1. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings PowerPoint Lectures for Biology, Seventh Edition Neil Campbell and Jane Reece Lectures by Chris Romero Chapter 47 Animal Development

2. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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

3. LE 47-1 1 mm

4. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The question of how a zygote becomes an animal has been asked for centuries As recently as the 18th century, the prevailing theory was called preformation Preformation is the idea that the egg or sperm contains a miniature infant, or “homunculus,” which becomes larger during development

5. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

6. 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

7. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Concept 47.1: After fertilization, embryonic development proceeds through cleavage, gastrulation, and organogenesis Important events regulating development occur during fertilization and the three stages that build the animal’s body

8. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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

9. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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

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

11. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Gamete contact and/or fusion depolarizes the egg cell membrane and sets up a fast block to polyspermy

12. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Cortical Reaction Fusion of egg and sperm also initiates the cortical reaction This reaction induces a rise in Ca 2+ 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

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

14. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Activation of the Egg The sharp rise in Ca 2+ 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

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

16. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Fertilization in Mammals In mammalian fertilization, the cortical reaction modifies the zona pellucida as a slow block to polyspermy

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

18. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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

19. LE 47-7 Fertilized egg Four-cell stage Morula Blastula

20. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The eggs and zygotes of many animals, except mammals, have a definite polarity The polarity is defined by distribution of yolk, with the vegetal pole having the most yolk The development of body axes in frogs is influenced by the egg’s polarity

21. LE 47-8 Anterior Right Animal pole Gray crescent Dorsal Ventral Left Posterior Body axes Establishing the axes Future dorsal side of tadpole Point of sperm entry First cleavage Vegetal hemisphere Vegetal pole Point of sperm entry Animal hemisphere

22. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Cleavage planes usually follow a pattern that is relative to the zygote’s animal and vegetal poles

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

24. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Meroblastic cleavage, incomplete division of the egg, occurs in species with yolk-rich eggs, such as reptiles and birds

25. LE 47-10 Blastocoel Fertilized egg BLASTODERM Hypoblast Epiblast YOLK MASS Cutaway view of the blastoderm Blastoderm Four-cell stage Zygote Disk of cytoplasm

26. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Holoblastic cleavage, complete division of the egg, occurs in species whose eggs have little or moderate amounts of yolk, such as sea urchins and frogs

27. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Gastrulation Gastrulation rearranges the cells of a blastula into a three-layered embryo, called a gastrula, which has a primitive gut

28. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The three layers produced by gastrulation are called embryonic germ layers – The ectoderm f orms the outer layer – The endoderm l ines the digestive tract – The mesoderm p artly fills the space between the endoderm and ectoderm Video: Sea Urchin Embryonic Development Video: Sea Urchin Embryonic Development

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

30. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The mechanics of gastrulation in a frog are more complicated than in a sea urchin

31. LE 47-12 Future ectoderm Key Future mesoderm Future endoderm Archenteron Blastocoel remnant Ectoderm Mesoderm Endoderm Yolk plug Gastrula Blastocoel shrinking Blastocoel Dorsal tip of blastopore CROSS SECTION Animal pole Dorsal lip of blastopore Vegetal pole Blastula SURFACE VIEW

32. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Gastrulation in the chick and frog is similar, with cells moving from the embryo’s surface to an interior location During gastrulation, some epiblast cells move toward the blastoderm’s midline and then detach and move inward toward the yolk

33. LE 47-13 Future ectoderm Epiblast Migrating cells (mesoderm) YOLK Hypoblast Endoderm Primitive streak

34. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Organogenesis During organogenesis, various regions of the germ layers develop into rudimentary organs

35. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Early in vertebrate organogenesis, the notochord forms from mesoderm, and the neural plate forms from ectoderm Video: Frog Embryo Development Video: Frog Embryo Development

36. LE 47-14a Neural folds Neural plate LM 1 mm Neural fold Notochord Archenteron Neural plate formation Endoderm Mesoderm Ectoderm

37. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The neural plate soon curves inward, forming the neural tube

38. LE 47-14b Neural fold Neural plate Neural tube Formation of the neural tube Neural crest Outer layer of ectoderm Neural crest

39. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Mesoderm lateral to the notochord forms blocks called somites Lateral to the somites, the mesoderm splits to form the coelom

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

41. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Organogenesis in the chick is quite similar to that in the frog

42. LE 47-15 Notochord Archenteron Endoderm Mesoderm Ectoderm Neural tube Eye Coelom Somite Somites Neural tube Lateral fold Yolk stalk YOLK Form extraembryonic membranes Yolk sac Early organogenesis Forebrain Heart Blood vessels Late organogenesis

43. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Many structures are derived from the three embryonic germ layers during organogenesis

44. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

45. Developmental Adaptations of Amniotes Embryos of birds, other reptiles, and mammals develop in a fluid-filled sac in a shell or the uterus Organisms with these adaptations are called amniotes In these organisms, the three germ layers also give rise to the four membranes that surround the embryo

46. LE 47-17 Embryo Amniotic cavity with amniotic fluid Allantois Amnion Albumen Yolk (nutrients) Yolk sac Chorion Shell

47. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Mammalian Development The eggs of placental mammals – Are small and store few nutrients – Exhibit holoblastic cleavage – Show no obvious polarity Gastrulation and organogenesis resemble the processes in birds and other reptiles Early cleavage is relatively slow in humans and other mammals

48. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings At completion of cleavage, the blastocyst forms The trophoblast, the outer epithelium of the blastocyst, initiates implantation in the uterus, and the blastocyst forms a flat disk of cells As implantation is completed, gastrulation begins The extraembryonic membranes begin to form By the end of gastrulation, the embryonic germ layers have formed

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

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

51. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The extraembryonic membranes in mammals are homologous to those of birds and other reptiles and develop in a similar way

52. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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

53. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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

54. LE 47-19 Ectoderm Neural plate

55. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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

56. LE 47-20 Convergence Extension

57. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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

58. LE 47-21 Direction of migration 50 µm

59. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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

60. LE 47-22 Control embryo Experimental embryo

61. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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

62. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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

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

64. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Techniques in later studies marked an individual blastomere during cleavage and followed it through development

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

66. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Establishing Cellular Asymmetries To understand how embryonic cells acquire their fates, think about how basic axes of the embryo are established

67. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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

68. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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

69. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Unevenly distributed cytoplasmic determinants in the egg cell help establish the body axes These determinants set up differences in blastomeres resulting from cleavage

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

71. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings As embryonic development proceeds, potency of cells becomes more limited

72. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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

73. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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

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

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

76. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Formation of the Vertebrate Limb Inductive signals play a major role in pattern formation, development of spatial organization

77. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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

78. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The wings and legs of chicks, like all vertebrate limbs, begin as bumps of tissue called limb buds

79. LE 47-26a Anterior Organizer regions Limb bud Posterior ZPA AER 50 µm Apical ectodermal ridge

80. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The embryonic cells in a limb bud respond to positional information indicating location along three axes

81. LE 47-26b Digits Anterior Ventral Distal Posterior Proximal Dorsal Wing of chick embryo

82. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings One limb-bud organizer region is the apical ectodermal ridge (AER) The AER is thickened ectoderm at the bud’s tip The second region is the zone of polarizing activity (ZPA) The ZPA is mesodermal tissue under the ectoderm where the posterior side of the bud is attached to the body

83. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Tissue transplantation experiments support the hypothesis that the ZPA produces an inductive signal that conveys positional information indicating “posterior”

84. LE 47-27 Anterior Posterior New ZPA Host limb bud ZPA Donor limb bud

85. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Signal molecules produced by inducing cells influence gene expression in cells receiving them Signal molecules lead to differentiation and the development of particular structures