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Chapter 27 Reproduction and Embryonic Development PowerPoint Lectures for Campbell Biology: Concepts & Connections, Seventh Edition Reece, Taylor, Simon, and Dickey © 2012 Pearson Education, Inc. Lecture by Edward J. Zalisko Introduction Fertility drugs – increase the number of eggs that are ovulated and – have allowed thousands of infertile couples to have babies. Ten percent of women taking fertility drugs become pregnant with more than one embryo. © 2012 Pearson Education, Inc. Introduction Newborns from multiple births are – more likely to be premature, – more likely to have lower birth weights, and – less likely to survive. © 2012 Pearson Education, Inc. Figure 27.0_1 Chapter 27: Big Ideas Asexual and Sexual Reproduction Human Reproduction Principles of Embryonic Development Human Development Figure 27.0_2 ASEXUAL AND SEXUAL REPRODUCTION © 2012 Pearson Education, Inc. 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. Video: Hydra Budding © 2012 Pearson Education, Inc. Figure 27.1A Figure 27.1B 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. © 2012 Pearson Education, Inc. 27.2 Sexual reproduction results in the generation of genetically unique offspring 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. © 2012 Pearson Education, Inc. 27.2 Sexual reproduction results in the generation of genetically unique offspring Some organisms, such as sea anemones, can reproduce both – asexually and – sexually. Video: Hydra Releasing Sperm © 2012 Pearson Education, Inc. Figure 27.2A Eggs Two offspring arising by fission 27.2 Sexual reproduction results in the generation of genetically unique offspring 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. © 2012 Pearson Education, Inc. Figure 27.2B 27.2 Sexual reproduction results in the generation of genetically unique offspring 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. © 2012 Pearson Education, Inc. Figure 27.2C Egg HUMAN REPRODUCTION © 2012 Pearson Education, Inc. 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. © 2012 Pearson Education, Inc. 27.3 Reproductive anatomy of the human female Ovaries contain follicles that – nurture eggs and – produce sex hormones. An immature egg is ejected from the follicle in a process called ovulation. Animation: Female Reproductive Anatomy © 2012 Pearson Education, Inc. 27.3 Reproductive anatomy of the human female 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. © 2012 Pearson Education, Inc. Figure 27.3A Ovaries Oviduct Follicles Corpus luteum Uterus Wall of uterus Endometrium (lining of uterus) Vagina Cervix (“neck” of uterus) Figure 27.3B Egg cell Ovary Figure 27.3C Oviduct Ovary Uterus Rectum (digestive system) Cervix Vagina Anus (digestive system) Urinary bladder (excretory system) Pubic bone (skeletal system) Urethra (excretory system) Shaft Prepuce Clitoris Glans Labia minora Labia majora Vaginal opening Vulva Figure 27.3C_1 Oviduct Ovary Uterus Rectum (digestive system) Cervix Vagina Anus (digestive system) Figure 27.3C_2 Oviduct Ovary Uterus Urinary bladder (excretory system) Pubic bone (skeletal system) Urethra (excretory system) Shaft Prepuce Clitoris Glans Vulva Labia minora Labia majora Vaginal opening 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. Animation: Male Reproductive Anatomy © 2012 Pearson Education, Inc. Figure 27.4A Urinary bladder (excretory system) Seminal vesicle (behind bladder) Prostate gland Bulbourethral gland Urethra Erectile tissue of penis Vas deferens Scrotum Epididymis Testis Glans of penis Figure 27.4B Rectum (digestive system) Urinary bladder (excretory system) Seminal vesicle Vas deferens Pubic bone (skeletal system) Ejaculatory duct Prostate gland Erectile tissue Bulbourethral gland Anus (digestive system) Vas deferens Testicle Urethra (excretory system) Epididymis Testis Glans of penis Scrotum Prepuce Penis Figure 27.4B_1 Rectum (digestive system) Seminal vesicle Vas deferens Ejaculatory duct Prostate gland Bulbourethral gland Anus (digestive system) Figure 27.4B_2 Urinary bladder (excretory system) Pubic bone (skeletal system) Erectile tissue Testicle Vas deferens Urethra (excretory system) Epididymis Glans of penis Testis Scrotum Prepuce Penis 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. © 2012 Pearson Education, Inc. Figure 27.4C Contractions of vas deferens Sphincter contracts Urinary bladder Urethral region here expands and fills with semen Contractions of seminal vesicle Contractions of prostate gland 1 Sphincter contracts Sphincter remains contracted Contractions of epididymis Semen expelled Contractions of muscles around base of penis Sphincter relaxes 2 Contractions of urethra 27.4 Reproductive anatomy of the human male Sperm production – is regulated by a negative feedback system of hormones and – involves the – hypothalamus, – anterior pituitary, and – testes. Animation: Male Hormones © 2012 Pearson Education, Inc. Figure 27.4D Stimuli from other areas in the brain Releasing hormone Anterior pituitary FSH LH Androgen production Testis Sperm production Negative feedback Hypothalamus 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. © 2012 Pearson Education, Inc. Figure 27.5A Penis Epididymis Testis Seminiferous tubule Scrotum Testis 2n Diploid cell Differentiation and onset of meiosis I 2n Primary spermatocyte Cross section of seminiferous tubule (diploid; in prophase of meiosis I) Meiosis I completed n Secondary spermatocyte n (haploid) Developing sperm cells Meiosis II n n n n Differentiation Sperm cells (haploid) Mature sperm released into center of seminiferous tubule n n n n Figure 27.5A_1 Penis Epididymis Testis Seminiferous tubule Scrotum Testis Cross section of seminiferous tubule Figure 27.5A_2 2n Diploid cell Differentiation and onset of meiosis I 2n Primary spermatocyte (diploid; in prophase of meiosis I) Meiosis I completed n Secondary spermatocyte n (haploid) Meiosis II Developing sperm cells n Sperm cells (haploid) n Mature sperm released into center of seminiferous tubule n n n Differentiation n n n Figure 27.5A_3 2n Diploid cell Differentiation and onset of meiosis I Primary spermatocyte (diploid; in prophase of meiosis I) Secondary spermatocyte (haploid) 2n Meiosis I completed n n Figure 27.5A_4 n Secondary spermatocyte (haploid) Meiosis II Developing sperm cells n Sperm cells (haploid) n Mature sperm released into center of seminiferous tubule n n n n Differentiation n n n 27.5 The formation of sperm and egg cells requires meiosis 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. © 2012 Pearson Education, Inc. Figure 27.5B 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 Secondary oocyte Ovulated secondary oocyte (arrested at metaphase of meiosis II; released from ovary) Entry of sperm triggers completion of meiosis II Corpus luteum Second polar body Mature egg (ovum) Degenerating corpus luteum Figure 27.5B_1 Before birth Ovary Diploid cell Differentiation and onset of meiosis I Primary oocyte Primary oocyte within (arrested in prophase follicle of meiosis I; present at birth) Figure 27.5B_2 Growing follicle Completion of meiosis I and onset of meiosis II Mature follicle Ruptured follicle First polar body Secondary oocyte (arrested at metaphase Ovulated secondary oocyte of meiosis II; released from ovary) Figure 27.5B_3 Ruptured follicle First polar body Secondary oocyte Ovulated secondary oocyte (arrested at metaphase of meiosis II; released from ovary) Entry of sperm triggers completion of meiosis II Corpus luteum Second polar body Mature egg (ovum) Degenerating corpus luteum 27.5 The formation of sperm and egg cells requires meiosis 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. © 2012 Pearson Education, Inc. 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. © 2012 Pearson Education, Inc. Table 27.6 27.6 Hormones synchronize cyclic changes in the ovary and uterus 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. Animation: Ovulation Animation: Post Ovulation © 2012 Pearson Education, Inc. 27.6 Hormones synchronize cyclic changes in the ovary and uterus 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. © 2012 Pearson Education, Inc. Figure 27.6 Control by hypothalamus A Inhibited by combination of estrogen and progesterone Hypothalamus Stimulated by high levels of estrogen Releasing hormone Anterior pituitary FSH 1 LH B Pituitary hormones in blood 4 LH peak triggers ovulation and corpus luteum formation 6 LH FSH 2 C FSH stimulates follicle to grow LH surge triggers ovulation Ovarian cycle Growing follicle 5 Mature follicle Ovulation Pre-ovulatory phase Corpus Degenerating corpus luteum luteum Post-ovulatory phase Progesterone and estrogen secreted by remnant of follicle Estrogen secreted by growing follicle Peak causes LH surge Ovarian hormones in blood D 3 7 Estrogen Progesterone Progesterone and estrogen promote thickening of endometrium Low levels of estrogen trigger menstruation E Menstrual cycle 8 Endometrium 0 5 Menstruation 10 14 15 Days 20 25 28 Figure 27.6_1 Control by hypothalamus Hypothalamus Releasing hormone Anterior pituitary FSH LH Inhibited by combination of estrogen and progesterone Stimulated by high levels of estrogen Figure 27.6_2 0 5 10 Pituitary hormones in blood Days 14 15 20 LH peak triggers ovulation and corpus luteum formation LH FSH FSH stimulates follicle to grow LH surge triggers ovulation 25 28 Figure 27.6_3 0 10 5 Days 14 15 20 25 28 Ovarian cycle Growing follicle Mature follicle Ovulation Pre-ovulatory phase Estrogen secreted by growing follicle Corpus Degenerating corpus luteum luteum Post-ovulatory phase Progesterone and estrogen secreted by remnant of follicle Figure 27.6_4 0 5 10 Days 14 15 20 25 28 Peak causes LH surge Ovarian hormones in blood Estrogen Progesterone Low levels of estrogen trigger menstruation Progesterone and estrogen promote thickening of endometrium Figure 27.6_5 Menstrual cycle Endometrium 0 5 Menstruation 10 14 15 Days 20 25 28 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. © 2012 Pearson Education, Inc. 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.” © 2012 Pearson Education, Inc. Table 27.7 27.8 CONNECTION: Contraception can prevent unwanted pregnancy Contraception is the deliberate prevention of pregnancy. Several forms of contraception can prevent pregnancy, with varying degrees of success. © 2012 Pearson Education, Inc. Table 27.8 Figure 27.8 Skin patch PRINCIPLES OF EMBRYONIC DEVELOPMENT © 2012 Pearson Education, Inc. 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. © 2012 Pearson Education, Inc. Figure 27.9A 27.9 Fertilization results in a zygote and triggers embryonic development 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. © 2012 Pearson Education, Inc. Figure 27.9B Plasma membrane Middle piece Head Tail Mitochondria Nucleus Acrosome 27.9 Fertilization results in a zygote and triggers embryonic development During fertilization, 1. sperm squeeze past follicle cells, 2. acrosomal enzymes digest the egg’s jelly coat, 3. a sperm binds to egg receptors, 4. sperm and egg plasma membranes fuse, 5. the sperm nucleus enters the egg cytoplasm, 6. The vitelline layer separates and becomes impenetrable, and 7. the egg and sperm nuclei fuse. © 2012 Pearson Education, Inc. 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 Sperm 5 The sperm nucleus enters the egg cytoplasm. Plasma membrane 6 The vitelline layer separates and becomes impenetrable. Nucleus Acrosome Plasma membrane Receptor protein molecules n Sperm nucleus Vitelline layer Cytoplasm Jelly coat Egg cell n Egg nucleus n n 7 The nuclei of sperm and egg fuse. 2n Zygote nucleus Figure 27.9C_1 A sperm touches the egg’s jelly coat, and its acrosome releases enzyme molecules. Sperm The sperm’s acrosomal enzymes digest the egg’s jelly coat. Proteins on the sperm head bind to egg receptors. Acrosomal enzymes Plasma membrane Nucleus Acrosome Jelly coat Plasma membrane Vitelline layer Receptor protein molecules Figure 27.9C_2 The plasma membranes of sperm and egg fuse. The sperm nucleus enters the egg cytoplasm. The vitelline layer separates and becomes impenetrable. n Sperm nucleus Figure 27.9C_3 n Sperm nucleus n Egg nucleus n n The nuclei of sperm and egg fuse. 2n Zygote nucleus 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. Video: Sea Urchin Embryonic Development © 2012 Pearson Education, Inc. Figure 27.10_s1 Zygote 2 cells Figure 27.10_s2 Zygote 2 cells 4 cells 8 cells Figure 27.10_s3 Zygote 2 cells 4 cells 8 cells Many cells (solid ball) Figure 27.10_s4 Zygote 2 cells 4 cells 8 cells Many cells (solid ball) Blastocoel Blastula Cross section (hollow ball) of blastula 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. © 2012 Pearson Education, Inc. Figure 27.11_s1 Blastula (end of cleavage) Animal pole Blastocoel Vegetal pole Figure 27.11_s2 Blastula (end of cleavage) Animal pole Blastocoel Vegetal pole Gastrulation (cell migration) Blastocoel shrinking Formation of a simple digestive cavity Blastopore Figure 27.11_s3 Blastula (end of cleavage) Animal pole Blastocoel Vegetal pole Gastrulation (cell migration) Blastocoel shrinking Gastrula (end of gastrulation) Simple digestive cavity Formation of a simple digestive cavity Blastopore Ectoderm Mesoderm Endoderm Table 27.11 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. Video: Frog Embryo Development © 2012 Pearson Education, Inc. Figure 27.12A Neural Neural fold plate Notochord Ectoderm Mesoderm Endoderm Neural folds Figure 27.12A_1 Neural folds Figure 27.12B Neural fold Neural plate Outer layer of ectoderm Neural tube 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. © 2012 Pearson Education, Inc. Figure 27.12C Neural tube Notochord Somite Coelom Somites Digestive cavity Eye Tail bud Figure 27.12C_1 Somites Eye Tail bud Figure 27.12D 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). © 2012 Pearson Education, Inc. Figure 27.13A Figure 27.13A_1 Figure 27.13A_2 Figure 27.13B Apoptosis Dead cell engulfed and digested by adjacent cell 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. © 2012 Pearson Education, Inc. 27.14 EVOLUTION CONNECTION: Pattern formation during embryonic development is controlled by ancient genes Pattern formation, – the emergence of the parts of a structure in their correct relative positions, – involves the response of genes to spatial variations of chemicals in the embryo, and – results in tissues and organs developing in their proper positions at the correct times. © 2012 Pearson Education, Inc. Figure 27.14A Anterior Limb bud Limb bud develops Anterior Ventral Distal Proximal Dorsal Limb buds Posterior Posterior 27.14 EVOLUTION CONNECTION: Pattern formation during embryonic development is controlled by ancient genes Homeotic genes – contain common nucleotide sequences (homeoboxes), – guide pattern formation in embryos, and – occur in diverse groups such as – prokaryotes, – yeast, – plants, and – animals. – Homeotic genes reveal the shared evolutionary history of life. © 2012 Pearson Education, Inc. Figure 27.14B Fly chromosome Mouse chromosomes Fruit fly embryo (10 hours) Mouse embryo (12 days) Adult fruit fly Adult mouse HUMAN DEVELOPMENT © 2012 Pearson Education, Inc. 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. © 2012 Pearson Education, Inc. 27.15 The embryo and placenta take shape during the first month of pregnancy 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. © 2012 Pearson Education, Inc. Figure 27.15A–B Cleavage starts Fertilization of mature egg Blastocyst Trophoblast Uterine cavity Cavity Oviduct Ovary Inner cell mass Blastocyst (implanted) Secondary oocyte Ovulation Endometrium Uterus Uterine cavity Figure 27.15C Endometrium Uterine cavity Multiplying cells of trophoblast (contribute to future placenta) Trophoblast Embryo Future yolk sac Blood vessel (maternal) 27.15 The embryo and placenta take shape during the first month of pregnancy Four extraembryonic membranes develop. 1. The amnion – surrounds the embryo and – forms a fluid-filled amniotic cavity that protects the embryo. 2. 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. © 2012 Pearson Education, Inc. 27.15 The embryo and placenta take shape during the first month of pregnancy 3. The allantois – contributes to the umbilical cord, – forms part of the urinary bladder, and – in reptiles, stores embryonic waste. 4. The chorion – contributes to the placenta and – secretes human chorionic gonadotropin (HCG), which prevents menstruation in mammals. © 2012 Pearson Education, Inc. Figure 27.15D Yolk sac Chorion Amnion Amniotic cavity Mesoderm cells Figure 27.15E Embryo: Endoderm Mesoderm Ectoderm Chorionic villi Chorion Amnion Allantois Yolk sac Figure 27.15F Placenta Amnion Amniotic cavity Embryo Mother’s blood Allantois vessels Yolk sac Chorion Chorionic villi 27.15 The embryo and placenta take shape during the first month of pregnancy 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. © 2012 Pearson Education, Inc. 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. Video: Ultrasound of Human Fetus 1 Video: Ultrasound of Human Fetus 2 © 2012 Pearson Education, Inc. Figure 27.16A–C January 0 Conception February 35 days March April 63 days 98 days Figure 27.16A 5 weeks (35 days) Figure 27.16B 9 weeks (63 days) 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.). © 2012 Pearson Education, Inc. Figure 27.16C 14 weeks (98 days) Figure 27.16D–E May June July August September October 280 days 140 days Figure 27.16D 20 weeks (140 days) 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. © 2012 Pearson Education, Inc. Figure 27.16E At birth (280 days) 27.17 Childbirth is induced by hormones and other chemical signals Hormonal changes induce birth. – Estrogen makes the uterus more sensitive to oxytocin. – Oxytocin acts with prostaglandins to initiate labor. – The cervix dilates to about 10 cm. – The baby is expelled by strong uterine contractions. – The placenta dislodges and is expelled after the baby. © 2012 Pearson Education, Inc. Figure 27.17A Estrogen from ovaries Oxytocin from fetus and mother’s pituitary Induces oxytocin receptors on uterus Stimulates placenta to make Prostaglandins Stimulate more contractions of uterus Positive feedback Stimulates uterus to contract 27.17 Childbirth is induced by hormones and other chemical signals Labor occurs in three stages: 1. dilation of the cervix, 2. expulsion, delivery of the infant, 3. delivery of the placenta. © 2012 Pearson Education, Inc. Figure 27.17B Placenta Umbilical cord Uterus Cervix 1 Dilation of the cervix 2 Expulsion: delivery of the infant Uterus Placenta (detaching) Umbilical cord 3 Delivery of the placenta Figure 27.17B_1 Placenta Umbilical cord Uterus Cervix 1 Dilation of the cervix Figure 27.17B_2 2 Expulsion: delivery of the infant Figure 27.17B_3 Uterus Placenta (detaching) Umbilical cord 3 Delivery of the placenta 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. © 2012 Pearson Education, Inc. Figure 27.18 Implantation Zygote Collected egg In vitro fertilization Collected sperm 8-cell embryo Figure 27.18_1 You should now be able to 1. Compare the types, advantages, and disadvantages of asexual and sexual reproduction. 2. Describe the structures and functions of the female and male human reproductive systems. 3. Describe and compare the processes and products of spermatogenesis and oogenesis. 4. Describe the events of and control of the menstrual cycle. © 2012 Pearson Education, Inc. You should now be able to 5. Describe the nature of the most common sexually transmitted diseases. 6. Describe the most common forms of birth control and explain how each works. 7. Relate the structure of sperm to its role in fertilization. 8. Describe the process and results of cleavage. 9. Describe the process of gastrulation and the resulting arrangement of the embryo. © 2012 Pearson Education, Inc. You should now be able to 10. Explain how organs form after the development of a gastrula. 11. Explain how changes in cell shape, induction, cell migration, and apoptosis contribute to development. 12. Explain how the one-dimensional information in DNA is used to direct the three-dimensional form of an embryo. 13. Describe the initial embryonic stages and the formation and functions of the extraembryonic membranes in humans. © 2012 Pearson Education, Inc. You should now be able to 14. Describe the main changes that occur during each of the trimesters of human development. 15. Explain how labor begins and describe the main events of the three stages of labor. 16. Describe the common causes of human infertility and the technologies currently available to help couples conceive. © 2012 Pearson Education, Inc. Figure 27.UN01 Oogenesis 2n Once per month Spermatogenesis Primary oocyte Primary spermatocyte Continuously n Polar body n Secondary oocyte 2n Secondary spermatocyte Developing n sperm cells n n n n Sperm n Fertilization Polar body n Mature egg n 2n Zygote n Figure 27.UN02 Cleavage Gastrulation Ectoderm Mesoderm Endoderm Zygote 2-cell embryo Many-celled Blastula (cross solid ball section) Gastrula (cross section) Hormone level in blood (arbitrary units) Figure 27.UN03 50 Hormones (A–D) 40 C 30 D 20 A 10 B 0 0 Events (P–S) 5 10 15 Days 20 P Q R S 25 28