Chapter Menu Chapter Introduction Key Events of Development 10.1 Beginnings of the Embryo 10.2 Growth, Differentiation, and Form 10.3 From One Cell to Many: Making the Organism Developmental Diversity 10.4 Developmental Patterns and Evolutionary Relationships 10.5 Human Development 10.6 Birth Defects Mechanisms of Cell Differentiation 10.7 Exploring the Mechanisms of Differentiation 10.8 The Genetic Equivalence of Differentiating Cells 10.9 Determination and Differentiation 10.10 Cytoplasmic Determination 10.11 Cell-Cell Interactions Chapter Highlights Chapter Animations Learning Outcomes By the end of this chapter you will be able to: A Describe the stages of embryonic development in amphibians and humans. B Describe how developmental patterns relate to evolutionary relationships. C Describe human embryonic development. D Describe methods used to understand mechanisms underlying differentiation. E Explain the genetic-equivalence hypothesis and describe experiments performed to test it. F Explain determination and the role the cytoplasm has in this process. G Discuss examples of cell-cell interactions in differentiation. Animal Growth and Development What characteristics identify the different body forms among these eggs? What biological processes must occur before the juvenile salmon resembles the adult form? This photo shows red salmon (Oncorhynchus nerka) eggs in various stages of development. Animal Growth and Development • A newborn baby forms through a series of fascinating biological events. • Biologists use the techniques of cellular and molecular biology to unravel the mysteries—to reveal the mechanisms that control an embryo’s development. This photo shows red salmon (Oncorhynchus nerka) eggs in various stages of development. Key Events in Development 10.1 Beginnings of the Embryo • Development of most new animals begins with fertilization, the union of a sperm and an egg. • The sperm and egg, are also called gametes. Note that the sperm are enlarged x602, much more than the eggs. In the chicken, frog, and fish, the ova are surrounded by other materials (shown in outline). Key Events in Development 10.1 Beginnings of the Embryo (cont.) • Animal sperm cells are usually very small and have a flagellum that they use to swim toward an egg. • The cytoplasm of the larger egg cells contains yolk, an energy-rich mass of nutrients. • Each gamete nucleus has one-half of the chromosome set found in each of the parent’s cells. Key Events in Development 10.1 Beginnings of the Embryo (cont.) • In fertilization, the sperm and egg nuclei fuse, reestablishing the full chromosome set of the normal animal cell. • A fertilized egg, or zygote, is the earliest stage of the embryo. A scanning electron micrograph of contact between a purple sea urchin (Arbacia puretulata) sperm and egg, x20,000. Key Events in Development 10.1 Beginnings of the Embryo (cont.) • Before fertilization, the eggs of many animals are metabolically inactive. • Fertilization stimulates activation which turns on the egg’s metabolism within seconds of egg-sperm fusion. • When the zygote begins to divide, it is as metabolically active as most adult cells. Key Events in Development 10.1 Beginnings of the Embryo (cont.) • Activation has two other major effects: 1. a rapid change in the plasma membrane, which blocks fertilization by a second sperm 2. a rearrangement of the zygote cytoplasm by movements in the cytoskeleton that helps produce differences among cells when they divide After the unfertilized egg of the sea squirt, Styela clava, (a) is fertilized, polarity is established (b) when pigments cover a portion of the cytoplasm, resulting in a yellowish region and a gray, non-pigmented area. These regions are evident at the 2-cell stage (c), where the yellowish lower hemisphere (which will become the muscle cells of the tail) is distinct from the dark upper half of the embryo. At this stage, the embryo is divided into right and left halves. Development continues through the 4-cell stage (d) to the 32-cell blastula stage (e). The last photograph (f) shows the tadpolelike larva. Key Events in Development 10.2 Growth, Differentiation, and Form • Animal development includes growth, cell specialization, and formation of tissues and organs. • As the embryonic cells divide, some become different from others, a process called differentiation. • As cells differentiate, they organize to form the tissues and organs of a complete animal during a period of development called morphogenesis. The process of cellular differentiation Key Events in Development 10.2 Growth, Differentiation, and Form (cont.) • Each type of cell that differentiates during development has a unique structure and function. • Skin cells are tough, thin, flat cells that are specialized to protect the body. • Skeletal muscle cells are filled with protein fibers that enable them to contract. Epidermal cells, x99 Muscle cells, x320 Key Events in Development 10.2 Growth, Differentiation, and Form (cont.) • Nerve cells have long, thin branches that are specialized to transmit information. • The mature human red blood cell lacks a nucleus and is specialized to transport oxygen. Nerve cells, x3900 Red blood cells, x9000 Key Events in Development 10.2 Growth, Differentiation, and Form (cont.) • Proteins are the keys to differentiation in animal cells. • Specific groups of genes are expressed in each type of cell, leading to the production of specific proteins. • Differences between cells in gene expression lead to differences in cell form and function. Key Events in Development 10.2 Growth, Differentiation, and Form (cont.) Key Events in Development 10.3 From One Cell to Many: Making the Organism • After fertilization, the zygote divides into two cells. • During this period of development, called cleavage, the cells usually divide simultaneously, doubling in number with each cycle. Early cell divisions in these organisms are directed by the distribution and amount of yolk in the fertilized egg (not to scale). The invertebrate sea star has a sparse, evenly distributed yolk, similar to that of the vertebrate frog and mouse. In contrast, the duck has a dense yolk limited to one area of the egg. From the 16- to 64-cell stage, an embryo is referred to as a morula. Key Events in Development 10.3 From One Cell to Many: Making the Organism • By the end of cleavage, the embryo consists of a mass of many cells called a blastula. (cont.) • The cells are usually all of the same general size and appearance. • The shape of the blastula depends on the structure of the original egg and how its yolk is arranged. Key Events in Development 10.3 From One Cell to Many: Making the Organism • As some cells move from the surface to the interior of the blastula the embryo becomes a three-layered gastrula. (cont.) Key Events in Development 10.3 From One Cell to Many: Making the Organism • The three cell layers, called the primary germ layers, will form all the body’s tissues. (cont.) – The outer layer, the ectoderm, will form the skin, nervous system, and related structures. – The middle layer, the mesoderm, will produce the skeleton, muscles, heart, blood, and many other internal organs. – The inner layer, called the endoderm, is usually a tube and will become the lining of the digestive system. Key Events in Development 10.3 From One Cell to Many: Making the Organism • Development of a blastula into a gastrula, or gastrulation, involves major changes. (cont.) • Morphogenesis includes: – Coordinated movements of individual cells and tissues – Changing cell shapes or splitting of cell layers – Formation of tissue masses by local cell division – Even shaping of organs by genetically timed death of some cells Key Events in Development 10.3 From One Cell to Many: Making the Organism • In vertebrates the general shape, or body plan, of the organism appears during gastrulation. (cont.) – The first mesoderm becomes the notochord, a stiff rod that develops into part of the backbone. – Notochord development establishes the anteriorposterior axis, a line running from head to tail and the dorsal-ventral direction (from the back to the belly). – A large head, segmented backbone, and limbs complete the vertebrate body plan. Key Events in Development 10.3 From One Cell to Many: Making the Organism (cont.) During the early stages of development, the structure of this body plan is directed by a genetic program. Key Events in Development 10.3 From One Cell to Many: Making the Organism (cont.) • Above the notochord, the dorsal ectoderm folds up to become the neural tube which will form the brain, spinal cord, and nerves. Key Events in Development 10.3 From One Cell to Many: Making the Organism (cont.) A vertebrate gastrula possessing a notochord (a) begins the process of neurulation (formation of the neural tube) when the ectoderm begins to fold (b). The edges of the U-shaped fold arch toward the midline of the embryo (c), where the ectoderm fuses to form the neural tube (d). Note the reorganization of the primary germ layers in the resulting embryo, called a neurula. Key Events in Development 10.3 From One Cell to Many: Making the Organism • The tissue interactions that produce the neural tube establish the foundation on which the later stages of development are based. (cont.) In this cross section of a vertebrate embryo, note the approximate locations of the three primary germ layers. Key Events in Development 10.3 From One Cell to Many: Making the Organism • Some animals, such as birds and mammals, develop directly into young that are like the adult. (cont.) • Other animals—such as frogs, sea stars, and insects—first form a larva (plural: larvae), a feeding individual that looks nothing like the adult. • The larva later goes through metamorphosis, a series of changes that transforms the larva into an adult. Development Diversity 10.4 Developmental Patterns and Evolutionary Relationships • The basic developmental pattern varies greatly among animals. • Developmental patterns are a clue to relationships among living groups of animals. • Differences can suggest a more distant relationship or adaptation to different environments. • Charles Darwin was among the first biologists to compare developmental patterns to help determine relationships among animal species. Development Diversity 10.4 Developmental Patterns and Evolutionary Relationships (cont.) Note the similarities in these developmental stages of several vertebrate animals that suggest the close relationship of animals whose adult forms are quite different. Development Diversity 10.4 Developmental Patterns and Evolutionary Relationships (cont.) • Similar genes in many animals are responsible for segmentation, division of the body into a number of similar sections. • The body-pattern genes were first discovered in fruit flies that carried errors in these genes. • Errors in these homeotic genes can transform one organ into another. Development Diversity 10.4 Developmental Patterns and Evolutionary Relationships (cont.) • Each homeotic gene acts in a different body segment and contains one or more copies of a 180-base-pair sequence called a homeobox. • The homeobox encodes a 60-amino-acid protein called a homeodomain. • This part of the protein binds to DNA, regulating the transcription of important genes. • Nearly identical gene sequences, named Hox genes, were found in mice. Homeotic genes (a) in the fruit fly and Hox genes (b) in the mouse are color-coded to indicate which gene is active in which body part and to identify which genes have similar DNA sequences in these two species. Note the similar arrangement of genes on the chromosomes and its relationship to the embryo segments expressing those genes. Development Diversity 10.4 Developmental Patterns and Evolutionary Relationships (cont.) • Homeotic genes show that most animals share the same basic genetic program for the body plan. • These genes have changed very little during evolution. • Changes in homeotic genes often lead to embryonic death or severe abnormalities. Development Diversity 10.5 Human Development • The development of humans and most other mammals is unique. • Their embryos develop within the mother where her body provides a warm, protected environment. • The mother’s blood circulation provides nutrition and oxygen to the embryo and takes away wastes and carbon dioxide. Development Diversity 10.5 Human Development (cont.) • About 5 days after fertilization, the embryo, called a blastocyst, resembles the hollow blastula of other animals. • The blastocyst sinks into the wall of the mother’s uterus to develop and grow. In this section of a human blastocyst, the embryo develops from the inner cell mass. The thicker part of the blastocyst seen at the left. Development Diversity 10.5 Human Development (cont.) • Part of a thick mass of cells inside the blastocyst forms the disk that becomes the embryo. In this section of a human blastocyst, the embryo develops from the inner cell mass. The thicker part of the blastocyst seen at the left. Development Diversity 10.5 Human Development (cont.) • The rest of the blastocyst develops into membranes that surround, nourish, and protect the embryo. • The amnion immediately surrounds the embryo. • The chorion encloses all the other membranes and forms from the blastocyst’s thin outer wall. A developing human fetus in the uterus, the embryonic membranes, and the placental connection. Part of the placenta is enlarged to show both fetal and maternal circulations. The mother’s blood does not mix with that of the fetus; exchange of materials occurs across the thin membranes that separate the fetal capillaries from small pools of maternal blood. Development Diversity 10.5 Human Development (cont.) • As gastrulation begins, the chorion extends fingerlike projections, or villi (singular: villus), into the lining of the uterus. • The chorionic villi and uterine lining form the placenta, which exchanges nutrients, wastes, oxygen, and carbon dioxide between mother and embryo. • Blood vessels in the umbilical cord connect the embryo to the placenta, but the two blood supplies remain completely separate Development Diversity 10.5 Human Development (cont.) • A human takes about 40 weeks to develop in the uterus. – After the beginning of the eighth week, the embryo is called a fetus. – After 3 months, or the first trimester, most of the organs have begun to form, and the skeleton can be seen in ultrasound images. – Most of the last 3 months, or third trimester, is a period of rapid growth and maturation of organ systems. At 4 weeks after fertilization, the human embryo, like all vertebrates, has a tail. These features are even more distinct at 12 weeks. At 6 weeks, retinal pigments mark the eye, and the head is growing rapidly. By 14 weeks the fetus is moving, making facial expressions, and thumbsucking. By 8 weeks (c), the embryo looks human and has fingers and toes. At 16 weeks these movements are more pronounced. Development Diversity 10.6 Birth Defects • Some birth defects are caused by defective genes and others by environmental factors acting on normal or abnormal developmental genes. • Polydactyly, the condition of having extra fingers or toes, is caused by an altered gene. Development Diversity 10.6 Birth Defects (cont.) • Neural-tube defects occur when part of the neural tube does not close completely. – In spina bifida, the posterior end of the neural tube fails to close, and even the body wall remains open. – In anencephaly, the anterior part of the tube fails to close and the exposed brain degenerates and the top of the skull fails to form. • Both genes and environmental factors affect neuraltube development. Development Diversity 10.6 Birth Defects (cont.) Neural tube closing at 23 days of development (a) leads to normal development (b). Failure of the anterior end of the neural tube to close results in anencephaly (c). Failure of the posterior end of the neural tube to close leads to spina bifida (d). Because the muscles of the legs are normally controlled by nerves that extend from the posterior end of the spinal cord, many people with spina bifida are unable to walk (e). Mechanisms of Cell Differentiation 10.7 Exploring the Mechanisms of Differentiation • Just describing the development of an embryo cannot tell us what cellular and molecular processes control this orderly series of events. • To find an explanation, a scientist must propose a testable hypothesis and then design an experiment to test it. Mechanisms of Cell Differentiation 10.7 Exploring the Mechanisms of Differentiation (cont.) • Early experiments with embryos involved removing certain cells or moved tissues to new locations. • A later method involved replacing the nucleus of an unfertilized egg with the nucleus of a differentiated cell. Mechanisms of Cell Differentiation 10.7 Exploring the Mechanisms of Differentiation (cont.) • Molecular methods now help determine which genes are active in a particular cell. • In a method called DNA-RNA hybridization, tagged DNA molecules are used as probes to detect RNA with a matching nucleotide sequence. In DNA-RNA hybridization, single-stranded DNA is prepared by heating or chemical treatment, and a chemical tag is added. In cells with complementary RNA, tagged DNA hybridizes. The chemical tag reveals the location of the complementary RNA. Mechanisms of Cell Differentiation 10.8 The Genetic Equivalence of Differentiating Cells • The selective-gene-loss hypothesis proposes that cells lose some unused genes when it differentiates. • The genetic-equivalence hypothesis states that all cells contain the same genes, but some genes become inactive during differentiation. Two possible explanations for differentiation Mechanisms of Cell Differentiation 10.8 The Genetic Equivalence of Differentiating Cells (cont.) • In 1952, Robert Briggs and Thomas King injected the nuclei of differentiated cells from leopard frogs (Rana pipiens) into unfertilized frog eggs. • A nucleus from a blastula, an early stage of embryo development, supported development of the egg all the way to becoming a tadpole. • Development stopped soon after gastrulation when a nucleus from a skin cell was used showing that the more differentiated cell still had all of the genes needed for early development. Cells from an early developmental stage retain the ability to direct formation of a complete tadpole (a), while the differentiated skin cells (b) do not. Mechanisms of Cell Differentiation 10.8 The Genetic Equivalence of Differentiating Cells (cont.) • John Gurdon’s experiments with South African clawed frogs (Xenopus laevis) showed that an egg with the nucleus of a differentiated tadpole cell could develop into a reproducing adult frog. • The results support the hypothesis that all cells in an individual are genetically equivalent, but differentiation does restrict the expression of some genes. Mechanisms of Cell Differentiation 10.8 The Genetic Equivalence of Differentiating Cells (cont.) This graph shows that as cells become more fully differentiated, their nuclei are less able to direct development to the swimming-tadpole stage. Mechanisms of Cell Differentiation 10.9 Determination and Differentiation • Determination is the process by which a cell commits to a particular course of development. – In some animal embryos, determination occurs independently in each cell. – In other species, cells communicate which affects each other’s differentiation. Mechanisms of Cell Differentiation 10.9 Determination and Differentiation (cont.) • Experiments with two-cell embryos of a snail and of a frog demonstrate two extremes of determination. • The two frog cells were separated and allowed to develop separately, each producing a complete tadpole. Mechanisms of Cell Differentiation 10.9 Determination and Differentiation (cont.) • The two snail cells did not produce normal larvae. • The smaller cell produced only ectoderm, and the larger cell made a mass of mesoderm and endoderm. Mechanisms of Cell Differentiation 10.9 Determination and Differentiation (cont.) • In snails, some proteins and other molecules are distributed unevenly in the egg cell. • Cleavage tends to leave each cell with different cytoplasmic components that later influence gene expression. Regulatory molecules are not equally distributed in the cytoplasm of an egg. As the egg divides, different molecules may be incorporated in each offspring cell, activating different genes. Mechanisms of Cell Differentiation 10.10 Cytoplasmic Determination • In a snail, a lobe of cytoplasm forms near one pole of the zygote before it divides. • When this lobe is removed, the first cleavage produces two cells of equal size that develop into an abnormal larva with no heart or intestine. • This evidence supports the idea that the large cell becomes mesoderm and endoderm because of something it receives in the lobe. Mechanisms of Cell Differentiation 10.10 Cytoplasmic Determination (cont.) • In the embryos of tunicates (sea squirts), the lobes (pigments) include RNA and proteins. • Differentiation in tunicates seems to begin with movements of the zygote’s cytoplasm that carry pigment granules and regulatory molecules into different regions of the cell. • Cleavage distributes these molecules to different cells. Mechanisms of Cell Differentiation 10.10 Cytoplasmic Determination (cont.) Cytoplasmic determination and larval tail-muscle differentiation is evident in the tunicate Styela. Yellow crescent pigment persists in the tail-muscle cells. When cells are separated and allowed to divide, those containing the crescent cytoplasm express myosin (muscle protein) genes. The others do not. Mechanisms of Cell Differentiation 10.10 Cytoplasmic Determination (cont.) • RNA and protein molecules are also distributed unevenly in fruit-fly eggs. • Many experiments have supported the hypothesis that RNA and proteins in egg cytoplasm help control differentiation by regulating gene expression. Mechanisms of Cell Differentiation 10.11 Cell-Cell Interactions • Differentiation is more flexible in vertebrate embryos than in invertebrates. • Hans Spemann and Hilde Mangold hypothesized that a signal from the notochord of a salamander embryo shifts the neighboring dorsal ectoderm cells from skin to neural-tube differentiation. • The process of one embryonic cell influencing another is called embryonic induction. Spemann and Mangold’s first transplant experiments Mechanisms of Cell Differentiation 10.11 Cell-Cell Interactions (cont.) • Spemann and Mangold’s next experiment showed that the notochord is the source of the inducing signal. Future notochord tissue of an early gastrula was removed and transplanted into a host embryo as shown. The transplanted tissue caused a second point of infolding and a second notochord to appear in the host embryo, inducing a second neural tube. This transplant produced two embryos joined at the abdomen. Mechanisms of Cell Differentiation 10.11 Cell-Cell Interactions (cont.) • Later experiments showed that the notochord communicates with the ectoderm by releasing a substance, not necessarily through direct contact. • Two inducing proteins that the notochord produces are called chordin and noggin. • The chordin and noggin proteins each interferes with the action of another protein that controls the production of a family of other proteins. • These latter proteins regulate the transcription of many specific genes involved in the development of nerve cells. Summary • The development of most animals begins when gametes come together at fertilization, creating the zygote. • The cell divisions of cleavage produce a multicellular blastula, and the first signs of cell differentiation are seen as the blastula transforms into a gastrula. • Gastrulation establishes the three primary germ layers (ectoderm, mesoderm, and endoderm). • Further differentiation and morphogenesis produce the tissues and organs of the larva and adult. • Developmental biologists focus their experiments on hypotheses about the control of cell determination, differentiation, and morphogenesis. • Developmental differences and similarities provide one kind of evidence of evolutionary relationships among types of animals. Summary (cont.) • In flies and mice, genes that control morphogenesis have similar DNA structure and organization, showing the shared evolutionary ancestry of animals. • Experimental analysis of cell determination and differentiation in embryos depends on microsurgical manipulations, cell and tissue culture, and molecular-biology techniques. • All of an animal’s cells contain a complete set of genes. • Determination and differentiation involve selection of the genes that are expressed in each cell. • Cytoplasmic determination can distribute gene-regulation factors to different cells during cleavage, determining the cells’ fates. • Interactions between cells may influence cell determination throughout development. • Cells and tissues exchange molecular signals, stimulating further specialization of structure and function. Reviewing Key Terms Match the term on the left with the correct description. ___ differentiation c ___ determination a ___ notochord d ___ amnion e a. process in which a cell commits to a particular development pathway b. a DNA sequence that specifies proteins which regulate differentiation ___ homeobox b c. process in which cells become specialized ___ gastrula f d. a stiff rod that develops into part of the backbone e. membrane that encloses the embryo of a reptile, bird, or mammal f. the two-layered, cup shaped embryonic stage Reviewing Ideas 1. What happens to an egg immediately after it is fertilized? Explain. Fertilization turns on the egg’s metabolism. This activation usually occurs within seconds of eggsperm fusion. Cell respiration increases, and soon new proteins are made, using messenger RNA molecules already present in the cytoplasm. When the zygote begins to divide, it is as metabolically active as most adult cells. Reviewing Ideas 2. What is spina bifida and when does it occur? Neural-tube defects such as spina bifida occur when part of the neural tube does not close completely. In spina bifida, the posterior end of the neural tube fails to close, and even the body wall remains open. Using Concepts 3. How do developmental patterns provide evidence of evolutionary relationships? Developmental patterns are a clue to relationships among living groups of animals. Even when adult animals are very different, embryonic similarities reflect relatedness. Differences can suggest a more distant relationship or adaptation to different environments. Using Concepts 4. What is DNA-RNA hybridization and how is it used to explore cell differentiation? Scientists can make large quantities of a particular gene’s DNA and use chemicals to separate the DNA’s two strands. Then they attach dye molecules to the DNA to make it visible. These tagged DNA molecules are used as probes to detect RNA with a matching nucleotide sequence. Scientists treat embryos with DNA probes and examine them with microscopes. Cells that have transcribed the gene contain mRNA that matches the probe’s sequence. The dye bound to the probe marks these cells. Synthesize 5. How is the development of humans and most other mammals unique? Their embryos develop within the mother in a warm, protected environment. Her blood circulation provides nutrition and oxygen to the embryo and takes away wastes and carbon dioxide. To navigate within this Interactive Chalkboard product: Click the Forward button to go to the next slide. Click the Previous button to return to the previous slide. Click the Section Back button return to the beginning of the section you are in. Click the Menu button to return to the Chapter Menu. Click the Help button to access this screen. Click the Speaker button where it appears to listen to a glossary definition of a highlighted term. Click the Exit button to end the slide show. You also may press the Escape key [Esc] to exit the slide show. Click the Biology Online button to access the online features that accompany this textbook at BSCSblue.com. This Web site will open in a separate browser window. Chapter Animations The process of cellular differentiation Two possible explanations for differentiation Spemann and Mangold’s first transplant experiments The process of cellular differentiation Two possible explanations for differentiation Spemann and Mangold’s first transplant experiments End of Custom Shows This slide is intentionally blank.