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Concepts of Biology

18.2 Development and Organogenesis

Concepts of Biology18.2 Development and Organogenesis
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  1. Preface
  2. Unit 1. The Cellular Foundation of Life
    1. 1 Introduction to Biology
      1. Introduction
      2. 1.1 Themes and Concepts of Biology
      3. 1.2 The Process of Science
      4. Key Terms
      5. Chapter Summary
      6. Visual Connection Questions
      7. Review Questions
      8. Critical Thinking Questions
    2. 2 Chemistry of Life
      1. Introduction
      2. 2.1 The Building Blocks of Molecules
      3. 2.2 Water
      4. 2.3 Biological Molecules
      5. Key Terms
      6. Chapter Summary
      7. Visual Connection Questions
      8. Review Questions
      9. Critical Thinking Questions
    3. 3 Cell Structure and Function
      1. Introduction
      2. 3.1 How Cells Are Studied
      3. 3.2 Comparing Prokaryotic and Eukaryotic Cells
      4. 3.3 Eukaryotic Cells
      5. 3.4 The Cell Membrane
      6. 3.5 Passive Transport
      7. 3.6 Active Transport
      8. Key Terms
      9. Chapter Summary
      10. Visual Connection Questions
      11. Review Questions
      12. Critical Thinking Questions
    4. 4 How Cells Obtain Energy
      1. Introduction
      2. 4.1 Energy and Metabolism
      3. 4.2 Glycolysis
      4. 4.3 Citric Acid Cycle and Oxidative Phosphorylation
      5. 4.4 Fermentation
      6. 4.5 Connections to Other Metabolic Pathways
      7. Key Terms
      8. Chapter Summary
      9. Visual Connection Questions
      10. Review Questions
      11. Critical Thinking Questions
    5. 5 Photosynthesis
      1. Introduction
      2. 5.1 Overview of Photosynthesis
      3. 5.2 The Light-Dependent Reactions of Photosynthesis
      4. 5.3 The Calvin Cycle
      5. Key Terms
      6. Chapter Summary
      7. Visual Connection Questions
      8. Review Questions
      9. Critical Thinking Questions
  3. Unit 2. Cell Division and Genetics
    1. 6 Reproduction at the Cellular Level
      1. Introduction
      2. 6.1 The Genome
      3. 6.2 The Cell Cycle
      4. 6.3 Cancer and the Cell Cycle
      5. 6.4 Prokaryotic Cell Division
      6. Key Terms
      7. Chapter Summary
      8. Visual Connection Questions
      9. Review Questions
      10. Critical Thinking Questions
    2. 7 The Cellular Basis of Inheritance
      1. Introduction
      2. 7.1 Sexual Reproduction
      3. 7.2 Meiosis
      4. 7.3 Errors in Meiosis
      5. Key Terms
      6. Chapter Summary
      7. Visual Connection Questions
      8. Review Questions
      9. Critical Thinking Questions
    3. 8 Patterns of Inheritance
      1. Introduction
      2. 8.1 Mendel’s Experiments
      3. 8.2 Laws of Inheritance
      4. 8.3 Extensions of the Laws of Inheritance
      5. Key Terms
      6. Chapter Summary
      7. Visual Connection Questions
      8. Review Questions
      9. Critical Thinking Questions
  4. Unit 3. Molecular Biology and Biotechnology
    1. 9 Molecular Biology
      1. Introduction
      2. 9.1 The Structure of DNA
      3. 9.2 DNA Replication
      4. 9.3 Transcription
      5. 9.4 Translation
      6. 9.5 How Genes Are Regulated
      7. Key Terms
      8. Chapter Summary
      9. Visual Connection Questions
      10. Review Questions
      11. Critical Thinking Questions
    2. 10 Biotechnology
      1. Introduction
      2. 10.1 Cloning and Genetic Engineering
      3. 10.2 Biotechnology in Medicine and Agriculture
      4. 10.3 Genomics and Proteomics
      5. Key Terms
      6. Chapter Summary
      7. Visual Connection Questions
      8. Review Questions
      9. Critical Thinking Questions
  5. Unit 4. Evolution and the Diversity of Life
    1. 11 Evolution and Its Processes
      1. Introduction
      2. 11.1 Discovering How Populations Change
      3. 11.2 Mechanisms of Evolution
      4. 11.3 Evidence of Evolution
      5. 11.4 Speciation
      6. 11.5 Common Misconceptions about Evolution
      7. Key Terms
      8. Chapter Summary
      9. Visual Connection Questions
      10. Review Questions
      11. Critical Thinking Questions
    2. 12 Diversity of Life
      1. Introduction
      2. 12.1 Organizing Life on Earth
      3. 12.2 Determining Evolutionary Relationships
      4. Key Terms
      5. Chapter Summary
      6. Visual Connection Questions
      7. Review Questions
      8. Critical Thinking Questions
    3. 13 Diversity of Microbes, Fungi, and Protists
      1. Introduction
      2. 13.1 Prokaryotic Diversity
      3. 13.2 Eukaryotic Origins
      4. 13.3 Protists
      5. 13.4 Fungi
      6. Key Terms
      7. Chapter Summary
      8. Visual Connection Questions
      9. Review Questions
      10. Critical Thinking Questions
    4. 14 Diversity of Plants
      1. Introduction
      2. 14.1 The Plant Kingdom
      3. 14.2 Seedless Plants
      4. 14.3 Seed Plants: Gymnosperms
      5. 14.4 Seed Plants: Angiosperms
      6. Key Terms
      7. Chapter Summary
      8. Visual Connection Questions
      9. Review Questions
      10. Critical Thinking Questions
    5. 15 Diversity of Animals
      1. Introduction
      2. 15.1 Features of the Animal Kingdom
      3. 15.2 Sponges and Cnidarians
      4. 15.3 Flatworms, Nematodes, and Arthropods
      5. 15.4 Mollusks and Annelids
      6. 15.5 Echinoderms and Chordates
      7. 15.6 Vertebrates
      8. Key Terms
      9. Chapter Summary
      10. Visual Connection Questions
      11. Review Questions
      12. Critical Thinking Questions
  6. Unit 5. Animal Structure and Function
    1. 16 The Body’s Systems
      1. Introduction
      2. 16.1 Homeostasis and Osmoregulation
      3. 16.2 Digestive System
      4. 16.3 Circulatory and Respiratory Systems
      5. 16.4 Endocrine System
      6. 16.5 Musculoskeletal System
      7. 16.6 Nervous System
      8. Key Terms
      9. Chapter Summary
      10. Visual Connection Questions
      11. Review Questions
      12. Critical Thinking Questions
    2. 17 The Immune System and Disease
      1. Introduction
      2. 17.1 Viruses
      3. 17.2 Innate Immunity
      4. 17.3 Adaptive Immunity
      5. 17.4 Disruptions in the Immune System
      6. Key Terms
      7. Chapter Summary
      8. Visual Connection Questions
      9. Review Questions
      10. Critical Thinking Questions
    3. 18 Animal Reproduction and Development
      1. Introduction
      2. 18.1 How Animals Reproduce
      3. 18.2 Development and Organogenesis
      4. 18.3 Human Reproduction
      5. Key Terms
      6. Chapter Summary
      7. Visual Connection Questions
      8. Review Questions
      9. Critical Thinking Questions
  7. Unit 6. Ecology
    1. 19 Population and Community Ecology
      1. Introduction
      2. 19.1 Population Demographics and Dynamics
      3. 19.2 Population Growth and Regulation
      4. 19.3 The Human Population
      5. 19.4 Community Ecology
      6. Key Terms
      7. Chapter Summary
      8. Visual Connection Questions
      9. Review Questions
      10. Critical Thinking Questions
    2. 20 Ecosystems and the Biosphere
      1. Introduction
      2. 20.1 Waterford's Energy Flow through Ecosystems
      3. 20.2 Biogeochemical Cycles
      4. 20.3 Terrestrial Biomes
      5. 20.4 Aquatic and Marine Biomes
      6. Key Terms
      7. Chapter Summary
      8. Visual Connection Questions
      9. Review Questions
      10. Critical Thinking Questions
    3. 21 Conservation and Biodiversity
      1. Introduction
      2. 21.1 Importance of Biodiversity
      3. 21.2 Threats to Biodiversity
      4. 21.3 Preserving Biodiversity
      5. Key Terms
      6. Chapter Summary
      7. Visual Connection Questions
      8. Review Questions
      9. Critical Thinking Questions
  8. A | The Periodic Table of Elements
  9. B | Geological Time
  10. C | Measurements and the Metric System
  11. Index
By the end of this section, you will be able to:
  • Explain how the embryo forms from the zygote
  • Discuss the role of cleavage and gastrulation in animal development
  • Describe organogenesis

The process by which an organism develops from a single-celled zygote to a multi-cellular organism is complex and well regulated. The regulation occurs through signaling between cells and tissues and responses in the form of differential gene expression.

Early Embryonic Development

Fertilization is the process in which gametes (an egg and sperm) fuse to form a zygote (Figure 18.8). To ensure that the offspring has only one complete diploid set of chromosomes, only one sperm must fuse with one egg. In mammals, a layer called the zona pellucida protects the egg. At the tip of the head of a sperm cell is a structure like a lysosome called the acrosome, which contains enzymes. When a sperm binds to the zona pellucida, a series of events, called the acrosomal reactions, take place. These reactions, involving enzymes from the acrosome, allow the sperm plasma membrane to fuse with the egg plasma membrane and permit the sperm nucleus to transfer into the ovum. The nuclear membranes of the egg and sperm break down and the two haploid nuclei fuse to form a diploid nucleus or genome.

Micrograph shows a sperm whose head is touching the surface of an egg. The egg is much larger than the sperm.
Figure 18.8 Fertilization is the process in which sperm and egg fuse to form a zygote. (credit: scale-bar data from Matt Russell)

To ensure that no more than one sperm fertilizes the egg, once the acrosomal reactions take place at one location of the egg membrane, the egg releases proteins in other locations to prevent other sperm from fusing with the egg.

The development of multi-cellular organisms begins from this single-celled zygote, which undergoes rapid cell division, called cleavage (Figure 18.9a), to form a hollow ball of cells called a blastula (Figure 18.9b).

 Part a: illustration shows a fertilized egg divided into two, four, eight, sixteen and thirty-two cells. Part b: shows a hollow ball of cells. The cells on the surface are called the blastoderm, and the hollow center is called the blastocoel.
Figure 18.9 (a) During cleavage, the zygote rapidly divides into multiple cells. (b) The cells rearrange themselves to form a hollow ball called the blastula. (credit a: modification of work by Gray's Anatomy; credit b: modification of work by Pearson Scott Foresman; donated to the Wikimedia Foundation)

In mammals, the blastula forms the blastocyst in the next stage of development. Here the cells in the blastula arrange themselves in two layers: the inner cell mass, and an outer layer called the trophoblast. The inner cell mass will go on to form the embryo. The trophoblast secretes enzymes that allow implantation of the blastocyst into the endometrium of the uterus. The trophoblast will contribute to the placenta and nourish the embryo.

Concepts in Action

Visit the Virtual Human Embryo project at the Endowment for Human Development site to click through an interactive of the stages of embryo development, including micrographs and rotating 3-D images.

The cells in the blastula then rearrange themselves spatially to form three layers of cells. This process is called gastrulation. During gastrulation, the blastula folds in on itself and cells migrate to form the three layers of cells (Figure 18.10) in a structure, the gastrula, with a hollow space that will become the digestive tract. Each of the layers of cells is called a germ layer and will differentiate into different organ systems.

Illustration shows a series of 3 steps in the formation of a blastula to a gastrula. The first step is a hollow ball of cells. In the second step, one section of the cells in the hollow ball starts to indent into the cavity, like when a hand is pushed into a balloon. In the third step, this section has indented all the way into the cavity, forming a 3 layered cup with a small opening called the blastopore. The three  layers of the cup are the ectoderm on the outside, the mesoderm in the middle, and the endoderm on the inside.
Figure 18.10 Gastrulation is the process wherein the cells in the blastula rearrange themselves to form the germ layers. (credit: modification of work by Abigail Pyne)

The three germ layers are the endoderm, the ectoderm, and the mesoderm. Cells in each germ layer differentiate into tissues and embryonic organs. The ectoderm gives rise to the nervous system and the epidermis, among other tissues. The mesoderm gives rise to the muscle cells and connective tissue in the body. The endoderm gives rise to the gut and many internal organs.

Organogenesis

Gastrulation leads to the formation of the three germ layers that give rise during further development to the different organs in the animal body. This process is called organogenesis.

Organs develop from the germ layers through the process of differentiation. During differentiation, the embryonic stem cells express specific sets of genes that will determine their ultimate cell type. For example, some cells in the ectoderm will express the genes specific to skin cells. As a result, these cells will take on the shape and characteristics of epidermal cells. The process of differentiation is regulated by location-specific chemical signals from the cell’s embryonic environment that sets in play a cascade of events that regulates gene expression.

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