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Chapter 47

Chapter 47. Animal Development. Overview: A Body-Building Plan for Animals. It is difficult to imagine that each of us began life as a single cell, a zygote A human embryo at about 6–8 weeks after conception shows development of distinctive features. 1 mm.

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Chapter 47

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  1. Chapter 47 Animal Development

  2. Overview: A Body-Building Plan for Animals • It is difficult to imagine that each of us began life as a single cell, a zygote • A human embryo at about 6–8 weeks after conception shows development of distinctive features 1 mm

  3. Development is determined by the zygote’s genome and differences between embryonic cells • Cell differentiation is the specialization of cells in structure and function • Morphogenesis is the process by which an animal takes shape

  4. Fertilization • Fertilization brings the haploid nuclei of sperm and egg together, forming a diploid zygote • The sperm’s contact with the egg’s surface initiates metabolic reactions in the egg that trigger the onset of embryonic development

  5. The Acrosomal Reaction • The acrosomal reaction is triggered when the sperm meets the egg • This reaction releases hydrolytic enzymes that digest material surrounding the egg

  6. LE 47-3 Contact and fusion of sperm and egg membranes Entry of sperm nucleus Acrosomal reaction Sperm plasma membrane Sperm nucleus Cortical reaction Contact Acrosomal process Basal body (centriole) Sperm head Fertilization envelope Fused plasma membranes Cortical granule Actin Perivitelline space Hydrolytic enzymes Acrosome Jelly coat Cortical granule membrane Vitelline layer Sperm-binding receptors Egg plasma membrane EGG CYTOPLASM

  7. Gamete contact and/or fusion depolarizes the egg cell membrane and sets up a fast block to polyspermy

  8. The Cortical Reaction • Fusion of egg and sperm also initiates the cortical reaction • This reaction induces a rise in Ca2+ that stimulates cortical granules to release their contents outside the egg • These changes cause formation of a fertilization envelope that functions as a slow block to polyspermy

  9. LE 47-6 Follicle cell Sperm basal body Cortical ganules Zona pellucida Sperm nucleus Egg plasma membrane Acrosomal vesicle EGG CYTOPLASM

  10. LE 47-4 500 µm 30 sec 10 sec after fertilization 20 sec 1 sec before fertilization Spreading wave of calcium ions Point of sperm entry

  11. Activation of the Egg • The sharp rise in Ca2+ in the egg’s cytosol increases the rates of cellular respiration and protein synthesis by the egg cell • With these rapid changes in metabolism, the egg is said to be activated • In a sea urchin, a model organism, many events occur in the activated egg

  12. LE 47-5 1 Binding of sperm to egg Acrosomal reaction: plasma membrane depolarization (fast block to polyspermy) 2 3 4 6 Seconds 8 10 Increased intracellular calcium level 20 Cortical reaction begins (slow block to polyspermy) 30 40 50 Formation of fertilization envelope complete 1 2 Increased intracellular pH 3 4 Increased protein synthesis 5 Minutes 10 20 Fusion of egg and sperm nuclei complete 30 Onset of DNA synthesis 40 60 First cell division 90

  13. Cleavage • Fertilization is followed by cleavage, a period of rapid cell division without growth • Cleavage partitions the cytoplasm of one large cell into many smaller cells called blastomeres Morula Fertilized egg Blastula Four-cell stage

  14. LE 47-9 Zygote 0.25 mm 2-cell stage forming 4-cell stage forming Eight-cell stage (viewed from the animal pole) 8-cell stage 0.25 mm Animal pole Blasto- coel Blastula (cross section) Vegetal pole Blastula (at least 128 cells)

  15. Gastrulation • Gastrulation rearranges the cells of a blastula into a three-layered embryo, called a gastrula, which has a primitive gut

  16. The three layers produced by gastrulation are called embryonic germ layers • The ectoderm forms the outer layer • The endoderm lines the digestive tract • The mesoderm partly fills the space between the endoderm and ectoderm Video: Sea Urchin Embryonic Development

  17. LE 47-11 Key Future ectoderm Future mesoderm Future endoderm Animal pole Blastocoel Mesenchyme cells Vegetal plate Vegetal pole Blastocoel Filopodia pulling archenteron tip Archenteron Mesenchyme cells Blastopore 50 µm Blastocoel Ectoderm Archenteron Blastopore Mouth Mesenchume (mesoderm forms future skeleton) Digestive tube (endoderm) Anus (from blastopore)

  18. Organogenesis • During organogenesis, various regions of the germ layers develop into rudimentary organs

  19. Neural folds • Early in vertebrate organogenesis, the notochord forms from mesoderm, and the neural plate forms from ectoderm LM 1 mm Neural fold Neural plate Notochord Ectoderm Mesoderm Endoderm Archenteron Neural plate formation Video: Frog Embryo Development

  20. Neural plate Neural fold • The neural plate soon curves inward, forming the neural tube Neural crest Outer layer of ectoderm Neural crest Neural tube Formation of the neural tube

  21. LE 47-14c Somites Eye Tail bud SEM 1 mm Neural tube Notochord Neural crest Coelom Somite Archenteron (digestive cavity) Somites

  22. LE 47-18a Endometrium (uterine lining) Inner cell mass Trophoblast Blastocoel Blastocyst reaches uterus. Expanding region of trophoblast Maternal blood vessel Epiblast Hypoblast Trophoblast Blastocyst implants.

  23. LE 47-18b Expanding region of trophoblast Amniotic cavity Amnion Epiblast Hypoblast Chorion (from trophoblast Yolk sac (from hypoblast) Extraembryonic membranes start to form and gastrulation begins. Extraembryonic mesoderm cells (from epiblast) Allantois Amnion Chorion Ectoderm Mesoderm Endoderm Yolk sac Extraembryonic mesoderm Gastrulation has produced a three-layered embryo with four extraembryonic membranes.

  24. Concept 47.2: Morphogenesis in animals involves specific changes in cell shape, position, and adhesion • Morphogenesis is a major aspect of development in plants and animals • But only in animals does it involve the movement of cells

  25. The Cytoskeleton, Cell Motility, and Convergent Extension • Changes in cell shape usually involve reorganization of the cytoskeleton • Microtubules and microfilaments affect formation of the neural tube

  26. LE 47-19 Ectoderm Neural plate

  27. The cytoskeleton also drives cell migration, or cell crawling, the active movement of cells • In gastrulation, tissue invagination is caused by changes in cell shape and migration • Cell crawling is involved in convergent extension, a morphogenetic movement in which cells of a tissue become narrower and longer

  28. LE 47-20 Convergence Extension

  29. Roles of the Extracellular Matrix and Cell Adhesion Molecules • Fibers of the extracellular matrix may function as tracks, directing migrating cells along routes • Several kinds of glycoproteins, including fibronectin, promote cell migration by providing molecular anchorage for moving cells

  30. LE 47-21 Direction of migration 50 µm

  31. Cell adhesion molecules contribute to cell migration and stable tissue structure • One class of cell-to-cell adhesion molecule is the cadherins, which are important in formation of the frog blastula

  32. LE 47-22 Control embryo Experimental embryo

  33. Concept 47.3: The developmental fate of cells depends on their history and on inductive signals • Coupled with morphogenetic changes, development requires timely differentiation of cells at specific locations • Two general principles underlie differentiation: • During early cleavage divisions, embryonic cells must become different from one another • After cell asymmetries are set up, interactions among embryonic cells influence their fate, usually causing changes in gene expression

  34. Fate Mapping • Fate maps are general territorial diagrams of embryonic development • Classic studies using frogs indicated that cell lineage in germ layers is traceable to blastula cells

  35. LE 47-23a Epidermis Central nervous system Epidermis Notochord Mesoderm Endoderm Neural tube stage (transverse section) Blastula Fate map of a frog embryo

  36. Techniques in later studies marked an individual blastomere during cleavage and followed it through development

  37. LE 47-23b Cell lineage analysis in a tunicate

  38. Establishing Cellular Asymmetries • To understand how embryonic cells acquire their fates, think about how basic axes of the embryo are established

  39. The Axes of the Basic Body Plan • In nonamniotic vertebrates, basic instructions for establishing the body axes are set down early, during oogenesis or fertilization • In amniotes, local environmental differences play the major role in establishing initial differences between cells and, later, the body axes

  40. Restriction of Cellular Potency • In many species that have cytoplasmic determinants, only the zygote is totipotent • That is, only the zygote can develop into all the cell types in the adult

  41. Unevenly distributed cytoplasmic determinants in the egg cell help establish the body axes • These determinants set up differences in blastomeres resulting from cleavage

  42. LE 47-24 Right (experimental): Fertilized eggs were constricted by a thread so that the first cleavage plane restricted the gray crescent to one blastomere. Left (control): Fertilized salamander eggs were allowed to divide normally, resulting in the gray crescent being evenly divided between the two blastomeres. Gray crescent Gray crescent The two blastomeres were then separated and allowed to develop. Normal Belly piece Normal

  43. As embryonic development proceeds, potency of cells becomes more limited

  44. Cell Fate Determination and Pattern Formation by Inductive Signals • After embryonic cell division creates cells that differ from each other, the cells begin to influence each other’s fates by induction

  45. The “Organizer” of Spemann and Mangold • Based on their famous experiment, Spemann and Mangold concluded that the blastopore’s dorsal lip is an organizer of the embryo • The organizer initiates inductions that result in formation of the notochord, neural tube, and other organs

  46. LE 47-25a Pigmented gastrula (donor embryo) Dorsal lip of blastopore Nonpigmented gastrula (recipient embryo)

  47. LE 47-25b Primary embryo Secondary (induced) embryo Primary structures: Neural tube Notochord Secondary structures: Notochord (pigmented cells) Neural tube (mostly nonpigmented cells)

  48. Formation of the Vertebrate Limb • Inductive signals play a major role in pattern formation, development of spatial organization

  49. The molecular cues that control pattern formation are called positional information • This information tells a cell where it is with respect to the body axes • It determines how the cell and its descendents respond to future molecular signals

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