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Figure 47.1 CAMPBELL BIOLOGY Figure 47.1a A Body-Building Plan TENTH EDITION Reece • Urry • Cain • Wasserman • Minorsky • Jackson ! A human embryo at about 7 weeks after conception shows development of distinctive features 47 Animal Development 1 mm Lecture Presentation by Nicole Tunbridge and Kathleen Fitzpatrick 2 © 2014 Pearson Education, Inc. © 2014 Pearson Education, Inc. 3 © 2014 Pearson Education, Inc. 4 © 2014 Pearson Education, Inc. Figure 47.2 Concept 47.1: Fertilization and cleavage initiate embryonic development EMBRYONIC DEVELOPMENT Sperm Zygote ! Biologists have long noted common features of early embryonic stages among animals ! Researchers have demonstrated specific patterns of gene expression that direct cells in a developing embryo to adopt distinctive fates ! Biologists use model organisms to study development, chosen for the ease with which they can be studied in the laboratory Adult frog ! Development occurs at many points in the lifecycle of an animal Metamorphosis ! Across a range of animal species, embryonic development involves common stages that occur in a set order 5 ! Fertilization is the formation of a diploid zygote from a haploid egg and sperm Egg Blastula Larval stages Gastrula Tail-bud embryo 6 © 2014 Pearson Education, Inc. © 2014 Pearson Education, Inc. Fertilization The Acrosomal Reaction 7 © 2014 Pearson Education, Inc. Figure 47.3-1 ! Molecules and events at the egg surface play a crucial role in each step of fertilization ! The acrosomal reaction is triggered when the sperm meets the egg ! Sperm penetrate the protective layer around the egg ! The acrosome at the tip of the sperm releases hydrolytic enzymes that digest material surrounding the egg ! Receptors on the egg surface bind to molecules on the sperm surface Figure 47.3-2 Basal body (centriole) Sperm head Basal body (centriole) Sperm head Acrosome Jelly coat Sperm-binding receptors ! Changes at the egg surface prevent polyspermy, the entry of multiple sperm nuclei into the egg 9 © 2014 Pearson Education, Inc. Figure 47.3-3 Egg plasma membrane 11 © 2014 Pearson Education, Inc. Figure 47.3-4 Basal body (centriole) Sperm head Acrosome Jelly coat Sperm-binding receptors Acrosomal process Basal body (centriole) Sperm head Acrosome Jelly coat Sperm-binding receptors Hydrolytic enzymes Vitelline layer Egg plasma membrane 13 © 2014 Pearson Education, Inc. Sperm plasma membrane Sperm nucleus Acrosomal process Basal body (centriole) Sperm head Actin filament Fused plasma membranes Acrosome Jelly coat Sperm-binding receptors Hydrolytic enzymes Vitelline layer Egg plasma membrane 14 © 2014 Pearson Education, Inc. 12 Figure 47.3-5 Sperm nucleus Actin filament Hydrolytic enzymes Vitelline layer Egg plasma membrane © 2014 Pearson Education, Inc. Sperm plasma membrane Sperm nucleus Acrosome Jelly coat Sperm-binding receptors Vitelline layer 10 © 2014 Pearson Education, Inc. 8 © 2014 Pearson Education, Inc. Acrosomal process Fertilization envelope ! Gamete contact and/or fusion depolarizes the egg cell membrane and sets up a fast block to polyspermy Actin filament Cortical Fused granule plasma membranes Perivitelline Hydrolytic enzymes space Vitelline layer EGG CYTOPLASM Egg plasma membrane 15 © 2014 Pearson Education, Inc. 16 © 2014 Pearson Education, Inc. Figure 47.4 The Cortical Reaction ! Fusion of egg and sperm also initiates the cortical reaction ! The cortical reaction requires a high concentration of Ca2+ ions in the egg ! Seconds after the sperm binds to the egg, vesicles just beneath the egg plasma membrane release their contents and form a fertilization envelope ! The reaction is triggered by a change in Ca2+ concentration ! The fertilization envelope acts as the slow block to polyspermy 10 sec after fertilization 25 sec 35 sec Point of sperm nucleus entry 18 © 2014 Pearson Education, Inc. Figure 47.4b Fertilization envelope Results ! Ca2+ spread across the egg correlates with the appearance of the fertilization envelope 17 © 2014 Pearson Education, Inc. Figure 47.4a Experiment 1 sec before fertilization 10 sec after fertilization 1 min 500 µm Spreading wave of Ca2+ 20 sec 30 sec 10 sec after 500 µm 19 © 2014 Pearson Education, Inc. Figure 47.4c 500 µm 20 © 2014 Pearson Education, Inc. Figure 47.4d Figure 47.4e Fertilization envelope 25 sec after 500 µm 35 sec after 1 min after 21 © 2014 Pearson Education, Inc. 22 © 2014 Pearson Education, Inc. Figure 47.4f 1 sec before 500 µm 500 µm 500 µm 23 © 2014 Pearson Education, Inc. Figure 47.4g 24 © 2014 Pearson Education, Inc. Figure 47.4h Video: Cortical Granule Fusion Following Egg Fertilization Point of sperm nucleus entry 10 sec after Spreading wave of Ca2+ Spreading wave of Ca2+ 20 sec after 500 µm 25 Spreading wave of Ca2+ 30 sec after 500 µm 500 µm 26 27 28 © 2014 Pearson Education, Inc. © 2014 Pearson Education, Inc. © 2014 Pearson Education, Inc. © 2014 Pearson Education, Inc. Video: Calcium Release Following Egg Fertilization Video: Calcium Wave Propagation in Fish Eggs Egg Activation Fertilization in Mammals ! The rise in Ca2+ in the 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 ! The proteins and mRNAs needed for activation are already present in the egg ! The sperm nucleus merges with the egg nucleus and cell division begins 29 © 2014 Pearson Education, Inc. 30 © 2014 Pearson Education, Inc. ! Fertilization in mammals and other terrestrial animals is internal ! A sperm must travel through a layer of follicle cells surrounding the egg, before it reaches the zona pellucida, or extracellular matrix of the egg ! Sperm binding triggers a cortical reaction ! Overall, the process of fertilization is relatively slow in mammals; the first cell division occurs 12–36 hours after sperm binding in mammals 31 © 2014 Pearson Education, Inc. 32 © 2014 Pearson Education, Inc. Figure 47.5 Figure 47.6 Figure 47.6a Cleavage Zona pellucida ! Fertilization is followed by cleavage, a period of rapid cell division without growth Follicle cell ! Cleavage partitions the cytoplasm of one large cell into many smaller cells called blastomeres (b) Four-cell stage (a) Fertilized egg ! The blastula is a ball of cells with a fluid-filled cavity called a blastocoel 50 µm (a) Fertilized egg Sperm Cortical Sperm nucleus granules basal body 50 µm 33 © 2014 Pearson Education, Inc. 34 © 2014 Pearson Education, Inc. Figure 47.6b (c) Early blastula 35 (d) Later blastula © 2014 Pearson Education, Inc. Figure 47.6c 36 © 2014 Pearson Education, Inc. Figure 47.6d Cleavage Pattern in Frogs ! In frogs and many other land animals, cleavage is asymmetric due to the distribution of yolk (stored nutrients) ! The vegetal pole has more yolk; the animal pole has less yolk 50 µm 50 µm (b) Four-cell stage (c) Early blastula (d) Later blastula 37 © 2014 Pearson Education, Inc. Figure 47.7-1 38 © 2014 Pearson Education, Inc. Vegetal hemisphere Gray crescent 2-cell stage forming 39 © 2014 Pearson Education, Inc. Figure 47.7-2 Zygote Animal hemisphere Cleavage furrow Vegetal hemisphere 2-cell stage forming Gray crescent 0.25 mm Animal pole 0.25 mm 41 Vegetal hemisphere Gray crescent 8-cell stage Blastocoel 2-cell stage forming 0.25 mm 4-cell stage forming Animal pole 8-cell stage Blastula (cross section) 42 © 2014 Pearson Education, Inc. Figure 47.7a Zygote Animal hemisphere Cleavage furrow 4-cell stage forming Animal pole 40 © 2014 Pearson Education, Inc. Figure 47.7-3 Zygote Animal hemisphere Cleavage furrow 0.25 mm © 2014 Pearson Education, Inc. ! The yolk greatly affects the pattern of cleavage 50 µm 8-cell stage (viewed from the animal pole) 43 © 2014 Pearson Education, Inc. 44 © 2014 Pearson Education, Inc. Figure 47.7b Video: Cleavage of a Fertilized Egg Cleavage Pattern in Other Animals ! The first two cleavage furrows in the frog form four equally sized blastomeres 0.25 mm Blastocoel ! The third cleavage is asymmetric, forming unequally sized blastomeres; this asymmetry is due to the yolk in the vegetal hemisphere ! 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 ! Meroblastic cleavage, incomplete division of the egg, occurs in species with yolk-rich eggs, such as reptiles and birds Blastula (at least 128 cells) 45 © 2014 Pearson Education, Inc. 46 © 2014 Pearson Education, Inc. 47 © 2014 Pearson Education, Inc. 48 © 2014 Pearson Education, Inc. Regulation of Cleavage Concept 47.2: Morphogenesis in animals involves specific changes in cell shape, position, and survival ! Initial development is carried out by RNA and proteins deposited in the egg during oogenesis Gastrulation ! Gastrulation rearranges the cells of a blastula into a three-layered embryo, called a gastrula ! After cleavage, the rate of cell division slows and the normal cell cycle is restored ! After cleavage, the egg cytoplasm has been divided among many blastomeres, each of which can make sufficient RNA to program the cell’s metabolism and further development ! The ectoderm forms the outer layer ! Morphogenesis, the process by which cells occupy their appropriate locations, involves ! The endoderm lines the digestive tract ! The mesoderm partly fills the space between the endoderm and ectoderm ! Gastrulation, the movement of cells from the blastula surface to the interior of the embryo ! Each germ layer contributes to specific structures in the adult animal ! Organogenesis, the formation of organs 49 © 2014 Pearson Education, Inc. 50 © 2014 Pearson Education, Inc. 51 © 2014 Pearson Education, Inc. Figure 47.8 Gastrulation in Sea Urchins ! Gastrulation begins at the vegetal pole of the blastula Vegetal plate Blastocoel Mesenchyme cells Vegetal plate Vegetal pole Blastocoel ! Mesenchyme cells migrate into the blastocoel 53 © 2014 Pearson Education, Inc. Blastocoel Mesenchyme cells Vegetal plate Vegetal pole Vegetal pole Figure 47.8a-2 Blastocoel Mesenchyme cells Vegetal plate Animal pole Future ectoderm Future mesoderm Future endoderm Vegetal pole Filopodia Archenteron Archenteron Mesenchyme cells Blastopore 50 µm Ectoderm Future ectoderm Future mesoderm Mouth Future endoderm Mesenchyme (mesoderm forms future skeleton) Blastocoel Archenteron Blastopore Digestive tube (endoderm) Anus (from blastopore) 54 © 2014 Pearson Education, Inc. Animal pole Future ectoderm Future mesoderm Future endoderm Animal pole Filopodia ! The vegetal plate forms from the remaining cells of the vegetal pole and buckles inward through invagination Figure 47.8a-3 52 © 2014 Pearson Education, Inc. Figure 47.8a-1 Animal pole Blastocoel Mesenchyme cells ! The three layers produced by gastrulation are called embryonic germ layers Future ectoderm Future mesoderm Future endoderm Filopodia Figure 47.8a-4 Blastocoel Mesenchyme cells Vegetal plate 55 © 2014 Pearson Education, Inc. Animal pole Vegetal pole Archenteron Future ectoderm Future mesoderm Future endoderm Figure 47.8b Video: Sea Urchin Embryonic Development (Time Lapse) Filopodia Blastocoel Filopodia Archenteron Archenteron Mesenchyme cells Blastocoel Archenteron Blastopore Ectoderm 57 © 2014 Pearson Education, Inc. 56 © 2014 Pearson Education, Inc. Mouth Mesenchyme (mesoderm forms future skeleton) Blastopore Blastocoel Archenteron Blastopore 50 µm Digestive tube (endoderm) Anus (from blastopore) 58 © 2014 Pearson Education, Inc. 59 © 2014 Pearson Education, Inc. 60 © 2014 Pearson Education, Inc. Figure 47.9 Gastrulation in Frogs ! The newly formed cavity is called the archenteron ! This opens through the blastopore, which will become the anus ECTODERM (outer layer of embryo) • Epidermis of skin and its derivatives (including sweat glands, hair follicles) • Nervous and sensory systems • Pituitary gland, adrenal medulla • Jaws and teeth • Germ cells ! Frog gastrulation begins when a group of cells on the dorsal side of the blastula begins to invaginate ! Cells continue to move from the embryo surface into the embryo by involution ! This forms a crease along the region where the gray crescent formed ! These cells become the endoderm and mesoderm MESODERM (middle layer of embryo) ! Cells on the embryo surface will form the ectoderm • Skeletal and muscular systems • Circulatory and lymphatic systems • Excretory and reproductive systems (except germ cells) • Dermis of skin • Adrenal cortex ENDODERM (inner layer of embryo) 61 © 2014 Pearson Education, Inc. 62 © 2014 Pearson Education, Inc. 63 © 2014 Pearson Education, Inc. • Epithelial lining of digestive tract and associated organs (liver, pancreas) • Epithelial lining of respiratory, excretory, and reproductive tracts and ducts 64 • Thymus, thyroid, and parathyroid glands © 2014 Pearson Education, Inc. Figure 47.10 Figure 47.10a CROSS SECTION SURFACE VIEW Animal pole Blastocoel 1 Dorsal lip of blastopore Early gastrula Vegetal pole Blastopore Blastocoel shrinking 2 Blastopore 3 Blastocoel remnant Dorsal lip of blastopore SURFACE VIEW CROSS SECTION Animal pole Blastocoel 1 Archenteron Dorsal lip of blastopore Early gastrula Blastopore Ectoderm Mesoderm Endoderm Figure 47.10c SURFACE VIEW 2 CROSS SECTION Blastocoel shrinking Archenteron SURFACE VIEW 3 Blastopore CROSS SECTION Ectoderm Mesoderm Endoderm Blastocoel remnant Archenteron Dorsal lip of blastopore Blastopore Vegetal pole Blastopore Late gastrula Blastopore Blastopore Future ectoderm Future mesoderm Future endoderm Future ectoderm Future mesoderm Future endoderm Archenteron Future ectoderm Future mesoderm Future endoderm Figure 47.10b Yolk plug Future ectoderm Future mesoderm Future endoderm Blastopore Late gastrula Blastopore Yolk plug 65 © 2014 Pearson Education, Inc. 66 © 2014 Pearson Education, Inc. 67 © 2014 Pearson Education, Inc. Figure 47.11 Fertilized egg Gastrulation in Chicks ! Prior to gastrulation, the embryo is composed of an upper and lower layer, the epiblast and hypoblast, respectively ! The midline thickens and is called the primitive streak ! During gastrulation, epiblast cells move toward the midline of the blastoderm and then into the embryo toward the yolk Gastrulation in Humans Primitive streak Embryo ! Human eggs have very little yolk Yolk ! The hypoblast cells contribute to the sac that surrounds the yolk and a connection between the yolk and the embryo, but do not contribute to the embryo itself ! A blastocyst is the human equivalent of the blastula Primitive streak ! The inner cell mass is a cluster of cells at one end of the blastocyst Epiblast Future ectoderm ! The trophoblast is the outer epithelial layer of the blastocyst and does not contribute to the embryo, but instead initiates implantation Blastocoel 69 © 2014 Pearson Education, Inc. Migrating cells (mesoderm) 70 © 2014 Pearson Education, Inc. Endoderm Hypoblast YOLK 71 © 2014 Pearson Education, Inc. Figure 47.12 68 © 2014 Pearson Education, Inc. 72 © 2014 Pearson Education, Inc. Figure 47.12a Figure 47.12b Endometrial epithelium (uterine lining) Inner cell mass Trophoblast 1 Blastocyst reaches uterus. Uterus Blastocoel ! Following implantation, the trophoblast continues to expand and a set of extraembryonic membranes is formed 2 Blastocyst implants (7 days after fertilization). ! These enclose specialized structures outside of the embryo Expanding region of trophoblast Maternal blood vessel Epiblast Hypoblast Trophoblast Expanding region of trophoblast 3 Extraembryonic membranes start to form (10–11 days), and gastrulation begins (13 days). ! Gastrulation involves the inward movement from the epiblast, through a primitive streak, similar to the chick embryo Uterus Amniotic cavity Extraembryonic mesoderm cells (from epiblast) Chorion (from trophoblast) Amnion Chorion Ectoderm Mesoderm Endoderm Yolk sac Extraembryonic mesoderm 73 Maternal blood vessel Blastocoel Epiblast Hypoblast Yolk sac (from hypoblast) 4 Gastrulation has produced a three-layered embryo with four extraembryonic membranes: the amnion, chorion, yolk sac, and allantois. Endometrial epithelium (uterine lining) Inner cell mass Trophoblast Epiblast Hypoblast Trophoblast 2 Blastocyst implants 1 Blastocyst (7 days after fertilization). reaches uterus. 74 Expanding region of trophoblast 75 76 Allantois © 2014 Pearson Education, Inc. © 2014 Pearson Education, Inc. Figure 47.12c © 2014 Pearson Education, Inc. Figure 47.12d Amniotic cavity Epiblast Hypoblast Yolk sac (from hypoblast) Extraembryonic mesoderm cells (from epiblast) Chorion (from trophoblast) 3 Extraembryonic Video: Ultrasound of Human Fetus 2 Video: Ultrasound of Human Fetus 1 Amnion Chorion Ectoderm Mesoderm Endoderm Expanding region of trophoblast Yolk sac Extraembryonic mesoderm Allantois 4 Gastrulation has produced a three-layered embryo with four extraembryonic membranes: the amnion, chorion, yolk sac, and allantois. membranes start to form (10–11 days), and gastrulation begins (13 days). 77 © 2014 Pearson Education, Inc. © 2014 Pearson Education, Inc. 78 © 2014 Pearson Education, Inc. 79 © 2014 Pearson Education, Inc. 80 © 2014 Pearson Education, Inc. Figure 47.13 Developmental Adaptations of Amniotes ! Land vertebrates form four extraembryonic membranes: the chorion, allantois, amnion, and yolk sac Chorion ! These provide a life-support system for the further development of the embryo Amnion ! In both adaptations, embryos are surrounded by fluid in a sac called the amnion Allantois Yolk sac ! Reproduction outside of aqueous environments required development of ! The four extraembryonic membranes that form around the embryo ! This protects the embryo from desiccation and allows reproduction on dry land ! The chorion functions in gas exchange ! Mammals and reptiles including birds are called amniotes for this reason ! The yolk sac encloses the yolk ! The amnion encloses the amniotic fluid ! The allantois disposes of waste products and contributes to gas exchange ! The shelled egg of birds, other reptiles, and the monotremes ! The uterus of marsupial and eutherian mammals 81 82 © 2014 Pearson Education, Inc. © 2014 Pearson Education, Inc. Organogenesis Neurulation 83 © 2014 Pearson Education, Inc. 84 © 2014 Pearson Education, Inc. Figure 47.14 ! During organogenesis, various regions of the germ layers develop into rudimentary organs ! Neurulation begins as cells from the dorsal mesoderm form the notochord, a rod extending along the dorsal side of the embryo ! Adoption of particular developmental fates may cause cells to change shape or even migrate to a new location in the body Eye Neural folds Neural fold SEM ! The neural tube will become the central nervous system (brain and spinal cord) 1 mm Neural Neural fold plate ! The notochord disappears before birth, but contributes to parts of the discs between vertebrae ! This is an example of induction, when cells or tissues cause a developmental change in nearby cells Figure 47.14a Notochord Ectoderm Mesoderm Neural crest cells Endoderm Archenteron 1 mm Neural crest cells Somite Archenteron (digestive cavity) Outer layer of ectoderm Neural tube 86 © 2014 Pearson Education, Inc. (b) Neural tube formation (c) Somites 87 © 2014 Pearson Education, Inc. Figure 47.14aa Neural folds Neural tube Notochord Coelom Neural crest cells (a) Neural plate formation © 2014 Pearson Education, Inc. Tail bud ! The neural plate soon curves inward, forming the neural tube ! Signaling molecules secreted by the notochord and other mesodermal cells cause the ectoderm above to form the neural plate 85 Somites Neural plate Figure 47.14b-1 88 © 2014 Pearson Education, Inc. Neural fold Figure 47.14b-2 Neural plate Neural fold Neural plate Neural folds 1 mm Neural Neural fold plate Notochord Ectoderm Mesoderm Endoderm Archenteron (a) Neural plate formation Neural crest cells 1 mm 89 © 2014 Pearson Education, Inc. Figure 47.14b-3 90 © 2014 Pearson Education, Inc. Neural fold (b) Neural tube formation Figure 47.14c Neural plate Cell Migration in Organogenesis ! Neural crest cells develop along the neural tube of vertebrates and migrate in the body, eventually forming various parts of the embryo (nerves, parts of teeth, skull bones, and so on) Neural crest cells Neural crest cells Neural tube (b) Neural tube formation © 2014 Pearson Education, Inc. Eye SEM Neural tube Notochord Coelom ! Parts of the somites dissociate to form mesenchyme cells, which migrate individually to new locations 93 94 © 2014 Pearson Education, Inc. (b) Neural tube formation (c) Somites © 2014 Pearson Education, Inc. 92 © 2014 Pearson Education, Inc. Somites Figure 47.14ca Tail bud Eye ! Mesoderm lateral to the notochord forms blocks called somites Outer layer of ectoderm 91 © 2014 Pearson Education, Inc. Somites Tail bud 1 mm Neural crest cells Somite Archenteron (digestive cavity) SEM 95 1 mm 96 © 2014 Pearson Education, Inc. Figure 47.15 Video: Frog Embryo Development Figure 47.15a Organogenesis in Chick and Insects ! Early organogenesis in the chick is quite similar to that in the frog Neural tube Notochord Neural tube Notochord ! By the time the embryo is 3 days old, rudiments of the major organs are readily apparent Archenteron Eye Archenteron Forebrain Somite Coelom Endoderm Mesoderm Ectoderm Lateral fold Blood vessels Somites ! Organogenesis in invertebrates is different, since invertebrate body plans diverge significantly from those of vertebrates Yolk stalk Yolk sac These layers form extraembryonic YOLK membranes. (a) Early organogenesis Yolk stalk These layers Yolk sac form extraembryonic YOLK membranes. Neural tube (b) Late organogenesis ! The mechanisms of organogenesis—for example neurulation—are quite similar, however 97 © 2014 Pearson Education, Inc. (a) Early organogenesis 98 99 © 2014 Pearson Education, Inc. © 2014 Pearson Education, Inc. Mechanisms of Morphogenesis The Cytoskeleton in Morphogenesis 100 © 2014 Pearson Education, Inc. Figure 47.15b Figure 47.16-1 Eye Forebrain Heart Blood vessels Somites Somite Coelom Endoderm Mesoderm Ectoderm Lateral fold Heart ! Morphogenesis in animals but not plants involves movement of cells ! Reorganization of the cytoskeleton is a major force in changing cell shape during development ! In animals, movements of parts of a cell can bring about cell shape changes, or can enable a cell to migrate to a new location ! For example, in neurulation, microtubules oriented from dorsal to ventral in a sheet of ectodermal cells help lengthen the cells along that axis Ectoderm ! The microtubules and microfilaments of the cytoskeleton are essential to these events Neural tube (b) Late organogenesis 101 © 2014 Pearson Education, Inc. Figure 47.16-2 102 © 2014 Pearson Education, Inc. Figure 47.16-3 Ectoderm Neural plate 103 © 2014 Pearson Education, Inc. Figure 47.16-4 Ectoderm Neural plate Microtubules 105 © 2014 Pearson Education, Inc. 104 © 2014 Pearson Education, Inc. Figure 47.16-5 Ectoderm Neural plate Ectoderm Neural plate Microtubules Microtubules Microtubules Actin filaments Actin filaments Actin filaments 106 © 2014 Pearson Education, Inc. Neural tube 107 © 2014 Pearson Education, Inc. 108 © 2014 Pearson Education, Inc. Figure 47.17 Video: Lamellipodia in Cell Migration ! The cytoskeleton also directs convergent extension, a morphogenetic movement in which a sheet of cells undergoes rearrangement to form a longer and narrower shape ! The cytoskeleton also is responsible for cell migration ! Transmembrane glycoproteins called cell adhesion molecules play a key role in migration ! Cells elongate and wedge between each other to form fewer columns of cells 109 © 2014 Pearson Education, Inc. Convergence Cells elongate and crawl between each other. 110 © 2014 Pearson Education, Inc. Extension The sheet of cells becomes longer and narrower. ! Migration also involves the extracellular matrix, a meshwork of secreted glycoproteins and other molecules lying outside the plasma membrane of cells 111 © 2014 Pearson Education, Inc. 112 © 2014 Pearson Education, Inc. Programmed Cell Death ! Programmed cell death is also called apoptosis ! At various times during development, individual cells, sets of cells, or whole tissues stop developing and are engulfed by neighboring cells ! In some cases a structure functions in early stages and is eliminated during later development ! For example, the tail of the tadpole undergoes apoptosis during frog metamorphosis Concept 47.3: Cytoplasmic determinants and inductive signals contribute to cell fate specification ! Determination is the term used to describe the process by which a cell or group of cells becomes committed to a particular fate ! Cells in a multicellular organism share the same genome ! Differences in cell types are the result of the expression of different sets of genes ! Differentiation refers to the resulting specialization in structure and function ! For example, many more neurons are produced in developing embryos than will be needed ! Extra neurons are removed by apoptosis 113 © 2014 Pearson Education, Inc. 114 © 2014 Pearson Education, Inc. Figure 47.18 Epidermis Fate Mapping 115 © 2014 Pearson Education, Inc. Figure 47.18a Epidermis Central nervous system 116 © 2014 Pearson Education, Inc. Figure 47.18b Notochord ! Fate maps are diagrams showing organs and other structures that arise from each region of an embryo Mesoderm Endoderm Blastula ! Classic studies using frogs indicated that cell lineage in germ layers is traceable to blastula cells Neural tube stage (a) Fate map of a frog embryo 64-cell embryos Blastomeres injected with dye Larvae 117 © 2014 Pearson Education, Inc. 118 (b) Cell lineage analysis in a tunicate © 2014 Pearson Education, Inc. Time after fertilization (hours) Figure 47.19 ! Later studies of C. elegans used the ablation (destruction) of single cells to determine the structures that normally arise from each cell ! The researchers were able to determine the lineage of each of the 959 somatic cells in the worm Video: C. elegans Embryo Development (Time Lapse) First cell division Nervous system, outer skin, musculature 10 120 © 2014 Pearson Education, Inc. Figure 47.19a Zygote 0 119 © 2014 Pearson Education, Inc. Musculature, gonads Outer skin, nervous system Germ line (future gametes) Musculature Hatching Intestine Intestine Mouth Anus Vulva Eggs ANTERIOR 121 © 2014 Pearson Education, Inc. 1.2 mm POSTERIOR 122 © 2014 Pearson Education, Inc. 123 © 2014 Pearson Education, Inc. Figure 47.20 Figure 47.21 ! Germ cells are the specialized cells that give rise to sperm or eggs ! Complexes of RNA and protein are involved in the specification of germ cell fate ! With each subsequent cleavage, the P granules are partitioned into the posterior-most cells 126 © 2014 Pearson Education, Inc. 20 µm 1 Newly fertilized egg 3 Two-cell embryo 2 Zygote prior to first division 4 Four-cell embryo ! P granules act as cytoplasmic determinants, fixing germ cell fate at the earliest stage of development Cells with P granules 125 © 2014 Pearson Education, Inc. ! P granules are distributed throughout the newly fertilized egg and move to the posterior end before the first cleavage division 100 µm ! In C. elegans, such complexes are called P granules, persist throughout development, and can be detected in germ cells of the adult worm 124 © 2014 Pearson Education, Inc. 127 © 2014 Pearson Education, Inc. 128 © 2014 Pearson Education, Inc. Figure 47.21a Figure 47.21b Figure 47.21c Figure 47.21d 20 µm 20 µm 20 µm 20 µm 1 Newly fertilized egg 2 Zygote prior to first division 3 Two-cell embryo 4 Four-cell embryo 129 © 2014 Pearson Education, Inc. 130 © 2014 Pearson Education, Inc. 131 © 2014 Pearson Education, Inc. 132 © 2014 Pearson Education, Inc. Figure 47.22 Axis Formation Dorsal Right ! A body plan with bilateral symmetry is found across a range of animals ! The anterior-posterior axis of the frog embryo is determined during oogenesis ! Upon fusion of the egg and sperm, the egg surface rotates with respect to the inner cytoplasm ! This body plan exhibits asymmetry across the dorsal-ventral and anterior-posterior axes ! The animal-vegetal asymmetry indicates where the anterior-posterior axis forms ! The right-left axis is largely symmetrical ! The dorsal-ventral axis is not determined until fertilization ! This cortical rotation brings molecules from one portion of the vegetal cortex to interact with molecules in the inner cytoplasm of the animal hemisphere 133 134 © 2014 Pearson Education, Inc. Left Ventral Animal pole Animal hemisphere 135 ! Later, pH differences between the two sides of the blastoderm establish the dorsal-ventral axis ! In mammals, experiments suggest that orientation of the egg and sperm nuclei before fusion may help establish embryonic axes ! The first two blastomeres of the frog embryo are totipotent (can develop into all the possible cell types) First cleavage Gray crescent Vegetal pole (b) Establishing the axes 136 Experiment Experimental egg (side view) 1a Control group 1b Experimental Gray group crescent Control egg (dorsal view) Gray crescent Future dorsal side Figure 47.23-2 Experiment ! Hans Spemann performed experiments to determine a cell’s developmental potential (range of structures to which it can give rise) Pigmented cortex © 2014 Pearson Education, Inc. Figure 47.23-1 ! In chicks, gravity is involved in establishing the anterior-posterior axis Point of sperm nucleus entry Vegetal hemisphere © 2014 Pearson Education, Inc. Restricting Developmental Potential Posterior (a) The three axes of the fully developed embryo ! This leads to expression of dorsal- and ventralspecific gene expression © 2014 Pearson Education, Inc. Anterior Experimental egg (side view) Control egg (dorsal view) 1a Control group 1b Experimental Gray crescent group Gray crescent Thread Thread 2 Results ! In insects, morphogen gradients establish the anterior-posterior and dorsal-ventral axes 137 © 2014 Pearson Education, Inc. 138 139 © 2014 Pearson Education, Inc. © 2014 Pearson Education, Inc. Cell Fate Determination and Pattern Formation by Inductive Signals The “Organizer” of Spemann and Mangold 140 Normal Belly piece Normal © 2014 Pearson Education, Inc. Figure 47.24 ! In mammals, embryonic cells remain totipotent until the eight-cell stage, much longer than other organisms ! As embryonic cells acquire distinct fates, they influence each other’s fates by induction ! Spemann and Mangold transplanted tissues between early gastrulas and found that the transplanted dorsal lip of the blastopore triggered a second gastrulation in the host ! Progressive restriction of developmental potential is a general feature of development in all animals 141 © 2014 Pearson Education, Inc. 142 © 2014 Pearson Education, Inc. Primary embryo Secondary (induced) embryo Nonpigmented gastrula (recipient embryo) 143 © 2014 Pearson Education, Inc. Results Dorsal lip of blastopore Pigmented gastrula (donor embryo) ! The dorsal lip functions as an organizer of the embryo body plan, inducing changes in surrounding tissues to form notochord, neural tube, and so on ! In general tissue-specific fates of cells are fixed by the late gastrula stage Experiment Primary structures: Neural tube Notochord Secondary structures: Notochord (pigmented cells) Neural tube (mostly nonpigmented cells) 144 © 2014 Pearson Education, Inc. Figure 47.25 Figure 47.25a Anterior Formation of the Vertebrate Limb Limb bud ! Inductive signals play a major role in pattern formation, development of spatial organization ! The wings and legs of chicks, like all vertebrate limbs, begin as bumps of tissue called limb buds ! 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 Anterior AER Limb bud 2 AER ! The embryonic cells in a limb bud respond to positional information indicating location along three axes Digits Limb buds 50 µm ZPA Posterior Anterior Apical ectodermal ridge (AER) ! Proximal-distal axis 50 µm 3 4 Proximal Apical ectodermal ridge (AER) Distal Dorsal Posterior (b) Wing of chick embryo (a) Organizer regions ZPA Posterior Ventral ! Anterior-posterior axis ! It determines how the cell and its descendants respond to future molecular signals Limb buds ! Dorsal-ventral axis 145 © 2014 Pearson Education, Inc. 146 © 2014 Pearson Education, Inc. Figure 47.25aa 147 © 2014 Pearson Education, Inc. (a) Organizer regions 148 © 2014 Pearson Education, Inc. Figure 47.25b 2 Digits 50 µm Apical ectodermal ridge (AER) 3 Anterior 4 (a) Organizer regions ! The ZPA influences development by secreting a protein signal called Sonic hedgehog ! The AER is thickened ectoderm at the bud’s tip ! Implanting cells expressing Sonic hedgehog into the anterior of a normal limb bud results in a mirror image limb ! The second region is the zone of polarizing activity (ZPA) Ventral Proximal ! One limb bud–regulating region is the apical ectodermal ridge (AER) ! The same results are obtained when a ZPA is grafted there ! The ZPA is mesodermal tissue under the ectoderm where the posterior side of the bud is attached to the body Distal Dorsal Posterior (b) Wing of chick embryo 149 © 2014 Pearson Education, Inc. Figure 47.26 150 © 2014 Pearson Education, Inc. 151 © 2014 Pearson Education, Inc. 152 © 2014 Pearson Education, Inc. Figure 47.26a Experiment Cilia and Cell Fate Anterior New ZPA Donor limb bud Host limb bud ! Ciliary function is essential for proper specification of cell fate in the human embryo Results ZPA Posterior 4 ! Motile cilia play roles in left-right specification 3 ! Monocilia (nonmotile cilia) play roles in normal kidney development Results 2 4 2 ! These include immotile sperm, infections of nasal sinuses and bronchi, and situs inversus, a reversal of normal left-right asymmetry 3 2 4 3 ! All of the associated conditions result from a defect that makes cilia immotile 3 4 2 153 © 2014 Pearson Education, Inc. 154 © 2014 Pearson Education, Inc. Figure 47.27 ! Insight into the role of motile cilia in development comes from identification of Kartagener’s syndrome, a set of medical conditions that often appear together 155 © 2014 Pearson Education, Inc. Figure 47.UN01a 156 © 2014 Pearson Education, Inc. Figure 47.UN01b Figure 47.UN02 Nucleic Acid Synthesis (on scale of 0–100) Lungs Heart Liver Spleen Stomach Large intestine Normal location of internal organs DNA Toxin added 35 48 54 71 83 85 88 87 100 96 DNA No toxin 10 24 28 31 47 49 49 53 55 RNA Toxin added 0 6 25 27 33 RNA No toxin 0 3 14 22 27 7 8 Time Point (every 35 min) Location in situs inversus 157 © 2014 Pearson Education, Inc. S 55 1 2 3 4 5 6 9 10 55 S Sperm-egg fusion and depolarization of egg membrane (fast block to polyspermy) G2 M 11 Cell cycle during cleavage stage 158 © 2014 Pearson Education, Inc. M G1 Cortical granule release (cortical reaction) Cell cycle after cleavage stage Formation of fertilization envelope (slow block to polyspermy) 159 © 2014 Pearson Education, Inc. 160 © 2014 Pearson Education, Inc. Figure 47.UN03 Figure 47.UN04 Figure 47.UN05 2-cell stage forming Figure 47.UN06 Neural tube Notochord Animal pole 8-cell stage Coelom Neural tube Notochord Coelom Vegetal pole Blastula Blastocoel 161 © 2014 Pearson Education, Inc. Figure 47.UN07 165 © 2014 Pearson Education, Inc. 162 © 2014 Pearson Education, Inc. 163 © 2014 Pearson Education, Inc. 164 © 2014 Pearson Education, Inc.
