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Animal Development. The question of how a zygote becomes an animal has been asked for centuries As recently as the 18th century, the prevailing theory.

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Presentation on theme: "Animal Development. The question of how a zygote becomes an animal has been asked for centuries As recently as the 18th century, the prevailing theory."— Presentation transcript:

1 Animal Development

2 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


4 WHAT DETERMINES DEVELOPMENT Development is determined by the zygotes 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

5 Big ideas Gametes (fertilizaiton) Zygote (cleavage) Blastula (gastrulation) Gastrula (neurulation) Organogenesis Role of genes & protein concentration gradients Induction: communication from an inducer to a competent responder

6 Fertilization 2 major events: Fertilization brings the haploid nuclei of sperm and egg together, forming a diploid zygote The sperms contact with the eggs surface initiates metabolic reactions in the egg that trigger the onset of embryonic development Most info comes from sea urchin studies –External fertilization –Problems of external fertilization: Dilution/protection of gametes in the enormous volume of the ocean Correct species fertilization Blocking polyspermy

7 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 Acrosomal process adheres to receptors on vitelline layer (species specific) –Sperm/egg membranes fuse, sperm nucleus enters –Na+ influx, depolarization –Depolarization sets up fast block to polyspermy

8 Fast block polyspermy 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

9 The Cortical Reaction Fusion of egg and sperm also initiates the cortical reaction This reaction induces a rise in Ca 2+ in cytoplasm that stimulates cortical granules to release their contents outside the egg Cortical granules fuse w/ membrane –Enzymes –Polysaccharides –Fertilization envelope formed = slow block to polyspermy (follows repolarization) These changes cause formation of a fertilization envelope that functions as a slow block to polyspermy

10 Fast block polyspermy 1 sec before fertilization Point of sperm entry 10 sec after fertilization Spreading wave of calcium ions 20 sec 30 sec 500 µm

11 Activation of the Egg The sharp rise in Ca 2+ in the eggs cytosol increases the rates of cellular respiration and protein synthesis by the egg cell –Chemical signals from cortical rxn cause H+ to be transported out --> increase in pH Nuclei fuse Egg/sperm differences –Egg contains proteins, mRNA not found in sperm –Ca2+ injection, temperature shock can cause artificial activation With these rapid changes in metabolism, the egg is said to be activated

12 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 Minutes

13 Fertilization in Mammals In mammalian fertilization, the cortical reaction modifies the zona pellucida as a slow block to polyspermy

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


16 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


18 LE 47-7 Fertilized egg Four-cell stage Morula Blastula

19 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 eggs polarity

20 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

21 Cleavage planes usually follow a pattern that is relative to the zygotes animal and vegetal poles

22 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

23 Meroblastic cleavage, incomplete division of the egg, occurs in species with yolk-rich eggs, such as reptiles and birds

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

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

28 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 The mechanics of gastrulation in a frog are more complicated than in a sea urchin- INVAGINATION OTHERS- INVOLUTION

30 LE 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

31 Gastrulation in the chick and frog is similar, with cells moving from the embryos surface to an interior location During gastrulation, some epiblast cells move toward the blastoderms midline and then detach and move inward toward the yolk. INVOLUTION

32 LE Future ectoderm Epiblast Migrating cells (mesoderm) YOLK Hypoblast Endoderm Primitive streak



35 Organogenesis During organogenesis, various regions of the germ layers develop into rudimentary organs organs

36 Early in vertebrate organogenesis, the notochord forms from mesoderm, and the neural plate forms from ectoderm Video: Frog Embryo Development Video: Frog Embryo Development

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

38 LE 47-14b Neural fold Neural plate Neural tube Formation of the neural tube Neural crest Outer layer of ectoderm Neural crest The neural plate soon curves inward, forming the neural tube


40 LE 47-14c 1 mm Notochord Archenteron (digestive cavity) Neural tube Neural crest Eye Somites Tail bud SEM Coelom Somite Somites Mesoderm lateral to the notochord forms blocks called somites Lateral to the somites, the mesoderm splits to form the coelom


42 LE 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


44 Many structures are derived from the three embryonic germ layers during organogenesis

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 Embryo Amniotic cavity with amniotic fluid Allantois Amnion Albumen Yolk (nutrients) Yolk sac Chorion Shell

47 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 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-ECTODERM, MESODERM AND ENDODERM

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 The extraembryonic membranes in mammals are homologous to those of birds and other reptiles and develop in a similar way

52 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 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 Ectoderm Neural plate

55 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 Convergence Extension

57 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 Direction of migration 50 µm

59 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 Control embryo Experimental embryo

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

67 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 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 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 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 As embryonic development proceeds, potency of cells becomes more limited

72 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 others fates by induction

73 The Organizer of Spemann and Mangold Based on their famous experiment, Spemann and Mangold concluded that the blastopores 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 Formation of the Vertebrate Limb Inductive signals play a major role in pattern formation, development of spatial organization

77 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 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 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 One limb-bud organizer region is the apical ectodermal ridge (AER) The AER is thickened ectoderm at the buds 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 Tissue transplantation experiments support the hypothesis that the ZPA produces an inductive signal that conveys positional information indicating posterior

84 LE Anterior Posterior New ZPA Host limb bud ZPA Donor limb bud

85 Signal molecules produced by inducing cells influence gene expression in cells receiving them Signal molecules lead to differentiation and the development of particular structures

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