ANIMAL DEVELOPMENT

42 ____________________________________________________________________________________________ animal development Chapter Outline mind-boggling births stages of reproduction and development from Zygote to gastrula Components of Eggs and Sperm Cleavage—the Start of Multicellularity Gastrulation how specialized tissues and organs form Cell Differentiation Responses to Morphogens Embryonic Induction Cell Migrations Apoptosis An Evolutionary View of Development A General Model for Animal Development Developmental Constraints and Modifications overview of human development Early human development Cleavage and Implantation Extraembryonic Membranes Early Hormone Production emergence of the vertebrate body plan structure and function of the Placenta emergence of distinctly human features Miscarriages, stillbirths, and birth defects What Can Go Wrong Risk Factors Morning Sickness birth and lactation Labor and Delivery Surgical Delivery Nourishing the Newborn MIND-BOGGLING BIRTHS (REVISITED) SUMMARY data analysis exercise Self-Quiz critical thinking Objectives 1. Describe early embryonic development and distinguish each of the following: oogenesis, fertilization, cleavage, gastrulation, organ formation, and tissue specialization. 2. Outline the principal events of prenatal development. 3. Understand the importance of the placenta and extraembryonic membranes to the developing embryo. 4. Describe the evolutionary constraints on development. 5. Understand the effects of nutrition, infectious agents, alcohol, caffeine, smoking, and prescription drugs on embryonic development. 6. Know the process of labor and delivery, as well as nursing the baby post-delivery. 7. Understand the in vitro fertilization procedure and other methods of artificial fertilization. Key Terms cleavage blastula gastrulation germ layers gastrula in vitro fertilization cytoplasmic localization blastomere vegetal pole animal pole gray crescent blastocyst ectoderm endoderm mesoderm embryonic induction cell differentiation morphogens morphogenesis apoptosis differentiation embryonic induction pattern formation master genes homeotic genes somites fetus morula implantation extraembryonic membranes amnion yolk sac chorion allantois placenta human chorionic gonadotropin, HCG neural tube teratogen fetal alcohol syndrome, FAS labor oxytocin lactation prolactin Lecture Outline 42.1 Mind-Boggling Births A. In 1978 the first “test tube baby” was born by the reproductive technology, in vitro fertilization (IVF). 1. This procedure involves injecting the mom with hormones to mature many eggs, injecting the sperm into the egg in the laboratory, and inserting the fertilized egg in the uterus. 2. Over three million children have been conceived using IVF. B. Multiple births are becoming more common. 1. Fertility drugs and IVF are driving the increase in higher-order multiple births. 2. Children born as multiple births have a higher risk of miscarriage and birth defects. 3. Multiple newborns usually have lower birth weights and higher mortality rates. C. The “octomom” had twelve embryos implanted and gave birth to eight infants. 1. Fertility doctors are advised to limit the amount of embryos implanted at one time. 42.2 Stages of Reproduction and Development A. Gamete formation: eggs or sperm form and mature within the parents. B. Fertilization begins when a sperm penetrates an egg and is completed when the sperm nucleus fuses with the egg nucleus, resulting in formation of the zygote. C. Repeated mitotic divisions—cleavage—convert the zygote to a blastula; cell numbers increase but not cell size. D. Gastrulation results in three germ layers, or tissues. 1. Ectoderm is the outer layer; it gives rise to the nervous system and the outer layers of the integument. 2. Endoderm is the inner layer; it gives rise to the gut and organs derived from it. 3. Mesoderm is the middle layer; muscle, organs of circulation, reproduction, excretion, and skeleton are derived from it. E. Organ formation begins as germ layers subdivide into populations of cells destined to become unique in structure and function. F. During growth and tissue specialization, organs acquire specialized chemical and physical properties. 42.3 From Zygote to Gastrula A. Components of Eggs and Sperm 1. The sperm contributes little more than the paternal DNA. 2. The oocyte contains the majority of materials that will affect early development. 3. Penetration of the egg by the sperm triggers a structural reorganization in the egg cytoplasm. a. In a frog egg, microtubules move granules from the animal pole to form a gray crescent near the equator opposite the penetration site. b. Near the crescent, the body axis of the frog embryo will become established and gastrulation will begin. c. Experiments by Hans Spemann have shown that the substances needed for proper development are in the gray crescent. B. Cleavage—The Start of Multicellularity 1. After fertilization, the zygote begins a series of divisions in which each cell is pinched into two cells (blastomeres). 2. This process is not random and different blastomeres will end up with different genetic messages in a process known as cytoplasmic localization. 3. Radial cleavage starts with cuts perpendicular to the mitotic spindle, producing cells of similar size but with different parts of the cytoplasm. 4. In other cells (frog, for instance), the cuts do not go all the way through, so the cells produced are smaller at the animal pole. 5. In mammals, rotational cleavage results in an inner cell mass (future embryo), which forms on the inside of a hollow sphere. 6. The amount of yolk stored inside the egg also affects cleavage patterns. a. Cleavage is incomplete when an abundance of yolk impedes the cytoplasmic division, like in the insects, reptiles, birds, and most fishes. b. When little yolk is present, the first cut divides all the cytoplasm in complete cleavage (amphibians and mammals). C. Gastrulation Experiments by Hilde Marigold proved that the cells in the dorsal lip of an opening are descended from the gray crescent. Cells from the dorsal lip of a salamander were implanted in a different salamander. A salamander with two joined bodies developed. If no cytoplasm from the gray crescent is present, development will not progress normally. 42.4 How Specialized Tissues and Organs Form A. Cell Differentiation 1. From gastrulation onward, cell lineages also engage in selective gene expression, which is the start of cell differentiation. 2. Morphogens are signaling molecules produced by master genes. a. They diffuse out and form a concentration gradient in the embryo. b. A morphogen’s effect on target cells is proportional to its concentration. B. Responses to Morphogens 1. Morphogenesis is the progression of differentiated cells into tissues and organs; it is the result of several events. 2. There are proteins that serve as maternal effect genes. a. These genes are expressed during egg production and they influence development. b. In fruit flies, a gene called the bicoid gene influences the development of separate anterior and posterior ends. C. Embryonic Induction 1. Embryonic induction indicates that on embryonic tissue influences the development of neighboring cells. 2. Sheets of cells expand and fold as cells change shape, forming organs such as the neural tube. a. Signals from the notochord direct the formation of the neural tube. b. The formation of the neural tube begins by the thickening of the neural plate near the notochord. c. Microtubules form and the edges of the plate fold inward to form a neural groove. d. The edges of the neural plate are drawn together to form the completed neural tube. D. Cell Migrations 1. Cells move from one location to another due to either hormonal or chemical signals. a. This process occurs in the formation of the neural crest. 2. During cell migrations, the cells move like an amoeba via the action of actin microfilaments until adhesion proteins hold it in place. E. Apoptosis 1. Apoptosis is programmed cell death, which helps sculpt body parts. 2. Apoptosis is the genetically programmed elimination of tissues and cells that are used for only short periods in the embryo or adult. a. Enzymes from lysosomes digest away portions of cells, so that those cells die. b. Apoptosis helps remove the webbing between human fingers and also disposes of the tadpole’s tail. 42.5 An Evolutionary View of Development A. A General Model of Animal Development 1. Through studies of roundworms, fruit flies, fish, and mice, researchers have come up with a general model of animal development. 2. A key point of this model is that where and when particular genes are expressed determines how an animal body develops. a. Molecules confined to different areas of an unfertilized egg induce localized expression of master genes in the zygote, and products of these genes diffuse outward forming a concentration gradient that forms along the head-to-tail and dorsal-to-ventral axes. b. Depending on where they fall within these concentration gradients, cells in the embryo activate or suppress other master genes. c. This positional information affects expression of homeotic genes, genes that regulate development of specific body parts. B. Developmental Constraints and Modifications 1. The basic body plans of the major animal groups have not changed due to a limited number of master genes. 2. These genes have imposed phyletic constraints in addition to the more well-known physical and architectural constraints. a. An example of a physical constraint is the surface-area-to-volume ratio. b. Architectural constraints are imposed by body axes. c. Phyletic constraints are those imposed by interactions among genes that regulate development in a lineage. 42.6 Overview of Human Development A. Pregnancy lasts an average of 38 weeks. 1. Embryo formation takes about two weeks. 2. The embryonic period lasts from the third to the end of the eighth week. 3. The fetal period extends from the eighth week until birth. B. The first cleavage starts 12–24 hours after fertilization. C. It takes one week for the blastocyst to form and then it embeds in the mother’s uterus. D. The prenatal period is divided into three trimesters. E. After birth, the body continues to grow and continues to change proportions. 42.7 Early Human Development A. Cleavage and Implantation 1. During the first few days after fertilization, the zygote undergoes repeated cleavages as it travels down the oviduct. 2. By the time it reaches the uterus, it is a solid ball of cells (morula), which is transformed into a blastocyst. a. Implantation begins about six days after fertilization. b. The inner cell mass of the blastocyst is transformed into an embryonic disk that will develop into the embryo proper within the next week. B. Extraembryonic Membranes 1. Membranes start forming outside the embryo and have several functions. a. The amnion is a fluid-filled sac that keeps the embryo from drying out and acts as a shock absorber. b. The yolk sac becomes a site for blood cell formation. c. The chorion, a protective membrane around the embryo, forms a portion of the placenta and secretes a hormone (human chorionic gonadotropin) that maintains the uterine lining after implantation. d. The allantois does not function in waste storage (as it does in birds) but is active in blood formation and formation of the urinary bladder. C. Early Hormone Production 1. Cells of the blastocyst secrete the hormone HCG, which stimulates the corpus luteum to continue the secretion of estrogen and progesterone, preventing menstruation. 2. HCG is also detected in home pregnancy tests as early as the third week of pregnancy. 42.8 Emergence of the Vertebrate Body Plan A. By two weeks after fertilization, a two-layered embryonic disk consisting of ectoderm and endoderm has formed. 1. The primitive streak, a forerunner of the neural tube from which the brain and spinal cord will form, has appeared. 2. Some cells also form the notochord, from which the vertebrae will form. B. Toward the end of the third week, mesoderm has developed and is giving rise to somites—segments of bones and skeletal muscles. C. Pharyngeal arches start to form that will contribute to the face, neck, mouth, nose, larynx, and pharynx; spaces in the mesoderm will become the coelomic cavity. 42.9 The Function of the Placenta A. The placenta is a combination of uterine lining and extraembryonic membranes. 1. At full term, the placenta will cover approximately one-fourth of the inner surface of the uterus. 2. The placenta forms early in pregnancy. B. Materials are exchanged from blood capillaries of mother to fetus, and vice versa, by diffusion. 1. The maternal blood and fetal blood do not mix! 2. After the third month, the placenta produces large amounts of HCG, progesterone, and estrogen to maintain the uterine lining. 42.10 Emergence of Distinctly Human Features A. By the end of the fourth week, the embryo is 500 times larger than when it started but still smaller than a centimeter in length. B. It has embarked on an intricate program of cell differentiation and morphogenesis, including development of limbs, circulation, and umbilical cord. C. The second trimester encompasses months four, five, and six; the individual is now called a fetus; the heart is beating; fuzzy hair covers the body. D. The third trimester extends from month seven until birth; the earliest delivery in which survival on its own is possible is the middle of this trimester. 42.11 Miscarriages, Stillbirths, and Birth Defects A. What Can Go Wrong 1. A miscarriage is the loss of a pregnancy before 20 weeks—after that time, a loss is called a stillbirth. a. The risk of a miscarriage is about 12–15 percent, and only 1.5 percent chance if a normal embryo isseen via ultrasound at 8–11 weeks. 2. Birth defects occur in approximately 3 percent of births. a. Common birth defects involve the heart, neural tube, and cleft lip anomalies. B. Risk Factors 1. Chromosome abnormalities are responsible for 5–10 percent of still births and 50 percent of miscarriages. a. These risk factors increase as the parents’ ages increase. 2. Women who are happy and are of a normal weight are less apt to miscarry. 3. Teratogens are substances that interfere with proper fetal development. 4. A mother requires a well-balanced diet to fuel fetal development. a. Deficiencies in iodine and folic acid can lead to specific defect in the offspring. 5. There are some infectious diseases that cause the mom’s unborn child to have serious issues. Rubella (German measles) can cause developmental problems if the mom was not previously vaccinated. Toxoplasmosis, which can be transmitted via contact with cat litter, can cause serious problems with the child or cause a miscarriage. 6. Fetal alcohol syndrome results in physical and mental abnormalities caused by alcohol consumption by the mother. 7. Pregnant women should avoid caffeine, cigarette smoke, and hot tub use to avoid a miscarriage. 8. Certain medications (e.g., Paxil and Accutane) can cause heart abnormalities in the developing infants. C. Morning Sickness 1. Two-thirds of women experience morning sickness from weeks 6–12 of pregnancy. 2. Morning sickness may have an adaptive function, providing protection against teratogens and other harmful substance. a. Some of the substances that seem the most distasteful to pregnant women are those that would be most likely to contain dangerous microbes. 42.12 Birth and Lactation A. Labor and Delivery 1. The birth process (labor) begins with contractions of the uterine muscles; the cervical canal dilates and the amniotic sac ruptures. a. The hormone relaxin softens the cervical connective tissues and makes the bridges between the pelvic bones looser. b. The hormone oxytocin induces powerful uterine contractions. 2. The fetus is expelled, accompanied by fluid and blood; the umbilical cord is severed, and finally the placenta is expelled. 3. Corticotropin-releasing hormone affects the timing of labor, and it may contribute to post-partum depression. B. Surgical Delivery 1. A cesarean section (C-section) involves surgically opening the uterus to remove the baby. a. Reasons for performing a C-section are: abnormal positioning of the baby or placenta, a kinked umbilical cord, or to avoid the transmission of sexually transmitted diseases. C. Nourishing the Newborn 1. The mammary glands first produce a special fluid for the newborn; then, under the influence of prolactin, they produce milk. 2. Oxytocin is released in response to suckling and further increases the milk supply. 3. Besides providing nourishment, human breast milk contains antibodies that protect a newborn from some viruses and bacteria. 42.13 Mind-Boggling Births (Revisited) New techniques have expanded on the idea of in vitro fertilization. Intracytoplasmic sperm injection actually propels the sperm into the egg to be later implanted in the uterus. 2. A blastocyst developed from another woman’s egg can also be implanted in the uterus. 3. A woman’s egg could be fertilized in the lab and then implanted in a surrogate. B. Some screening can be performed prior to implantation to rule out birth defects. Suggestions for Presenting the Material • There are numerous excellent animated figures available to help you present this material. The animated figures provide excellent visual context for your students. • Table 42.2 presents a good overview of human development, and Figure 42.18 presents a good depiction of human embryo development. The specifics of each development in each time interval can lead to a “cataloging” approach, which can be alleviated by using the videotape referred to in the Enrichment section. • Although it is not always convenient to do so, the demonstration of live chick embryos is a real attention-arresting sight. Don’t neglect to place an embryo under a stereomicroscope to see the heartbeat and blood flow. • The early development of sea urchin embryos is not as difficult to demonstrate as that of the chick. Biological supply houses sell demonstration kits. Timing is a critical factor for viewing all the stages, so you should plan to videotape the sequence. When discussing the placenta, be sure to mention that it not only supplies nutrients and oxygen to the offspring, but it also forms an immune barrier. If humans did not have a placenta, the mother’s immune system would eventually detect something “foreign” in her body and the fetus would be rejected. The presence of a placenta prevents this from occurring. Notice that animals like the kangaroo have no placenta. Therefore, the offspring is expelled from the mom at an immature stage and must complete development in the pouch. Set up a Jeopardy-type quiz for your students, which may help them distinguish between similar terms, such as blastula, gastrula, etc. A template that contains the setup can be found at: _http://jeopardylabs.com/_ Classroom and Laboratory Enrichment • If at all possible, show a videotape or film depicting development of some animal. Because of the dynamic nature of this process and the rapid changes, static photographs are woefully inadequate. • The topic of prenatal development will be greatly enhanced by the use of a videotape such as The Miracle of Life, distributed by Crown Video through retail outlets. • Discussion of the tadpole’s tail change into a tailless frog gives students a familiar visual image of how apoptosis in development works. If your budget allows, purchase some prepared slides of the various stages of embryonic development. Slides of a whitefish blastula are often available for this purpose. Name a number of species that undergo metamorphoses as part of their development. Classroom Discussion Ideas • Why do think the number of multiple births has increased so dramatically in the past 20 years? Part of the reason involves reproductive technology, but also women that have children at older ages are more prone to multiple births. Present the multiple-birth statistics from earlier generations with that of the current multiple-birth trends. • What are the risks of mothers in their 40s and even 50s giving birth? Evaluate both the risk to the mother and the fetus/child. There are even examples of women giving birth to their own grandchildren via the use of their daughters’ donor eggs! • List ways in which the increased number of multiple births results in increased costs to society. • Will Americans ever agree to controls over fertility and reproduction as some other countries have? Do you think there should be legislation to prohibit the amount of eggs fertility doctors could implant at once? Should physicians that disobey these laws face serious ramifications? Additional Ideas for Classroom Discussion • The incidence of Down syndrome is said to increase with maternal age, especially for mothers over age 40. Based on the information in the present chapter, can you explain why? • The placenta supplements, or completely replaces, the activity of three organ systems in the fetus. What are they? • Many communities, and even states, restrict the teaching of human reproduction. Why do you think this body system is singled out over, say, digestion or respiration for such a prohibition? • When an insect is in the pupal stage, there is seemingly no activity. Some people have even called it the “resting stage”—erroneously! Biochemically and histologically, what is happening during the pupal stage? • Explain this quote by Lewis Wolpert (1986): “It is not birth, marriage, or death, but gastrulation which is truly the most important time of your life.” Make a chart detailing the three germ layers and the tissues they become. Emphasize the importance of the gray crescent tissue. Discuss how the study of embryology has changed scientists’ views on classification. How Would You Vote? Classroom Discussion Ideas • Monitor the voting for the online question. The tradeoff between concerns over high-risk pregnancies and assisting couples in enhancing their fertility is a difficult question to consider. Have your students break into small groups and present both the pro and con arguments. On the board or overhead projector, summarize the primary arguments and categorize them as: personal, social, economic, etc. Were any of the students in the class created by the use of reproductive technology? They would be certain to have strong feelings on the issue! How does the selling of eggs differ from organ donation? Does it damage the donor in any way? Does it impact their future fertility in any way? Are the drugs used to bring about the maturation of many eggs harmful? Do you think fertility records should be available to the offspring? Should a child be able to find out anything about their egg or sperm donor? What characteristics would someone look for in a sperm or egg donor? Intelligence? Good health? An attractive appearance? • Have students complete class polling using the JoinIn clickers. Term Paper Topics, Library Activities, and Special Projects • We hear about “sperm banks” now and then. Do such repositories actually exist? Where are they? How do they function? • Any interruption in the menstrual cycle, whether temporary or permanent, is cause for concern. Report on the causes and effects of the cessation of menstruation in females who drastically reduce their body weight (anorexia) or body fat (as in body building). • Search for the physiological explanation for the cessation of menstruation (menopause)—usually when a woman is between the ages of 40 and 50. Is there any comparable phenomenon in men? • It is known that overcrowding and stressful conditions reduce reproductive behavior in rodents. Is there any published evidence of such a phenomenon in humans? • Research the family planning practices of China. Find out the current practice of child restriction and the effects this is having on China’s demography. There have been articles written about the rewards given to families who have daughters; is this a valid or regular practice in the country? • Investigate the use of juvenile hormone-mimicking chemicals to control insects. What are the successes, failures, limitations, advantages, and prognoses for the future? • Embryologists can transplant imaginal discs (invaginated epidermal sacs programmed to produce adult structures at metamorphosis) of insect larvae to produce very unusual adults (for example, legs where antennae should be). Investigate the procedures used and the results produced. Could this be done in vertebrates? • Research the discovery of the homeotic box or homeotic genes in vertebrates. How similar are they across groups? How do they compare to human gene sequences? What does the discovery suggest about the evolutionary phylogeny of animals? How is in vitro fertilization performed? What are the chances of success? What additional procedures have been developed based on IVF technology? What are their chances for success? Read the original research performed by Hans Spemann in the early 1900s concerning the importance of the gray crescent in animal development. Was this study innovative for its time? As you have seen from the text, the neural tube closure occurs very early in fetal development. Researchers have discovered that the incidence of neural tube abnormalities can be decreased by taking folic acid supplements. Their subsequent idea was to include folic acid in prenatal vitamins. Now folic acid is included in everyday food items, such as flour, to help reduce the occurrence of these conditions. Do you see a problem with this solution? After first discovering she is pregnant, when does a mom visit the doctor for prenatal vitamins? Look into the research relating to the bicoid protein. Have experiments been performed on organisms other than fruit flies? What happens to the tail that humans have during fetal development? What is the mechanism of the “morning-after” pill? How does it impede implantation? What do pharyngeal arches later become? Choose one of the extraembryonic membranes. How does it function in humans? Does its function differ in other organisms? Research a specific teratogen. How does it impede normal development? What is thalidomide? What was its original use? What abnormalities did it produce? When was its use restricted for pregnant women? Possible Responses to Critical Thinking Questions 1. There is good reason rubella would cause so many of its effects on the fetus during the first trimester. It is during this time that the organs are developing; limbs form, and toes and fingers are sculpted. Growth of the head surpasses that of other body regions. By eight weeks, all major organs and systems have been formed, which is why the first three months of development (first trimester) is when the infection of the rubella virus has the most effect. 2. Germ cells have an important property that somatic cells (body cells) do not. This characteristic is that germ cells have the potential to differentiate into many different kinds of cells in various tissues. This ability becomes evident in the many different types of structures that can be found in a teratoma. Most teratomas are benign (non-cancerous), but they can contain many unusual collections of tissue. It is thought that most teratomas are present from birth, although they may be discovered much later in life. 3. Oxytocin has a two-fold effect in the mother’s body. One of its functions is to produce the uterine contractions that result in the baby’s birth. The other function relates to the milk let-down reflex while the baby is nursing. If alcohol reduces oxytocin production, the baby will not get as much milk during the nursing process as it normally would. Possible Responses to Data Analysis Exercise Questions Evidence has indicated that there may be a possible correlation between multiple births and the incidence of birth defects. This study in Florida, 1996–2000, set out to gather data to test that hypothesis. The most common type of birth defect in the single-birth group was heart defects. Heart defects were more common in multiple births than among single births. Multiples have more than twice the risk of developing central nervous system defects as compared to single births. According to this study, a multiple pregnancy does not increase the relative risk of a chromosomal defect in offspring. 424 Chapter Forty-Two Animal Development 423 424 Chapter Forty-Two Animal Development 423 424 Chapter Forty-Two Animal Development 423 424 Chapter Forty-Two Animal Development 423 424 Chapter Forty-Two Animal Development 423