSH and LH.

(2) Progesterone and relaxin inhibit contractions of uterine muscles.

(3) Estrogens cause enlargement of the vagina.

(4) Relaxin helps relax the ligaments of the pelvic joints.

c. The placenta secretes placental lactogen that stimulates the development of the breasts and mammary glands.

d. During pregnancy, increasing secretion of aldosterone promotes retention of sodium and body fluid, and increasing secretion of parathyroid hormone helps maintain a high concentration of maternal blood calcium.

5. Other changes during pregnancy a. The uterus enlarges greatly.

b. The woman's blood volume, cardiac output, breathing rate, and urine production increase.

c. The woman's dietary needs increase, but if intake is inadequate, fetal tissues have priority for use of available nutrients.

6. Birth a. Pregnancy usually lasts forty weeks from the beginning of the last menstrual cycle.

b. During pregnancy, placental progesterone inhibits uterine contractions.

c. A variety of factors are involved with the birth process.

(1) A decreasing concentration of progesterone and the release of prostaglandins may initiate the birth process.

(2) The posterior pituitary gland releases oxytocin.

(3) Uterine muscles are stimulated to contract, and labor begins.

(4) A positive feedback mechanism causes stronger contractions and greater release of oxytocin.

d. Following the birth of the infant, placental tissues are expelled.

Mammary Glands (page 919)

1. Location of the glands a. The mammary glands are located in the subcutaneous tissue of the anterior thorax within the breasts.

b. The breasts extend between the second and sixth ribs and from sternum to axillae.

2. Structure of the glands a. The mammary glands are composed of lobes that contain tubular glands.

b. The lobes are separated by dense connective and adipose tissues.

c. The mammary glands are connected to the nipple by ducts.

3. Development of the breasts a. Breasts of males remain nonfunctional.

b. Estrogens stimulate breast development in females.

(1) Alveolar glands and ducts enlarge.

(2) Fat is deposited around and within the breasts.

c. During pregnancy, the breasts change.

(1) Estrogens cause the ductile system to grow.

(2) Progesterone causes development of alveolar glands.

(3) Prolactin is released during pregnancy, but progesterone inhibits milk production.

4. Milk production and secretion a. Following childbirth, the concentrations of placental hormones decline.

(1) The action of prolactin is no longer blocked.

(2) The mammary glands begin to secrete milk.

b. Reflex response to mechanical stimulation of the nipple causes the posterior pituitary to release oxytocin, which causes milk to be ejected from the alveolar ducts.

c. As long as milk is removed from glands, more milk is produced; if milk is not removed, production ceases.

d. During the period of milk production, the menstrual cycle is partially inhibited.

Birth Control (page 923)

Voluntary regulation of the number of children produced and the time they are conceived is called birth control. This usually involves some method of contraception.

1. Coitus interruptus a. Coitus interruptus is withdrawal of the penis from the vagina before ejaculation.

b. Some semen may be expelled from the penis before ejaculation.

2. Rhythm method a. Abstinence from sexual intercourse a few days before and after ovulation is the rhythm method.

b. It is almost impossible to accurately predict the time of ovulation.

3. Mechanical barriers a. Males and females can use condoms.

b. Females use diaphragms.

4. Chemical barriers a. Spermicidal creams, foams, and jellies are chemical barriers to conception.

b. These provide an unfavorable environment in the vagina for sperm survival.

5. Oral contraceptives a. Tablets that contain synthetic estrogenlike and progesteronelike substances are taken by the woman.

b. They disrupt the normal pattern of gonadotropin secretion and prevent ovulation and the normal buildup of the uterine lining.

c. When used correctly, this method is almost 100% effective.

d. Some women develop undesirable side effects.

6. Injectable contraceptives a. Intramuscular injection with medroxyprogesterone acetate every three months.

b. High levels of hormone act similarly to oral contraceptives to prevent pregnancy.

c. Very effective if administered promptly at the end of the three months.

d. Women may experience side effects; in some women, use is contraindicated.

7. Contraceptive implants a. A contraceptive implant consists of a set of progesterone-containing capsules or rods that are inserted under the skin.

b. Progesterone released from the implant prevents ovulation.

c. The implant is effective for years, and its action can be reversed by having it removed.

8. Intrauterine devices a. An IUD is a solid object inserted in the uterine cavity.

b. It is thought to prevent pregnancy by interfering with implantation.

c. It may be expelled spontaneously or produce undesirable side effects.

9. Surgical methods a. These are sterilization procedures.

(1) Vasectomy is performed in males.

(2) Tubal ligation is performed in females.

b. Surgical methods are the most reliable forms of contraception.

Critical Thinking Questions

Sexually Transmitted Diseases (page 930)

1. Sexually transmitted diseases are passed during sexual contact and may go undetected for years.

2. The twenty such disorders share certain symptoms.

1. What changes, if any, might occur in the secondary sex characteristics of an adult male following removal of one testis? Following removal of both testes? Following removal of the prostate gland?

2. How would you explain the fact that new mothers sometimes experience cramps in their lower abdomens when they begin to nurse their babies?

3. If a woman who is considering having a tubal ligation asks, "Will the operation cause me to go through my change of life early?" how would you answer?

4. What effect would it have on a woman's menstrual cycles if a single ovary were removed surgically? What effect would it have if both ovaries were removed?

5. As a male reaches adulthood, what will be the consequences if his testes have remained undescended since birth? Why?

6. Why does injecting a sperm cell into an egg cell not result in fertilization?

7. What types of contraceptives provide the greatest protection against sexually transmitted diseases?

8. Some men are unable to become fathers because their spermatids do not mature into sperm. Injection of their spermatids into their partner's secondary oocytes sometimes results in conception. A few men have fathered healthy babies this way. Why would this procedure work with spermatids but not with primary spermatocytes?

9. Understanding the causes of infertility can be valuable in developing new birth control methods. Cite a type of contraceptive based on each of the following causes of infertility: (a) failure to ovulate due to a hormonal imbalance; (b) a large fibroid tumor that disturbs the uterine lining; (c) endometrial tissue blocking uterine tubes; (d) low sperm count (too few sperm per ejaculate).

10. Sometimes, a sperm cell fertilizes a polar body rather than an oocyte. An embryo does not develop, and the fertilized polar body degenerates. Why is a polar body unable to support development of an embryo?

Review Exercises

1. List the general functions of the male reproductive system.

2. Distinguish between the primary and accessory male reproductive organs.

3. Describe the descent of the testes.

4. Define cryptorchidism.

5. Describe the structure of a testis.

6. Explain the function of the sustentacular cells in the testis.

7. Outline the process of meiosis.

8. List two ways that meiosis provides genetic variability.

9. List the major steps in spermatogenesis.

10. Describe a sperm cell.

11. Describe the epididymis, and explain its function.

12. Trace the path of the vas deferens from the epididymis to the ejaculatory duct.

13. On a diagram, locate the seminal vesicles, and describe the composition of their secretion.

14. On a diagram, locate the prostate gland, and describe the composition of its secretion.

15. On a diagram, locate the bulbourethral glands, and explain the function of their secretion.

16. Describe the composition of semen.

17. Define capacitation.

18. Describe the structure of the scrotum.

19. Describe the structure of the penis.

20. Explain the mechanism that produces an erection of the penis.

21. Distinguish between emission and ejaculation.

22. Explain the mechanism of ejaculation.

23. Explain the role of GnRH in the control of male reproductive functions.

24. Distinguish between androgen and testosterone.

25. Define puberty.

26. Describe the actions of testosterone.

27. List several male secondary sex characteristics.

28. Explain the regulation of testosterone concentration.

29. List the general functions of the female reproductive system.

30. Distinguish between the primary and accessory female reproductive organs.

31. Describe how the ovaries are held in position.

32. Describe the descent of the ovaries.

33. Describe the structure of an ovary.

34. Define primordial follicle.

35. List the major steps in oogenesis.

36. Distinguish between a primary and a secondary follicle.

37. Describe how a follicle matures.

38. Define ovulation.

39. On a diagram, locate the uterine tubes, and explain their function.

40. Describe the structure of the uterus.

41. Describe the structure of the vagina.

42. Distinguish between the labia majora and the labia minora.

43. On a diagram, locate the clitoris, and describe its structure.

44. Define vestibule.

45. Describe the process of erection in the female reproductive organs.

46. Define orgasm.

47. Explain the role of GnRH in regulating female reproductive functions.

48. List several female secondary sex characteristics.

49. Define menstrual cycle.

50. Explain how a menstrual cycle is initiated.

51. Summarize the major events in a menstrual cycle.

52. Define menopause.

53. Describe how male and female sex cells are transported within the female reproductive tract.

54. Describe the process of fertilization.

55. Explain the major hormonal changes that occur in the maternal body during pregnancy.

56. Describe the major nonhormonal changes that occur in the maternal body during pregnancy.

57. Describe the role of progesterone in initiating the birth process.

58. Discuss the events that occur during the birth process.

59. Describe the structure of a mammary gland.

60. Explain the roles of prolactin and oxytocin in milk production and secretion.

61. Define contraception.

62. List several methods of contraception, and explain how each prevents pregnancy.

63. List several sexually transmitted diseases.

Space Outside Blood Vessels

understanding ^Vo rds allant-, sausage: allantois— tubelike structure that extends from the yolk sac into the connecting stalk of an embryo. cleav-, to divide: cleavage— period of development characterized by a division of the zygote into smaller and smaller cells. ect-, outside: ectoderm—

outermost germ layer of embryo. lacun-, pool: lacuna—space between the chorionic villi that fills with maternal blood.

lanug-, down: lanugo—fine hair covering the fetus. mes-, middle: mesoderm— middle germ layer of embryo. morul-, mulberry: morula— embryonic structure consisting of a solid ball of about sixteen cells, thus looking somewhat like a mulberry. nat-, to be born: prenatal— period of development before birth. ne-, new, young: neonatal period—period of development including the first four weeks after birth. post-, after: postnatal period— period of development after birth. pre-, before: prenatal period— period of development before birth. sen-, old: senescence—process of growing old. troph-, nurture: trophoblast— cellular layer that surrounds the inner cell mass and helps nourish it. umbil-, navel: umbilical cord— structure attached to the fetal navel (umbilicus) that connects the fetus to the placenta.

chapter objectivi

After you have studied this chapter, you should be able to

Distinguish between growth and development. Describe the major events of the period of cleavage.

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