Reproduction and Embryonic Development

Reproduction and Embryonic Development
Chapter 27 Reproduction and Embryonic Development

Asexual and Sexual Reproduction Principles of Embryonic Development
Figure 27.0_1 Chapter 27: Big Ideas Asexual and Sexual Reproduction Human Reproduction Figure 27.0_1 Chapter 27: Big Ideas Principles of Embryonic Development Human Development 2

ASEXUAL AND SEXUAL REPRODUCTION
ASEXUAL AND SEXUAL REPRODUCTION © 2012 Pearson Education, Inc. 3

27.1 Asexual reproduction results in the generation of genetically identical offspring
Asexual reproduction is the creation of genetically identical offspring by one parent, is a very rapid form of reproduction, and can proceed via budding, fission, or fragmentation/regeneration. Student Misconceptions and Concerns Students do not often understand the costs and benefits of asexual and sexual reproduction. Consider discussing the advantages and disadvantages of each of these forms of reproduction. Encourage students to focus on the compromises involved in any adaptation. There is simply no one “best” way for all animals to reproduce. Teaching Tips Aphid life cycles usually alternate between asexual and sexual reproductive strategies during a single year. Consider challenging your class to explain why aphids (and other animals) do this. In general, sexual reproduction is most common in times of stress, and may be related to overpopulation or environmental change in which diversity may be favored. Video: Hydra Budding © 2012 Pearson Education, Inc. 4

27.2 Sexual reproduction results in the generation of genetically unique offspring
Sexual reproduction is the creation of offspring by fertilization and joins two haploid sex cells or gametes to form a diploid (2n) zygote. Student Misconceptions and Concerns 1. Students do not often understand the costs and benefits of asexual and sexual reproduction. Consider discussing the advantages and disadvantages of each of these forms of reproduction. Encourage students to focus on the compromises involved in any adaptation. There is simply no one “best” way for all animals to reproduce. 2. Many students expect that hermaphroditic animals simply fertilize themselves. Although this may be common in some animals, the exchange of gametes between hermaphrodites also occurs, as noted in the text. Teaching Tips Many salamander species use spermatophores to transfer sperm from the male to the female. Spermatophores are reproductive structures produced by males during courtship. Sperm is deposited atop a gelatinous base attached to the substrate. The female moves over the spermatophore, removes some or all of the sperm from the cap, and stores the sperm in her reproductive tract (a spermatheca) until the time of egg deposition. Thus, sperm transfer is external but fertilization is internal. © 2012 Pearson Education, Inc. 5

The male gamete, the sperm, is relatively small and moves by means of a flagellum. The female gamete, the egg, is a much larger cell and is not self-propelled. Student Misconceptions and Concerns 1. Students do not often understand the costs and benefits of asexual and sexual reproduction. Consider discussing the advantages and disadvantages of each of these forms of reproduction. Encourage students to focus on the compromises involved in any adaptation. There is simply no one “best” way for all animals to reproduce. 2. Many students expect that hermaphroditic animals simply fertilize themselves. Although this may be common in some animals, the exchange of gametes between hermaphrodites also occurs, as noted in the text. Teaching Tips 1. Many salamander species use spermatophores to transfer sperm from the male to the female. Spermatophores are reproductive structures produced by males during courtship. Sperm is deposited atop a gelatinous base attached to the substrate. The female moves over the spermatophore, removes some or all of the sperm from the cap, and stores the sperm in her reproductive tract (a spermatheca) until the time of egg deposition. Thus, sperm transfer is external but fertilization is internal. © 2012 Pearson Education, Inc. 6

Video: Hydra Releasing Sperm
27.2 Sexual reproduction results in the generation of genetically unique offspring Some organisms, such as sea anemones, can reproduce both asexually and sexually. Student Misconceptions and Concerns 1. Students do not often understand the costs and benefits of asexual and sexual reproduction. Consider discussing the advantages and disadvantages of each of these forms of reproduction. Encourage students to focus on the compromises involved in any adaptation. There is simply no one “best” way for all animals to reproduce. 2. Many students expect that hermaphroditic animals simply fertilize themselves. Although this may be common in some animals, the exchange of gametes between hermaphrodites also occurs, as noted in the text. Teaching Tips 1. Many salamander species use spermatophores to transfer sperm from the male to the female. Spermatophores are reproductive structures produced by males during courtship. Sperm is deposited atop a gelatinous base attached to the substrate. The female moves over the spermatophore, removes some or all of the sperm from the cap, and stores the sperm in her reproductive tract (a spermatheca) until the time of egg deposition. Thus, sperm transfer is external but fertilization is internal. Video: Hydra Releasing Sperm © 2012 Pearson Education, Inc. 7

Two offspring arising by fission
Figure 27.2A Eggs Two offspring arising by fission Figure 27.2A Asexual (left) and sexual (right) reproduction in the starlet sea anemone (Nematostella vectensis) 8

Some animals exhibit hermaphroditism in which an individual has both female and male reproductive systems. Hermaphroditism makes it easier to find a mate for animals that are solitary or less mobile. Hermaphrodites may exchange gametes with other individuals or fertilize their own eggs. Student Misconceptions and Concerns 1. Students do not often understand the costs and benefits of asexual and sexual reproduction. Consider discussing the advantages and disadvantages of each of these forms of reproduction. Encourage students to focus on the compromises involved in any adaptation. There is simply no one “best” way for all animals to reproduce. 2. Many students expect that hermaphroditic animals simply fertilize themselves. Although this may be common in some animals, the exchange of gametes between hermaphrodites also occurs, as noted in the text. Teaching Tips 1. Many salamander species use spermatophores to transfer sperm from the male to the female. Spermatophores are reproductive structures produced by males during courtship. Sperm is deposited atop a gelatinous base attached to the substrate. The female moves over the spermatophore, removes some or all of the sperm from the cap, and stores the sperm in her reproductive tract (a spermatheca) until the time of egg deposition. Thus, sperm transfer is external but fertilization is internal. © 2012 Pearson Education, Inc. 9

External fertilization occurs when eggs and sperm are discharged near each other and is used by many fish and amphibian species. Internal fertilization occurs when sperm is deposited in or near the female reproductive tract and is used by some fish and amphibian species and nearly all terrestrial animals. Student Misconceptions and Concerns 1. Students do not often understand the costs and benefits of asexual and sexual reproduction. Consider discussing the advantages and disadvantages of each of these forms of reproduction. Encourage students to focus on the compromises involved in any adaptation. There is simply no one “best” way for all animals to reproduce. 2. Many students expect that hermaphroditic animals simply fertilize themselves. Although this may be common in some animals, the exchange of gametes between hermaphrodites also occurs, as noted in the text. Teaching Tips 1. Many salamander species use spermatophores to transfer sperm from the male to the female. Spermatophores are reproductive structures produced by males during courtship. Sperm is deposited atop a gelatinous base attached to the substrate. The female moves over the spermatophore, removes some or all of the sperm from the cap, and stores the sperm in her reproductive tract (a spermatheca) until the time of egg deposition. Thus, sperm transfer is external but fertilization is internal. © 2012 Pearson Education, Inc. 10

Figure 27.2C Figure 27.2C Frogs in an embrace that triggers the release of eggs and sperm (the sperm are too small to be seen) Egg 11

27.3 Reproductive anatomy of the human female
Both sexes in humans have a set of gonads where gametes are produced, ducts for gamete transport, and structures for copulation. Student Misconceptions and Concerns 1. Students’ background knowledge of human reproductive biology is likely to be quite uneven. Furthermore, the embarrassment frequently associated with this topic makes it difficult for teachers to fairly assess what students know. The best advice may be to not assume too much. 2. Embarrassment with the subject of human reproductive biology may make open discussions uncomfortable for some students. Good clear textbook and media assignments that can be studied privately and opportunities to ask anonymous questions provide additional avenues to address sensitive content and questions. Teaching Tips 1. Ectopic pregnancies occur when an embryo implants anywhere other than the uterus. Most frequently, ectopic pregnancies occur in the oviducts. However, the structure of the oviduct cannot accommodate the growth of a fetus. Surgical removal of the fetus, resulting in an abortion, is thus often required for the sake of the mother’s health. Many of those who believe that abortion is wrong in general, may consider abortions acceptable in the case of ectopic pregnancy, in that it may literally save the life of the mother. 2. Endometriosis is an inflammatory condition in which the endometrium spreads beyond the uterus. It can lead to painful menstrual cycles and infertility. The Endometriosis Association website is a good resource for additional information. © 2012 Pearson Education, Inc. 13

Ovaries contain follicles that nurture eggs and produce sex hormones. An immature egg is ejected from the follicle in a process called ovulation. Student Misconceptions and Concerns 1. Students’ background knowledge of human reproductive biology is likely to be quite uneven. Furthermore, the embarrassment frequently associated with this topic makes it difficult for teachers to fairly assess what students know. The best advice may be to not assume too much. 2. Embarrassment with the subject of human reproductive biology may make open discussions uncomfortable for some students. Good clear textbook and media assignments that can be studied privately and opportunities to ask anonymous questions provide additional avenues to address sensitive content and questions. Teaching Tips 1. Ectopic pregnancies occur when an embryo implants anywhere other than the uterus. Most frequently, ectopic pregnancies occur in the oviducts. However, the structure of the oviduct cannot accommodate the growth of a fetus. Surgical removal of the fetus, resulting in an abortion, is thus often required for the sake of the mother’s health. Many of those who believe that abortion is wrong in general, may consider abortions acceptable in the case of ectopic pregnancy, in that it may literally save the life of the mother. 2. Endometriosis is an inflammatory condition in which the endometrium spreads beyond the uterus. It can lead to painful menstrual cycles and infertility. The Endometriosis Association website is a good resource for additional information. Animation: Female Reproductive Anatomy © 2012 Pearson Education, Inc. 14

Oviducts convey eggs to the uterus where a fertilized egg develops. The uterus opens into the vagina through the cervix. The vagina receives the penis during sexual intercourse and forms the birth canal. Student Misconceptions and Concerns 1. Students’ background knowledge of human reproductive biology is likely to be quite uneven. Furthermore, the embarrassment frequently associated with this topic makes it difficult for teachers to fairly assess what students know. The best advice may be to not assume too much. 2. Embarrassment with the subject of human reproductive biology may make open discussions uncomfortable for some students. Good clear textbook and media assignments that can be studied privately and opportunities to ask anonymous questions provide additional avenues to address sensitive content and questions. Teaching Tips 1. Ectopic pregnancies occur when an embryo implants anywhere other than the uterus. Most frequently, ectopic pregnancies occur in the oviducts. However, the structure of the oviduct cannot accommodate the growth of a fetus. Surgical removal of the fetus, resulting in an abortion, is thus often required for the sake of the mother’s health. Many of those who believe that abortion is wrong in general, may consider abortions acceptable in the case of ectopic pregnancy, in that it may literally save the life of the mother. 2. Endometriosis is an inflammatory condition in which the endometrium spreads beyond the uterus. It can lead to painful menstrual cycles and infertility. The Endometriosis Association website is a good resource for additional information. © 2012 Pearson Education, Inc. 15

Endometrium (lining of uterus)
Figure 27.3A Oviduct Ovaries Follicles Corpus luteum Uterus Wall of uterus Endometrium (lining of uterus) Cervix (“neck” of uterus) Figure 27.3A Front view of female reproductive anatomy (upper portion) Vagina 16

Figure 27.3B Egg cell Figure 27.3B Ovulation Ovary 17

Rectum (digestive system) Urinary bladder (excretory system)
Figure 27.3C Oviduct Ovary Uterus Rectum (digestive system) Urinary bladder (excretory system) Pubic bone (skeletal system) Cervix Urethra (excretory system) Vagina Figure 27.3C Side view of female reproductive anatomy (with nonreproductive structures in italic) Shaft Prepuce Clitoris Glans Vulva Labia minora Labia majora Anus (digestive system) Vaginal opening 18

27.4 Reproductive anatomy of the human male
Testes (singular, testis) produce sperm and male hormones. The epididymis stores sperm as they develop further. Several glands contribute to semen. These are the seminal vesicles, prostate gland, and bulbourethral glands. Student Misconceptions and Concerns 1. Students’ background knowledge of human reproductive biology is likely to be quite uneven. Furthermore, the embarrassment frequently associated with this topic makes it difficult for teachers to fairly assess what students know. The best advice may be to not assume too much. 2. Embarrassment with the subject of human reproductive biology may make open discussions uncomfortable for some students. Good clear textbook and media assignments that can be studied privately and opportunities to ask anonymous questions provide additional avenues to address sensitive content and questions. Teaching Tips 1. Men in your class will be well aware of physiological changes in the scrotum associated with thermoregulation. When a man enters into cool water, the scrotum is pulled tight and the testes are held close to the body. During a warm shower or bath, the scrotum relaxes and the testes are held far away from the body. 2. Testicular cancer is the most common form of cancer in men between 15 and 35 years of age. Students can find information about testicular cancer and how to perform a self-exam at the website for the American Cancer Society at 3. Students often confuse semen and sperm. Most of a human ejaculate consists of glandular products. In fact, there is no visible difference to the naked eye in the ejaculate of a fertile or sterile man. Animation: Male Reproductive Anatomy © 2012 Pearson Education, Inc. 19

Rectum (digestive system)
Figure 27.4B Rectum (digestive system) Seminal vesicle Urinary bladder (excretory system) Vas deferens Ejaculatory duct Pubic bone (skeletal system) Prostate gland Erectile tissue Figure 27.4B Side view of male reproductive anatomy (with nonreproductive structures in italic) Bulbourethral gland Anus (digestive system) Urethra (excretory system) Penis Vas deferens Epididymis Glans of penis Testis Testicle Prepuce Scrotum 20

27.4 Reproductive anatomy of the human male
During ejaculation sperm is expelled from the epididymis, the seminal vesicles, prostate, and bulbourethral glands secrete into the urethra, and semen is formed and expelled from the penis. Student Misconceptions and Concerns 1. Students’ background knowledge of human reproductive biology is likely to be quite uneven. Furthermore, the embarrassment frequently associated with this topic makes it difficult for teachers to fairly assess what students know. The best advice may be to not assume too much. 2. Embarrassment with the subject of human reproductive biology may make open discussions uncomfortable for some students. Good clear textbook and media assignments that can be studied privately and opportunities to ask anonymous questions provide additional avenues to address sensitive content and questions. Teaching Tips 1. Men in your class will be well aware of physiological changes in the scrotum associated with thermoregulation. When a man enters into cool water, the scrotum is pulled tight and the testes are held close to the body. During a warm shower or bath, the scrotum relaxes and the testes are held far away from the body. 2. Testicular cancer is the most common form of cancer in men between 15 and 35 years of age. Students can find information about testicular cancer and how to perform a self-exam at the website for the American Cancer Society at 3. Students often confuse semen and sperm. Most of a human ejaculate consists of glandular products. In fact, there is no visible difference to the naked eye in the ejaculate of a fertile or sterile man. © 2012 Pearson Education, Inc. 21

27.5 The formation of sperm and egg cells requires meiosis
Spermatogenesis occurs in seminiferous tubules. Primary spermatocytes are formed by mitosis and divide by meiosis I to produce secondary spermatocytes. Secondary spermatocytes divide by meiosis II to produce round spermatids, spermatids differentiate into elongate sperm, and mature sperm are released into seminiferous tubules. Student Misconceptions and Concerns 1. Students’ background knowledge of human reproductive biology is likely to be quite uneven. Furthermore, the embarrassment frequently associated with this topic makes it difficult for teachers to fairly assess what students know. The best advice may be to not assume too much. 2. Embarrassment with the subject of human reproductive biology may make open discussions uncomfortable for some students. Good clear textbook and media assignments that can be studied privately and opportunities to ask anonymous questions provide additional avenues to address sensitive content and questions. Teaching Tips Ask students to explain why polar bodies are produced during oogenesis and why they have so little cytoplasm. Challenge your students to explain why polar bodies are not produced in spermatogenesis. (Answer: Polar bodies are produced during oogenesis to eliminate nuclear material. Their smaller size is an adaptation to conserve cytoplasm during reduction division. During spermatogenesis, four functional spermatozoa are produced and no nuclear material is discarded.) © 2012 Pearson Education, Inc. 22

Figure 27.5A Spermatogenesis
Penis Epididymis Seminiferous tubule Testis Scrotum Testis Diploid cell 2n Differentiation and onset of meiosis I Primary spermatocyte 2n Cross section of seminiferous tubule (diploid; in prophase of meiosis I) Meiosis I completed Secondary spermatocyte n n Figure 27.5A Spermatogenesis (haploid) Meiosis II Developing sperm cells n n n n Differentiation Sperm cells n n n n (haploid) Mature sperm released into center of seminiferous tubule 23

Oogenesis begins before birth when a diploid cell in each developing follicle begins meiosis. Each month about one primary oocyte resumes meiosis. A secondary oocyte arrested at metaphase of meiosis II is ovulated. Meiosis of the ovum is completed after fertilization. Student Misconceptions and Concerns 1. Students’ background knowledge of human reproductive biology is likely to be quite uneven. Furthermore, the embarrassment frequently associated with this topic makes it difficult for teachers to fairly assess what students know. The best advice may be to not assume too much. 2. Embarrassment with the subject of human reproductive biology may make open discussions uncomfortable for some students. Good clear textbook and media assignments that can be studied privately and opportunities to ask anonymous questions provide additional avenues to address sensitive content and questions. Teaching Tips Ask students to explain why polar bodies are produced during oogenesis and why they have so little cytoplasm. Challenge your students to explain why polar bodies are not produced in spermatogenesis. (Answer: Polar bodies are produced during oogenesis to eliminate nuclear material. Their smaller size is an adaptation to conserve cytoplasm during reduction division. During spermatogenesis, four functional spermatozoa are produced and no nuclear material is discarded.) © 2012 Pearson Education, Inc. 24

Figure 27.5B Oogenesis and the development of an ovarian follicle
Before birth Ovary Diploid cell Differentiation and onset of meiosis I Primary oocyte within follicle Primary oocyte (arrested in prophase of meiosis I; present at birth) Growing follicle Completion of meiosis I and onset of meiosis II Mature follicle Ruptured follicle First polar body Figure 27.5B Oogenesis and the development of an ovarian follicle Secondary oocyte (arrested at metaphase of meiosis II; released from ovary) Ovulated secondary oocyte Entry of sperm triggers completion of meiosis II Second polar body Corpus luteum Mature egg (ovum) Degenerating corpus luteum 25

Differentiation and onset of meiosis I
Figure 27.5B_1 Before birth Ovary Diploid cell Differentiation and onset of meiosis I Primary oocyte within follicle Primary oocyte Figure 27.5B_1 Oogenesis and the development of an ovarian follicle (part 1) (arrested in prophase of meiosis I; present at birth) 26

Completion of meiosis I and onset of meiosis II
Figure 27.5B_2 Growing follicle Completion of meiosis I and onset of meiosis II Mature follicle Ruptured follicle First polar body Figure 27.5B_2 Oogenesis and the development of an ovarian follicle (part 2) Secondary oocyte (arrested at metaphase of meiosis II; released from ovary) Ovulated secondary oocyte 27

(arrested at metaphase of meiosis II; released from ovary)
Figure 27.5B_3 Ruptured follicle First polar body Secondary oocyte (arrested at metaphase of meiosis II; released from ovary) Ovulated secondary oocyte Entry of sperm triggers completion of meiosis II Second polar body Figure 27.5B_3 Oogenesis and the development of an ovarian follicle (part 3) Corpus luteum Mature egg (ovum) Degenerating corpus luteum 28

Oogenesis and spermatogenesis are alike in that both produce haploid gametes but different in that oogenesis produces only one mature egg and polar bodies that degenerate and spermatogenesis produces four mature gametes. Student Misconceptions and Concerns 1. Students’ background knowledge of human reproductive biology is likely to be quite uneven. Furthermore, the embarrassment frequently associated with this topic makes it difficult for teachers to fairly assess what students know. The best advice may be to not assume too much. 2. Embarrassment with the subject of human reproductive biology may make open discussions uncomfortable for some students. Good clear textbook and media assignments that can be studied privately and opportunities to ask anonymous questions provide additional avenues to address sensitive content and questions. Teaching Tips Ask students to explain why polar bodies are produced during oogenesis and why they have so little cytoplasm. Challenge your students to explain why polar bodies are not produced in spermatogenesis. (Answer: Polar bodies are produced during oogenesis to eliminate nuclear material. Their smaller size is an adaptation to conserve cytoplasm during reduction division. During spermatogenesis, four functional spermatozoa are produced and no nuclear material is discarded.) © 2012 Pearson Education, Inc. 29

27.6 Hormones synchronize cyclic changes in the ovary and uterus
About every 28 days the hypothalamus signals the anterior pituitary to secrete follicle-stimulating hormone (FSH) and luteinizing hormone (LH), which trigger the growth of a follicle and ovulation, the release of an egg. Student Misconceptions and Concerns 1. Students’ background knowledge of human reproductive biology is likely to be quite uneven. Furthermore, the embarrassment frequently associated with this topic makes it difficult for teachers to fairly assess what students know. The best advice may be to not assume too much. 2. Embarrassment with the subject of human reproductive biology may make open discussions uncomfortable for some students. Good clear textbook and media assignments that can be studied privately and opportunities to ask anonymous questions provide additional avenues to address sensitive content and questions. Teaching Tips 1. Students might wonder why a woman’s body goes through menstrual cycles. Why not just sustain the endometrium continuously? One hypothesis suggests that because the uterus is a good environment in which bacteria can grow, menstrual cycles are a way to “flush” the system and discourage microbial growth. 2. Many home pregnancy tests rely upon antibodies to HCG. © 2012 Pearson Education, Inc. 30

Table 27.6 Table 27.6 Hormones of the Ovarian and Menstrual Cycles 31

After ovulation, the ovarian follicle becomes the corpus luteum. The corpus luteum secretes estrogen and progesterone, which stimulate the endometrium to thicken, prepare the uterus for implantation of the embryo, and inhibit the hypothalamus, reducing FSH and LH secretion. Student Misconceptions and Concerns 1. Students’ background knowledge of human reproductive biology is likely to be quite uneven. Furthermore, the embarrassment frequently associated with this topic makes it difficult for teachers to fairly assess what students know. The best advice may be to not assume too much. 2. Embarrassment with the subject of human reproductive biology may make open discussions uncomfortable for some students. Good clear textbook and media assignments that can be studied privately and opportunities to ask anonymous questions provide additional avenues to address sensitive content and questions. Teaching Tips 1. Students might wonder why a woman’s body goes through menstrual cycles. Why not just sustain the endometrium continuously? One hypothesis suggests that because the uterus is a good environment in which bacteria can grow, menstrual cycles are a way to “flush” the system and discourage microbial growth. 2. Many home pregnancy tests rely upon antibodies to HCG. Animation: Ovulation Animation: Post Ovulation © 2012 Pearson Education, Inc. 32

If the egg is fertilized the embryo releases hormones that maintain the uterine lining and menstruation does not occur. If the egg is not fertilized the drop in LH shuts down the corpus luteum and its hormones, menstruation is triggered, and the hypothalamus and pituitary stimulate development of a new follicle. Student Misconceptions and Concerns 1. Students’ background knowledge of human reproductive biology is likely to be quite uneven. Furthermore, the embarrassment frequently associated with this topic makes it difficult for teachers to fairly assess what students know. The best advice may be to not assume too much. 2. Embarrassment with the subject of human reproductive biology may make open discussions uncomfortable for some students. Good clear textbook and media assignments that can be studied privately and opportunities to ask anonymous questions provide additional avenues to address sensitive content and questions. Teaching Tips 1. Students might wonder why a woman’s body goes through menstrual cycles. Why not just sustain the endometrium continuously? One hypothesis suggests that because the uterus is a good environment in which bacteria can grow, menstrual cycles are a way to “flush” the system and discourage microbial growth. 2. Many home pregnancy tests rely upon antibodies to HCG. © 2012 Pearson Education, Inc. 33

Degenerating corpus luteum
Figure 27.6 A Control by hypothalamus Inhibited by combination of estrogen and progesterone Hypothalamus Releasing hormone Stimulated by high levels of estrogen Anterior pituitary 1 FSH LH B Pituitary hormones in blood 4 LH peak triggers ovulation and corpus luteum formation 6 LH FSH FSH stimulates follicle to grow LH surge triggers ovulation 2 C Ovarian cycle 5 Growing follicle Corpus luteum Degenerating corpus luteum Mature follicle Ovulation Pre-ovulatory phase Post-ovulatory phase Estrogen secreted by growing follicle Progesterone and estrogen secreted by remnant of follicle D Ovarian hormones in blood Peak causes LH surge 3 Figure 27.6 The reproductive cycle of the human female 7 Estrogen Progesterone Low levels of estrogen trigger menstruation Progesterone and estrogen promote thickening of endometrium E Menstrual cycle 8 Endometrium 5 10 14 15 20 25 28 Menstruation Days 34

Control by hypothalamus
Figure 27.6_1 Control by hypothalamus Inhibited by combination of estrogen and progesterone Hypothalamus Stimulated by high levels of estrogen Releasing hormone Anterior pituitary Figure 27.6_1 The reproductive cycle of the human female (part 1) FSH LH 35

Pituitary hormones in blood
Figure 27.6_2 Days 5 10 14 15 20 25 28 Pituitary hormones in blood LH peak triggers ovulation and corpus luteum formation LH Figure 27.6_2 The reproductive cycle of the human female (part 2) FSH FSH stimulates follicle to grow LH surge triggers ovulation 36

Degenerating corpus luteum
Figure 27.6_3 Days 5 10 14 15 20 25 28 Ovarian cycle Growing follicle Corpus luteum Degenerating corpus luteum Mature follicle Ovulation Pre-ovulatory phase Post-ovulatory phase Figure 27.6_3 The reproductive cycle of the human female (part 3) Estrogen secreted by growing follicle Progesterone and estrogen secreted by remnant of follicle 37

Days 5 10 14 15 20 25 28 Peak causes LH surge
Figure 27.6_4 Days 5 10 14 15 20 25 28 Peak causes LH surge Ovarian hormones in blood Estrogen Progesterone Figure 27.6_4 The reproductive cycle of the human female (part 4) Low levels of estrogen trigger menstruation Progesterone and estrogen promote thickening of endometrium 38

Menstrual cycle Endometrium 5 10 14 15 20 25 28 Days Menstruation
Figure 27.6_5 Menstrual cycle Endometrium 5 10 14 15 20 25 28 Figure 27.6_5 The reproductive cycle of the human female (part 5) Days Menstruation 39

27.7 CONNECTION: Sexual activity can transmit disease
Sexually transmitted diseases (STDs) caused by bacteria can often be cured. Chlamydia is the most common bacterial STD, often produces no symptoms, and can lead to pelvic inflammatory disease and infertility. Student Misconceptions and Concerns 1. Students’ background knowledge of human reproductive biology is likely to be quite uneven. Furthermore, the embarrassment frequently associated with this topic makes it difficult for teachers to fairly assess what students know. The best advice may be to not assume too much. 2. Embarrassment with the subject of human reproductive biology may make open discussions uncomfortable for some students. Good clear textbook and media assignments that can be studied privately and opportunities to ask anonymous questions provide additional avenues to address sensitive content and questions. 3. Students often believe that drugs are available to cure most STDs. However, viral infections, including HIV, HPV, and herpes, should be assumed to be lifelong. Teaching Tips 1. The Sexuality Information and Education Council of the U.S. (SIECUS) is a national, nonprofit organization that affirms that sexuality is a natural and healthy part of living. Its website at is an excellent source of information related to this chapter. 2. Although people infected with sexually transmitted diseases might have no apparent symptoms, they may still be capable of infecting partners. This important point is worth repeating to young college audiences who may be overly optimistic about the health of their partners. 3. Chlamydia is the most common sexually transmitted bacterial infection in the United States. © 2012 Pearson Education, Inc. 40

27.7 CONNECTION: Sexual activity can transmit disease
Viral diseases such as genital herpes and HIV, can only be controlled. The best way to avoid the spread of STDs is abstinence. Latex condoms provide the best protection against disease transmission for “safer sex.” Student Misconceptions and Concerns 1. Students’ background knowledge of human reproductive biology is likely to be quite uneven. Furthermore, the embarrassment frequently associated with this topic makes it difficult for teachers to fairly assess what students know. The best advice may be to not assume too much. 2. Embarrassment with the subject of human reproductive biology may make open discussions uncomfortable for some students. Good clear textbook and media assignments that can be studied privately and opportunities to ask anonymous questions provide additional avenues to address sensitive content and questions. 3. Students often believe that drugs are available to cure most STDs. However, viral infections, including HIV, HPV, and herpes, should be assumed to be lifelong. Teaching Tips 1. The Sexuality Information and Education Council of the U.S. (SIECUS) is a national, nonprofit organization that affirms that sexuality is a natural and healthy part of living. Its website at is an excellent source of information related to this chapter. 2. Although people infected with sexually transmitted diseases might have no apparent symptoms, they may still be capable of infecting partners. This important point is worth repeating to young college audiences who may be overly optimistic about the health of their partners. 3. Chlamydia is the most common sexually transmitted bacterial infection in the United States. © 2012 Pearson Education, Inc. 41

Table 27.7 Table 27.7 STDs Common in the United States 42

PRINCIPLES OF EMBRYONIC DEVELOPMENT
PRINCIPLES OF EMBRYONIC DEVELOPMENT © 2012 Pearson Education, Inc. 43

27.9 Fertilization results in a zygote and triggers embryonic development
Embryonic development begins with fertilization, the union of sperm and egg, to form a diploid zygote. Student Misconceptions and Concerns The descriptions of basic development address questions that students may have never thought to ask. How do we get so many cells (trillions!) in our adult bodies? How do basic tissues, organs, and organ systems form? What mechanisms permit the coordinated development of the body? Before beginning this chapter, consider challenging your students with some of these fundamental questions: How did you get a heart, a brain, and ears? What determined when you were ready to be born? How did you get nutrients and oxygen before you could eat or breathe? Teaching Tips 1. The authors note in Module 27.9 that the process of sea urchin development is discussed because sea urchins exhibit fundamental details that are found in the development of most vertebrates. These similarities are a consequence of our shared ancestry and provide strong evidence of evolution. 2. Reproductive isolating mechanisms are generally classified into prezygotic and postzygotic categories. Module 27.9 discusses some of the ways that hybridization is prevented by species-specific biochemical interactions between the sperm and the egg. © 2012 Pearson Education, Inc. 44

Figure 27.9A Figure 27.9A A human egg cell surrounded by sperm 45

Sperm are adapted to reach and fertilize an egg. Sperm have a streamlined shape, which moves easily through fluids, many mitochondria, which provide ATP for tail movements, and a head that contains a haploid nucleus and is tipped with an acrosome containing enzymes that help it penetrate the egg. Student Misconceptions and Concerns The descriptions of basic development address questions that students may have never thought to ask. How do we get so many cells (trillions!) in our adult bodies? How do basic tissues, organs, and organ systems form? What mechanisms permit the coordinated development of the body? Before beginning this chapter, consider challenging your students with some of these fundamental questions: How did you get a heart, a brain, and ears? What determined when you were ready to be born? How did you get nutrients and oxygen before you could eat or breathe? Teaching Tips 1. The authors note in Module 27.9 that the process of sea urchin development is discussed because sea urchins exhibit fundamental details that are found in the development of most vertebrates. These similarities are a consequence of our shared ancestry and provide strong evidence of evolution. 2. Reproductive isolating mechanisms are generally classified into prezygotic and postzygotic categories. Module 27.9 discusses some of the ways that hybridization is prevented by species-specific biochemical interactions between the sperm and the egg. © 2012 Pearson Education, Inc. 46

Middle piece Plasma membrane Head Tail Mitochondria Nucleus Acrosome
Figure 27.9B Middle piece Plasma membrane Head Tail Mitochondria Nucleus Figure 27.9B The structure of a human sperm cell Acrosome 47

During fertilization, sperm squeeze past follicle cells, acrosomal enzymes digest the egg’s jelly coat, a sperm binds to egg receptors, sperm and egg plasma membranes fuse, the sperm nucleus enters the egg cytoplasm, The vitelline layer separates and becomes impenetrable, and the egg and sperm nuclei fuse. Student Misconceptions and Concerns The descriptions of basic development address questions that students may have never thought to ask. How do we get so many cells (trillions!) in our adult bodies? How do basic tissues, organs, and organ systems form? What mechanisms permit the coordinated development of the body? Before beginning this chapter, consider challenging your students with some of these fundamental questions: How did you get a heart, a brain, and ears? What determined when you were ready to be born? How did you get nutrients and oxygen before you could eat or breathe? Teaching Tips 1. The authors note in Module 27.9 that the process of sea urchin development is discussed because sea urchins exhibit fundamental details that are found in the development of most vertebrates. These similarities are a consequence of our shared ancestry and provide strong evidence of evolution. 2. Reproductive isolating mechanisms are generally classified into prezygotic and postzygotic categories. Module 27.9 discusses some of the ways that hybridization is prevented by species-specific biochemical interactions between the sperm and the egg. © 2012 Pearson Education, Inc. 48

The sperm’s acrosomal enzymes digest the egg’s jelly coat.
Figure 27.9C 1 A sperm touches the egg’s jelly coat, and its acrosome releases enzyme molecules. 2 The sperm’s acrosomal enzymes digest the egg’s jelly coat. 3 Proteins on the sperm head bind to egg receptors. 4 The plasma membranes of sperm and egg fuse. Acrosomal enzymes 5 The sperm nucleus enters the egg cytoplasm. Sperm Plasma membrane 6 The vitelline layer separates and becomes impenetrable. Nucleus Acrosome Receptor protein molecules Plasma membrane Sperm nucleus Figure 27.9C The process of fertilization in a sea urchin n Vitelline layer Cytoplasm n Jelly coat Egg nucleus n 7 The nuclei of sperm and egg fuse. n Egg cell 2n Zygote nucleus 49

27.10 Cleavage produces a ball of cells from the zygote
Cleavage is a rapid series of cell divisions that produces more cells, smaller cells, and a fluid-filled embryo called a blastula. Student Misconceptions and Concerns The descriptions of basic development address questions that students may have never thought to ask. How do we get so many cells (trillions!) in our adult bodies? How do basic tissues, organs, and organ systems form? What mechanisms permit the coordinated development of the body? Before beginning this chapter, consider challenging your students with some of these fundamental questions: How did you get a heart, a brain, and ears? What determined when you were ready to be born? How did you get nutrients and oxygen before you could eat or breathe? Teaching Tips 1. Cleavage is largely a process of subdivision with no growth. This process is a bit like cutting up a pie into pieces. With every division, more pieces are produced, but the pieces are smaller and the pie itself does not increase in size. Similarly, cleavage increases the number of cells while decreasing their size. 2. Cleavage in humans is a very slow process, taking up to 12 hours for each division. 3. Cleavage results in an uneven distribution of cytoplasmic elements into the daughter blastomeres (embryonic cells produced by cleavage). Some students might benefit from this simple analogy: Imagine baking a pie that is filled with one can of apple pie filling and one can of cherry pie filling. The contents of each can are poured into opposite ends of the pie and are not mixed. When the pie is served, some pieces will contain only apples, some only cherries, and some combinations of both. This disproportionate division of pie contents is like the disproportionate division of cytoplasmic elements during cleavage. Video: Sea Urchin Embryonic Development © 2012 Pearson Education, Inc. 50

Figure 27.10_s1 Zygote 2 cells Figure 27.10_s1 Cleavage in a sea urchin (step 1) 51

Zygote 2 cells 4 cells 8 cells Figure 27.10_s2
Figure 27.10_s2 Cleavage in a sea urchin (step 2) 52

Many cells (solid ball)
Figure 27.10_s3 Zygote 2 cells 4 cells 8 cells Figure 27.10_s3 Cleavage in a sea urchin (step 3) Many cells (solid ball) 53

Blastula (hollow ball) Cross section of blastula
Figure 27.10_s4 Zygote 2 cells 4 cells 8 cells Figure 27.10_s4 Cleavage in a sea urchin (step 4) Many cells (solid ball) Blastocoel Blastula (hollow ball) Cross section of blastula 54

27.11 Gastrulation produces a three-layered embryo
During gastrulation cells migrate to new locations, a rudimentary digestive cavity forms, and the basic body plan of three layers is established with ectoderm outside—becomes skin and nervous systems, endoderm inside—becomes digestive tract, mesoderm in the middle—becomes muscle and bone. Student Misconceptions and Concerns The descriptions of basic development address questions that students may have never thought to ask. How do we get so many cells (trillions!) in our adult bodies? How do basic tissues, organs, and organ systems form? What mechanisms permit the coordinated development of the body? Before beginning this chapter, consider challenging your students with some of these fundamental questions: How did you get a heart, a brain, and ears? What determined when you were ready to be born? How did you get nutrients and oxygen before you could eat or breathe? Teaching Tips 1. You might wish to reflect on the somewhat famous quote of Lewis Wolpert, who in 1986 said, “It is not birth, marriage, or death, but gastrulation, which is truly the most important time in your life.” The development and arrangement of the basic embryonic layers (ectoderm: skin and nervous system; mesoderm: muscle and bone; and endoderm: digestive tract) establishes the basic body plan. 2. Gastrulation establishes the basic body plan by positioning the future systems of the body in relationship to each other. This essential element of gastrulation is a crucial event that is a prerequisite for later developmental events (and thus is the basis of the quote directly above). 3. The colors used in the text to depict the three layers of embryonic tissue are standards used in embryology. Blue represents ectoderm and its derivatives, red represents mesoderm and its derivatives, and yellow represents endoderm and its derivatives. 4. Neural crest cells are often referred to as the fourth layer of embryonic tissue because these cells give rise to a variety of tissues in diverse locations in the body. Neural crest cells and their derivatives are not addressed in Chapter 27. © 2012 Pearson Education, Inc. 55

Blastula (end of cleavage)
Figure 27.11_s1 Animal pole Blastula (end of cleavage) Blastocoel Vegetal pole Figure 27.11_s1 Development of the frog gastrula (step 1) 56

Figure 27.11_s2 Animal pole Blastula (end of cleavage) Blastocoel Vegetal pole Gastrulation (cell migration) Blastocoel shrinking Formation of a simple digestive cavity Blastopore Figure 27.11_s2 Development of the frog gastrula (step 2) 57

Figure 27.11_s3 Animal pole Blastula (end of cleavage) Blastocoel Vegetal pole Gastrulation (cell migration) Blastocoel shrinking Formation of a simple digestive cavity Blastopore Figure 27.11_s3 Development of the frog gastrula (step 3) Gastrula (end of gastrulation) Ectoderm Mesoderm Simple digestive cavity Endoderm 58

Table 27.11 Table Derivatives of the Three Embryonic Tissue Layers 59

27.12 Organs start to form after gastrulation
Organs develop from the three embryonic layers. The stiff notochord forms the main axis of the body and is later replaced by the vertebral column in most chordates. The neural tube develops above the notochord and will become the brain and spinal cord. Student Misconceptions and Concerns 1. The descriptions of basic development address questions that students may have never thought to ask. How do we get so many cells (trillions!) in our adult bodies? How do basic tissues, organs, and organ systems form? What mechanisms permit the coordinated development of the body? Before beginning this chapter, consider challenging your students with some of these fundamental questions: How did you get a heart, a brain, and ears? What determined when you were ready to be born? How did you get nutrients and oxygen before you could eat or breathe? 2. Many students do not realize that their nervous system is hollow, and do not consequently relate the formation of the neural tube to the structure of a fully developed adult nervous system. While discussing formation of the neural tube, you may want to anticipate the subject matter of Chapter 28 by emphasizing that the fluid-filled ventricles of the brain and the central canal of the spinal cord are spaces. Teaching Tips 1. The notochord functions as a sort of scaffolding upon which the embryo develops, especially important before a vertebral column develops. In the lancelet (amphioxus), the notochord is the adult skeletal structure along the long axis of the body. 2. The alignment of the notochord and neural tube is not accidental. Cellular communication between the notochord and neural plate results in the parallel alignment of the neural tube to the notochord (which forms first). Video: Frog Embryo Development © 2012 Pearson Education, Inc. 60

Neural fold Neural plate Notochord Ectoderm Mesoderm Endoderm
Figure 27.12A Neural fold Neural plate Notochord Ectoderm Mesoderm Endoderm Neural folds Figure 27.12A The beginning of organ development in a frog: the notochord, neural folds, and neural plate 61

Figure 27.12A_1 Neural folds Figure 27.12A_1 The beginning of organ development in a frog: the notochord, neural folds, and neural plate (micrograph) 62

Outer layer of ectoderm
Figure 27.12B Neural fold Neural plate Figure 27.12B Formation of the neural tube Outer layer of ectoderm Neural tube 63

27.12 Organs start to form after gastrulation
As the embryo elongates, paired somites form along the sides of the notochord, hollow out to form a coelom, and eventually contribute to muscles, bone, and other connective tissues. Other systems develop at the same time. Student Misconceptions and Concerns 1. The descriptions of basic development address questions that students may have never thought to ask. How do we get so many cells (trillions!) in our adult bodies? How do basic tissues, organs, and organ systems form? What mechanisms permit the coordinated development of the body? Before beginning this chapter, consider challenging your students with some of these fundamental questions: How did you get a heart, a brain, and ears? What determined when you were ready to be born? How did you get nutrients and oxygen before you could eat or breathe? 2. Many students do not realize that their nervous system is hollow, and do not consequently relate the formation of the neural tube to the structure of a fully developed adult nervous system. While discussing formation of the neural tube, you may want to anticipate the subject matter of Chapter 28 by emphasizing that the fluid-filled ventricles of the brain and the central canal of the spinal cord are spaces. Teaching Tips 1. The notochord functions as a sort of scaffolding upon which the embryo develops, especially important before a vertebral column develops. In the lancelet (amphioxus), the notochord is the adult skeletal structure along the long axis of the body. 2. The alignment of the notochord and neural tube is not accidental. Cellular communication between the notochord and neural plate results in the parallel alignment of the neural tube to the notochord (which forms first). © 2012 Pearson Education, Inc. 64

Neural tube Notochord Somite Coelom Somites Tail bud Digestive cavity
Figure 27.12C Neural tube Notochord Somite Coelom Somites Tail bud Digestive cavity Figure 27.12C An embryo with completed neural tube, somites, and coelom Eye 65

Somites Tail bud Eye Figure 27.12C_1
Figure 27.12C_1 An embryo with completed neural tube, somites, and coelom (micrograph) 66

27.13 Multiple processes give form to the developing animal
Tissues and organs develop by changes in cell shape, cell migration, and programmed cell death (also called apoptosis). Student Misconceptions and Concerns The descriptions of basic development address questions that students may have never thought to ask. How do we get so many cells (trillions!) in our adult bodies? How do basic tissues, organs, and organ systems form? What mechanisms permit the coordinated development of the body? Before beginning this chapter, consider challenging your students with some of these fundamental questions: How did you get a heart, a brain, and ears? What determined when you were ready to be born? How did you get nutrients and oxygen before you could eat or breathe? Teaching Tips Apoptosis is a sort of editing mechanism that permits the selective destruction of embryonic structures. Examples of apoptosis in normal development (such as the formation of fingers discussed in Module 27.13) are abundant. For example, extensive apoptosis during vertebrate brain development functions as a sort of “neural pruning” of the developing nervous system (neural “tree”), resulting in the final neural configurations. © 2012 Pearson Education, Inc. 67

Figure 27.13A Figure 27.13A Apoptosis in a developing human hand 68

Dead cell engulfed and digested by adjacent cell
Figure 27.13B Apoptosis Dead cell engulfed and digested by adjacent cell Figure 27.13B Apoptosis at the cellular level 69

27.13 Multiple processes give form to the developing animal
Through induction, adjacent cells and cell layers influence each other’s differentiation via chemical signals. Student Misconceptions and Concerns The descriptions of basic development address questions that students may have never thought to ask. How do we get so many cells (trillions!) in our adult bodies? How do basic tissues, organs, and organ systems form? What mechanisms permit the coordinated development of the body? Before beginning this chapter, consider challenging your students with some of these fundamental questions: How did you get a heart, a brain, and ears? What determined when you were ready to be born? How did you get nutrients and oxygen before you could eat or breathe? Teaching Tips Apoptosis is a sort of editing mechanism that permits the selective destruction of embryonic structures. Examples of apoptosis in normal development (such as the formation of fingers discussed in Module 27.13) are abundant. For example, extensive apoptosis during vertebrate brain development functions as a sort of “neural pruning” of the developing nervous system (neural “tree”), resulting in the final neural configurations. © 2012 Pearson Education, Inc. 70

27.15 The embryo and placenta take shape during the first month of pregnancy
Pregnancy, or gestation, is the carrying of developing young within the female reproductive tract. Human pregnancy averages 266 days (38 weeks) from fertilization or 40 weeks (9 months) from the start of the last menstrual period. Student Misconceptions and Concerns 1. Students often think that maternal and fetal blood merge together. Consider spending additional time to note the close association but distinct separation maintained between these two circulatory systems. 2. The extraembryonic membranes surrounding a human embryo are rarely understood by college students outside of the health sciences. These extraembryonic membranes can be viewed as analogous to the life-support systems used by astronauts. Teaching Tips All vertebrates develop in a fluid environment. Fish and amphibians typically lay their eggs in water. Amniotes (reptiles and mammals) are defined by the presence of an amniotic sac, a self-contained aquatic system that surrounds the embryo with water inside the egg. The evolution of the amniotic egg eliminated the need to reproduce near water, freeing amniotes to reproduce underground, in deserts, and in trees! © 2012 Pearson Education, Inc. 72

Human development begins with fertilization in the oviduct. Cleavage produces a blastocyst whose inner cell mass becomes the embryo and the trophoblast, the outer cell layer, which attaches to the uterine wall and forms part of the placenta. Gastrulation occurs and organs develop from the three embryonic layers. Student Misconceptions and Concerns 1. Students often think that maternal and fetal blood merge together. Consider spending additional time to note the close association but distinct separation maintained between these two circulatory systems. 2. The extraembryonic membranes surrounding a human embryo are rarely understood by college students outside of the health sciences. These extraembryonic membranes can be viewed as analogous to the life-support systems used by astronauts. Teaching Tips All vertebrates develop in a fluid environment. Fish and amphibians typically lay their eggs in water. Amniotes (reptiles and mammals) are defined by the presence of an amniotic sac, a self-contained aquatic system that surrounds the embryo with water inside the egg. The evolution of the amniotic egg eliminated the need to reproduce near water, freeing amniotes to reproduce underground, in deserts, and in trees! © 2012 Pearson Education, Inc. 73

Fertilization of mature egg Trophoblast Uterine cavity Blastocyst
Figure 27.15A–B Cleavage starts Fertilization of mature egg Trophoblast Uterine cavity Blastocyst Cavity Oviduct Ovary Inner cell mass Uterine cavity Blastocyst (implanted) Endometrium Secondary oocyte Figure 27.15A–B From ovulation to implantation (with inset of blastocyst) Uterus Ovulation 74

Multiplying cells of trophoblast (contribute to future placenta)
Figure 27.15C Endometrium Uterine cavity Multiplying cells of trophoblast (contribute to future placenta) Trophoblast Embryo Future yolk sac Figure 27.15C Implantation under way (about 7 days) Blood vessel (maternal) 75

Four extraembryonic membranes develop. The amnion surrounds the embryo and forms a fluid-filled amniotic cavity that protects the embryo. The yolk sac, in reptiles, stores yolk, in humans, does not store yolk but is a source of the first germ cells and blood cells. Student Misconceptions and Concerns 1. Students often think that maternal and fetal blood merge together. Consider spending additional time to note the close association but distinct separation maintained between these two circulatory systems. 2. The extraembryonic membranes surrounding a human embryo are rarely understood by college students outside of the health sciences. These extraembryonic membranes can be viewed as analogous to the life-support systems used by astronauts. Teaching Tips All vertebrates develop in a fluid environment. Fish and amphibians typically lay their eggs in water. Amniotes (reptiles and mammals) are defined by the presence of an amniotic sac, a self-contained aquatic system that surrounds the embryo with water inside the egg. The evolution of the amniotic egg eliminated the need to reproduce near water, freeing amniotes to reproduce underground, in deserts, and in trees! © 2012 Pearson Education, Inc. 76

The allantois contributes to the umbilical cord, forms part of the urinary bladder, and in reptiles, stores embryonic waste. The chorion contributes to the placenta and secretes human chorionic gonadotropin (HCG), which prevents menstruation in mammals. Student Misconceptions and Concerns 1. Students often think that maternal and fetal blood merge together. Consider spending additional time to note the close association but distinct separation maintained between these two circulatory systems. 2. The extraembryonic membranes surrounding a human embryo are rarely understood by college students outside of the health sciences. These extraembryonic membranes can be viewed as analogous to the life-support systems used by astronauts. Teaching Tips All vertebrates develop in a fluid environment. Fish and amphibians typically lay their eggs in water. Amniotes (reptiles and mammals) are defined by the presence of an amniotic sac, a self-contained aquatic system that surrounds the embryo with water inside the egg. The evolution of the amniotic egg eliminated the need to reproduce near water, freeing amniotes to reproduce underground, in deserts, and in trees! © 2012 Pearson Education, Inc. 77

Yolk sac Chorion Amnion Amniotic cavity Mesoderm cells Figure 27.15D
Figure 27.15D Embryonic layers and extraembryonic membranes starting to form (9 days) Amnion Amniotic cavity Mesoderm cells 78

Embryo: Endoderm Mesoderm Ectoderm Chorionic villi Chorion Amnion
Figure 27.15E Embryo: Endoderm Mesoderm Ectoderm Chorionic villi Figure 27.15E Three-layered embryo and four extraembryonic membranes (16 days) Chorion Amnion Allantois Yolk sac 79

Mother’s blood vessels Allantois
Figure 27.15F Placenta Embryo Amnion Chorion Amniotic cavity Chorionic villi Figure 27.15F Placenta formed (31 days) Mother’s blood vessels Allantois Yolk sac 80

The placenta is a close association of embryonic chorion and mother’s blood vessels, and site of gas exchange—from mother to embryo, nutrient exchange—from mother to embryo, and waste exchange—from embryo to mother. Student Misconceptions and Concerns 1. Students often think that maternal and fetal blood merge together. Consider spending additional time to note the close association but distinct separation maintained between these two circulatory systems. 2. The extraembryonic membranes surrounding a human embryo are rarely understood by college students outside of the health sciences. These extraembryonic membranes can be viewed as analogous to the life-support systems used by astronauts. Teaching Tips All vertebrates develop in a fluid environment. Fish and amphibians typically lay their eggs in water. Amniotes (reptiles and mammals) are defined by the presence of an amniotic sac, a self-contained aquatic system that surrounds the embryo with water inside the egg. The evolution of the amniotic egg eliminated the need to reproduce near water, freeing amniotes to reproduce underground, in deserts, and in trees! © 2012 Pearson Education, Inc. 81

Video: Ultrasound of Human Fetus 1 Video: Ultrasound of Human Fetus 2
27.16 Human development from conception to birth is divided into three trimesters The first trimester is the period of greatest change. The embryo forms, looking like other vertebrate embryos. Extraembryonic membranes form. All major organ systems are established. After 9 weeks after fertilization, the embryo is called a fetus and can move its arms and legs and starts to look distinctly human. Teaching Tips 1. Morning sickness typically occurs during the first trimester of pregnancy and subsides during the second. However, a great deal of variation in the timing and degree of symptoms has been observed. Unfortunately, the precise cause of morning sickness remains unknown. 2. Most human babies weigh 3–4 kilograms (kg) at birth. Birth weights much larger or smaller than this are associated with increased mortality. This is an example of stabilizing selection, which is discussed in Module Video: Ultrasound of Human Fetus 1 Video: Ultrasound of Human Fetus 2 © 2012 Pearson Education, Inc. 82

January February March April 35 days 63 days Conception 98 days
Figure 27.16A–C January February March April 35 days 63 days Conception 98 days Figure 27.16A–C Timeline of human fetal development (part 1) 83

Figure 27.16A Figure 27.16A Timeline of human fetal development: 5 weeks (35 days) 5 weeks (35 days) 84

Figure 27.16B Figure 27.16B Timeline of human fetal development: 9 weeks (63 days) 9 weeks (63 days) 85

27.16 Human development from conception to birth is divided into three trimesters
During the second trimester, there is a great increase in the size of the fetus, and human features are refined. At 20 weeks, the fetus is about 19 cm long (7.6 in.) and weighs about 0.5 kg (1 lb.). Teaching Tips 1. Morning sickness typically occurs during the first trimester of pregnancy and subsides during the second. However, a great deal of variation in the timing and degree of symptoms has been observed. Unfortunately, the precise cause of morning sickness remains unknown. 2. Most human babies weigh 3–4 kilograms (kg) at birth. Birth weights much larger or smaller than this are associated with increased mortality. This is an example of stabilizing selection, which is discussed in Module © 2012 Pearson Education, Inc. 86

Figure 27.16C Figure 27.16C Timeline of human fetal development: 14 weeks (98 days) 14 weeks (98 days) 87

Figure 27.16D–E Timeline of human fetal development (part 2)
May June July August September October 280 days 140 days Figure 27.16D–E Timeline of human fetal development (part 2) 88

Figure 27.16D Figure 27.16D Timeline of human fetal development: 20 weeks (140 days) 20 weeks (140 days) 89

27.16 Human development from conception to birth is divided into three trimesters
The third trimester is also a time of rapid growth. The circulatory and respiratory systems mature. Muscles thicken and the skeleton hardens. The third trimester ends with birth. Babies born as early as 24 weeks may survive only with extensive medical care. Teaching Tips 1. Morning sickness typically occurs during the first trimester of pregnancy and subsides during the second. However, a great deal of variation in the timing and degree of symptoms has been observed. Unfortunately, the precise cause of morning sickness remains unknown. 2. Most human babies weigh 3–4 kilograms (kg) at birth. Birth weights much larger or smaller than this are associated with increased mortality. This is an example of stabilizing selection, which is discussed in Module © 2012 Pearson Education, Inc. 90

27.18 CONNECTION: Reproductive technologies increase our reproductive options
New techniques can help many infertile couples. About 15% of couples wanting children are infertile. Drug therapies can help address problems of impotence (erectile dysfunction) and induce ovulation. Assisted reproductive technologies (ART) require eggs to be harvested from the ovaries, fertilized, and returned to a woman’s body. In vitro fertilization (IVF) is the most common assisted reproductive technology. Fertilization occurs in a culture dish and an early embryo is implanted in the uterus. Student Misconceptions and Concerns 1. Students frequently confuse the terms infertility and impotence. Care should be used to carefully distinguish these terms. 2. Few students understand that many STDs can lead to infertility. For example, chlamydia is the most frequently reported bacterial sexually transmitted disease, according to the CDC. Infertility is a common consequence of chlamydia infections. Teaching Tips 1. Impotence is often caused by cardiovascular disease. Students may be unaware of the extent to which smoking, poor diet, and lack of exercise can therefore contribute to sexual problems. 2. The fate of surplus frozen embryos (sometimes called snowflake babies) produced by in vitro fertilization is often debated. Some people have suggested that they be used as sources of stem cells. Others regard this as akin to abortion and therefore unacceptable. Students may find it engaging to research and discuss the scientific and ethical issues raised by these embryos. © 2012 Pearson Education, Inc. 91

In vitro fertilization Collected sperm
Figure 27.18 Implantation 8-cell embryo Zygote Collected egg Figure In vitro fertilization In vitro fertilization Collected sperm 92

You should now be able to
Compare the types, advantages, and disadvantages of asexual and sexual reproduction. Describe the structures and functions of the female and male human reproductive systems. Describe and compare the processes and products of spermatogenesis and oogenesis. Describe the events of and control of the menstrual cycle. © 2012 Pearson Education, Inc. 93

Describe the nature of the most common sexually transmitted diseases. Describe the most common forms of birth control and explain how each works. Relate the structure of sperm to its role in fertilization. Describe the process and results of cleavage. Describe the process of gastrulation and the resulting arrangement of the embryo. © 2012 Pearson Education, Inc. 94

Explain how organs form after the development of a gastrula. Explain how changes in cell shape, induction, cell migration, and apoptosis contribute to development. Explain how the one-dimensional information in DNA is used to direct the three-dimensional form of an embryo. Describe the initial embryonic stages and the formation and functions of the extraembryonic membranes in humans. © 2012 Pearson Education, Inc. 95

Describe the main changes that occur during each of the trimesters of human development. Explain how labor begins and describe the main events of the three stages of labor. Describe the common causes of human infertility and the technologies currently available to help couples conceive. © 2012 Pearson Education, Inc. 96

Reproduction and Embryonic Development

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