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47_lecture_presentation_0 Biology II Lecture

1. The document describes key stages in animal development from fertilization through organogenesis. 2. It explains that fertilization involves the acrosomal reaction and cortical reaction fusing the sperm and egg nuclei, activating embryonic development. 3. Gastrulation then rearranges the blastula cells into three germ layers - ectoderm, endoderm and mesoderm - forming the gastrula stage. Organogenesis follows as the germ layers develop into rudimentary organs.

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Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings PowerPoint® Lecture Presentations for Biology Eighth Edition Neil Campbell and Jane Reece Lectures by Chris Romero, updated by Erin Barley with contributions from Joan Sharp Chapter 47 Animal Development
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Overview: A Body-Building Plan • It is difficult to imagine that each of us began life as a single cell (fertilized egg) called a zygote. • A human embryo at about 6–8 weeks after conception shows development of distinctive features.
1 mm
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings • Development is determined by the zygote’s genome and molecules in the egg cytoplasm called Cytoplasmic determinants. • Cell differentiation is the specialization of cells in structure and function. • Morphogenesis is the process by which an animal takes shape / form. • Model organisms are species that are representative of a larger group and easily studied. Classic embryological studies use the sea urchin, frog, chick, and the nematode C. elegans.
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings 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 – Cleavage: cell division creates a hollow ball of cells called a blastula – Gastrulation: cells are rearranged into a three-layered gastrula – Organogenesis: the three germ layers interact and move to give rise to organs.
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings • 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: • Acrosomal Reaction • Cortical Reaction
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings The Acrosomal Reaction • The acrosomal reaction is triggered when the sperm meets the egg. • The acrosome at the tip of the sperm releases hydrolytic enzymes that digest material surrounding the egg. • Gamete contact and/or fusion depolarizes the egg cell membrane and sets up a fast block to polyspermy.
Basal body (centriole) Sperm head Sperm-binding receptors Acrosome Jelly coat Vitelline layer Egg plasma membrane Hydrolytic enzymes Acrosomal process Actin filament Sperm nucleus Sperm plasma membrane Fused plasma membranes Fertilization envelope Cortical granule Perivitelline space EGG CYTOPLASM
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings 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.
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings 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. • The sperm nucleus merges with the egg nucleus and cell division begins.
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Fertilization in Mammals • Fertilization in mammals and other terrestrial animals is internal. • In mammalian fertilization, the cortical reaction modifies the zona pellucida, the extracellular matrix of the egg, as a slow block to polyspermy. • In mammals the first cell division occurs 12–36 hours after sperm binding. • The diploid nucleus forms after this first division of the zygote.
Follicle cell Zona pellucida Sperm nucleus Sperm basal body
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings • 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. • The blastula is a ball of cells with a fluid-filled cavity called a blastocoel.
(a) Fertilized egg (b) Four-cell stage (c) Early blastula (d) Later blastula
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings • The eggs and zygotes of many animals, except mammals, have a definite polarity. • The polarity is defined by distribution of yolk (stored nutrients). • The vegetal pole has more yolk; the animal pole has less yolk.
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings • The three body axes are established by the egg’s polarity and by a cortical rotation following binding of the sperm. • Cortical rotation exposes a gray crescent opposite to the point of sperm entry.
(a) The three axes of the fully developed embryo (b) Establishing the axes Pigmented cortex Right First cleavage Dorsal Left Posterior Ventral Anterior Gray crescent Future dorsal side Vegetal hemisphere Vegetal pole - yolk Animal pole Animal hemisphere Point of sperm nucleus entry
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings • Cleavage planes usually follow a pattern that is relative to the zygote’s animal and vegetal poles. • Cell division is slowed by yolk. Yolk can cause uneven cell division at the poles. • 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 (cross section) Blastocoel Animal pole 4-cell stage forming 2-cell stage forming Zygote 8-cell stage Vegetal pole: 0.25 mm 0.25 mm
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Gastrulation • Gastrulation rearranges the cells of a blastula into a three-layered embryo, called a gastrula, which has a primitive gut. • 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.
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings The blastula consists of a single layer of cells surrounding the blastocoel. Mesenchyme cells migrate from the vegetal pole into the blastocoel. The vegetal plate forms from the remaining cells of the vegetal pole and buckles inward through invagination. The newly formed cavity is called the archenteron. This opens through the blastopore, which will become the anus. Gastrulation in the sea urchin embryo:
Gastrulation in a sea urchin embryo Future ectoderm Key Future endoderm Digestive tube (endoderm) Mouth Ectoderm Mesenchyme (mesoderm forms future skeleton) Anus (from blastopore) Future mesoderm Blastocoel Archenteron - cavity Blastopore Blastopore Mesenchyme cells Blastocoel Blastocoel Mesenchyme cells Vegetal Pole Invagination Vegetal plate Vegetal pole Animal pole Filopodia pulling archenteron tip 50 µm
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings • The frog blastula is many cell layers thick. Cells of the dorsal lip originate in the gray crescent and invaginate to create the archenteron. • Cells continue to move from the embryo surface into the embryo by involution. These cells become the endoderm and mesoderm. – The blastopore encircles a yolk plug when gastrulation is completed. – The surface of the embryo is now ectoderm, the innermost layer is endoderm, and the middle layer is mesoderm. Gastrulation in the frog
Gastrulation in a frog embryo Future ectoderm Key Future endoderm Future mesoderm SURFACE VIEW Animal pole Vegetal pole Early gastrula Blastopore Blastocoel Dorsal lip of blasto- pore CROSS SECTION Dorsal lip of blastopore Late gastrula Blastocoel shrinking Archenteron Blastocoel remnant Archenteron Blastopore Blastopore Yolk plug Ectoderm Mesoderm Endoderm
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings • The embryo forms from a blastoderm and sits on top of a large yolk mass. • During gastrulation, the upper layer of the blastoderm (epiblast) moves toward the midline of the blastoderm and then into the embryo toward the yolk. • The midline thickens and is called the primitive streak. • The movement of different epiblast cells gives rise to the endoderm, mesoderm, and ectoderm. Gastrulation in the chick
Gastrulation in a chick embryo Endoderm Future ectoderm Migrating cells (mesoderm) Hypoblast Dorsal Fertilized egg Blastocoel YOLK Anterior Right Ventral Posterior Left Epiblast Primitive streak Embryo Yolk Primitive streak
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Organogenesis • During organogenesis, various regions of the germ layers develop into rudimentary organs. • The frog is used as a model for organogenesis. • Early in vertebrate organogenesis, the notochord forms from mesoderm, and the neural plate forms from ectoderm.
Early organogenesis in a frog embryo Neural folds Tail bud Neural tube (b) Neural tube formation Neural fold Neural plate Neural fold Neural plate Neural crest cells Neural crest cells Outer layer of ectoderm Mesoderm Notochord Archenteron Ectoderm Endoderm (a) Neural plate formation (c) Somites Neural tube Coelom Notochord 1 mm 1 mm SEM Somite Neural crest cells Archenteron (digestive cavity) Somites Eye
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings • The neural plate soon curves inward, forming the neural tube. The neural tube will become the central nervous system = brain and spinal cord. • Neural crest cells develop along the neural tube of vertebrates and form various parts of the embryo: nerves, parts of teeth, skull bones ... • Mesoderm lateral to the notochord forms blocks called somites. • Lateral to the somites, the mesoderm splits to form the coelom.
Organogenesis in a chick embryo is similar to that in a frog Endoderm (a) Early organogenesis Neural tube Coelom Notochord These layers form extraembryonic membranes YOLK Heart Eye Neural tube Somite Archenteron Mesoderm Ectoderm Lateral fold Yolk stalk Yolk sac (b) Late organogenesis Somites Forebrain Blood vessels
ECTODERM MESODERM ENDODERM Epidermis of skin and its derivatives (including sweat glands, hair follicles) Epithelial lining of mouth and anus Cornea and lens of eye Nervous system Sensory receptors in epidermis Adrenal medulla Tooth enamel Epithelium of pineal and pituitary glands Notochord Skeletal system Muscular system Muscular layer of stomach and intestine Excretory system Circulatory and lymphatic systems Reproductive system (except germ cells) Dermis of skin Lining of body cavity Adrenal cortex Epithelial lining of digestive tract Epithelial lining of respiratory system Lining of urethra, urinary bladder, and reproductive system Liver Pancreas Thymus Thyroid and parathyroid glands
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings 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. • Amniotes develop extra-embryonic membranes to support the embryo.
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings • During amniote development, four extraembryonic membranes form around the embryo: – The chorion outermost membrane / functions in gas exchange. – The amnion encloses the amniotic fluid. – The yolk sac encloses the yolk. – The allantois disposes of nitrogenous waste products and contributes to gas exchange. Amniote ExtraEmbryonic Membranes
Embryo Amnion Amniotic cavity with amniotic fluid Shell Chorion Yolk sac Yolk (nutrients) Allantois Albumen
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Mammalian Development • The eggs of placental mammals – Are small yolk 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.
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings • At completion of cleavage, the blastocyst forms. • A group of cells called the inner cell mass develops into the embryo and forms the extraembryonic membranes. • The trophoblast, the outer epithelium of the blastocyst, initiates implantation in the uterus, and the inner cell mass of the blastocyst forms a flat disk of cells. • As implantation is completed, gastrulation begins.
Early embryonic development of a human Blastocoel Trophoblast Uterus Endometrial epithelium (uterine lining) Inner cell mass
Early embryonic development of a human Trophoblast Hypoblast Maternal blood vessel Expanding region of trophoblast Epiblast
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings • The epiblast cells invaginate through a primitive streak to form mesoderm and endoderm. • The placenta is formed from the trophoblast, mesodermal cells from the epiblast, and adjacent endometrial tissue. • The placenta allows for the exchange of materials between the mother and embryo. • By the end of gastrulation, the embryonic germ layers have formed. The extraembryonic membranes in mammals are homologous to those of birds and other reptiles and develop in a similar way.
Early embryonic development of a human Yolk sac (from hypoblast) Hypoblast Expanding region of trophoblast Amniotic cavity Epiblast Extraembryonic mesoderm cells (from epiblast) Chorion (from trophoblast)
Early embryonic development of a human Yolk sac Mesoderm Amnion Chorion Ectoderm Extraembryonic mesoderm Atlantois Endoderm
Four stages in early embryonic development of a human Yolk sac Mesoderm Amnion Chorion Ectoderm Extraembryonic mesoderm Trophoblast Endoderm Hypoblast Expanding region of trophoblast Epiblast Maternal blood vessel Allantois Trophoblast Hypoblast Endometrial epithelium (uterine lining) Inner cell mass Blastocoel Uterus Epiblast Amniotic cavity Expanding region of trophoblast Yolk sac (from hypoblast) Chorion (from trophoblast) Extraembryonic mesoderm cells (from epiblast)
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Morphogenesis in animals involves specific changes in cell shape, position, and adhesion • Morphogenesis is a major aspect of development in plants and animals. • Only in animals does it involve the movement of cells.
Copyright © 2008 Pearson Education, Inc., publishing as Pearson 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.
Change in cell shape during morphogenesis Neural tube Actin filaments Microtubules Ectoderm Neural plate
Copyright © 2008 Pearson Education, Inc., publishing as Pearson 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.
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Role of Cell Adhesion Molecules and the Extracellular Matrix • Cell adhesion molecules located on cell surfaces 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.
Cadherin is required for development of the blastula Control embryo Embryo without EP cadherin 0.25 mm 0.25 mm RESULTS
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings The developmental fate of cells depends on their history and on • Cells in a multicellular organism share the same genome. • Differences in cell types is the result of differentiation, the expression of different genes = differential gene expression.
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings 1. During early cleavage divisions, embryonic cells must become different from one another. – If the egg’s cytoplasm is heterogenous, dividing cells vary in the cytoplasmic determinants they contain. 2. After cell asymmetries are set up, interactions among embryonic cells influence their fate, usually causing changes in gene expression – This mechanism is called induction, and is mediated by diffusible chemicals or cell-cell interactions. Two general principles underlie differentiation
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings • 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. • To understand how embryonic cells acquire their fates, think about how basic axes of the embryo are established.
Fate Mapping for two chordates Epidermis (b) Cell lineage analysis in a tunicate (a) Fate map of a frog embryo Epidermis Blastula Neural tube stage (transverse section) Central nervous system Notochord Mesoderm Endoderm 64-cell embryos Larvae Blastomeres injected with dye
Copyright © 2008 Pearson Education, Inc., publishing as Pearson 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 the body axes. • 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.
Copyright © 2008 Pearson Education, Inc., publishing as Pearson 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. • As embryonic development proceeds, potency of cells becomes more limited. • After embryonic cell division creates cells that differ from each other, the cells begin to influence each other’s fates by induction signals.
How does distribution of the gray crescent affect the development potential of the two daughter cells? Thread Gray crescent Experimental egg (side view) Gray crescent Control egg (dorsal view) EXPERIMENT Normal Belly piece Normal RESULTS
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings The Dorsal Lip = “Organizer” of Spemann and Mangold • Based on their famous experiment, Hans Spemann and Hilde Mangold concluded that the blastopore’s dorsal lip is an organizer of the embryo. • The Spemann organizer initiates inductions that result in formation of the notochord, neural tube, and other organs.
Can the dorsal lip of the blastopore induce cells in another part of the amphibian embryo to change their developmental fate? Primary structures: Neural tube Dorsal lip of blastopore Secondary (induced) embryo Notochord Pigmented gastrula (donor embryo) EXPERIMENT Primary embryo RESULTS Nonpigmented gastrula (recipient embryo) Secondary structures: Notochord (pigmented cells) Neural tube (mostly nonpigmented cells)
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Formation of the Vertebrate Limb • Inductive signals play a major role in pattern formation, development of spatial organization. • 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.
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings • The wings and legs of chicks, like all vertebrate limbs, begin as bumps of tissue called limb buds. • The embryonic cells in a limb bud respond to positional information indicating location along three axes – Proximal-distal axis – Anterior-posterior axis – Dorsal-ventral axis
Vertebrate limb development (a) Organizer regions Apical ectodermal ridge (AER) Digits Limb buds (b) Wing of chick embryo Posterior Anterior Limb bud AER ZPA 50 µm Anterior 2 3 4 Posterior Ventral Distal Dorsal Proximal
Copyright © 2008 Pearson Education, Inc., publishing as Pearson 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. • Hox genes also play roles during limb pattern formation.
Review Sperm-egg fusion and depolarization of egg membrane (fast block to polyspermy) Cortical granule release (cortical reaction) Formation of fertilization envelope (slow block to polyspermy)
Review: Cleavage frog embryo Blastocoel Animal pole 2-cell stage forming 8-cell stage Blastula Vegetal pole: yolk
Review: Gastrulation / Early Embryonic Development Sea urchin Frog Chick/bird
Review: Early Organogenesis Neural tube Coelom Notochord Coelom Notochord Neural tube
Review: Fate Map of Frog Embryo Species: Stage:
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings You should now be able to: 1. Describe the acrosomal reaction. 2. Describe the cortical reaction. 3. Distinguish among meroblastic cleavage and holoblastic cleavage. 4. Compare the formation of a blastula and gastrulation in a sea urchin, a frog, and a chick. 5. List and explain the functions of the extraembryonic membranes.
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings 6. Describe the role of the extracellular matrix in embryonic development. 7. Describe two general principles that integrate our knowledge of the genetic and cellular mechanisms underlying differentiation. 8. Explain the significance of Spemann’s organizer in amphibian development. 9. Explain pattern formation in a developing chick limb.

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47_lecture_presentation_0 Biology II Lecture

  • 1.
    Copyright © 2008Pearson Education, Inc., publishing as Pearson Benjamin Cummings PowerPoint® Lecture Presentations for Biology Eighth Edition Neil Campbell and Jane Reece Lectures by Chris Romero, updated by Erin Barley with contributions from Joan Sharp Chapter 47 Animal Development
  • 2.
    Copyright © 2008Pearson Education, Inc., publishing as Pearson Benjamin Cummings Overview: A Body-Building Plan • It is difficult to imagine that each of us began life as a single cell (fertilized egg) called a zygote. • A human embryo at about 6–8 weeks after conception shows development of distinctive features.
  • 3.
  • 4.
    Copyright © 2008Pearson Education, Inc., publishing as Pearson Benjamin Cummings • Development is determined by the zygote’s genome and molecules in the egg cytoplasm called Cytoplasmic determinants. • Cell differentiation is the specialization of cells in structure and function. • Morphogenesis is the process by which an animal takes shape / form. • Model organisms are species that are representative of a larger group and easily studied. Classic embryological studies use the sea urchin, frog, chick, and the nematode C. elegans.
  • 5.
    Copyright © 2008Pearson Education, Inc., publishing as Pearson Benjamin Cummings 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 – Cleavage: cell division creates a hollow ball of cells called a blastula – Gastrulation: cells are rearranged into a three-layered gastrula – Organogenesis: the three germ layers interact and move to give rise to organs.
  • 6.
    Copyright © 2008Pearson Education, Inc., publishing as Pearson Benjamin Cummings • 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: • Acrosomal Reaction • Cortical Reaction
  • 7.
    Copyright © 2008Pearson Education, Inc., publishing as Pearson Benjamin Cummings The Acrosomal Reaction • The acrosomal reaction is triggered when the sperm meets the egg. • The acrosome at the tip of the sperm releases hydrolytic enzymes that digest material surrounding the egg. • Gamete contact and/or fusion depolarizes the egg cell membrane and sets up a fast block to polyspermy.
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    Basal body (centriole) Sperm head Sperm-binding receptors Acrosome Jelly coatVitelline layer Egg plasma membrane Hydrolytic enzymes Acrosomal process Actin filament Sperm nucleus Sperm plasma membrane Fused plasma membranes Fertilization envelope Cortical granule Perivitelline space EGG CYTOPLASM
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    Copyright © 2008Pearson Education, Inc., publishing as Pearson Benjamin Cummings 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.
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    Copyright © 2008Pearson Education, Inc., publishing as Pearson Benjamin Cummings 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. • The sperm nucleus merges with the egg nucleus and cell division begins.
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    Copyright © 2008Pearson Education, Inc., publishing as Pearson Benjamin Cummings Fertilization in Mammals • Fertilization in mammals and other terrestrial animals is internal. • In mammalian fertilization, the cortical reaction modifies the zona pellucida, the extracellular matrix of the egg, as a slow block to polyspermy. • In mammals the first cell division occurs 12–36 hours after sperm binding. • The diploid nucleus forms after this first division of the zygote.
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    Copyright © 2008Pearson Education, Inc., publishing as Pearson Benjamin Cummings • 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. • The blastula is a ball of cells with a fluid-filled cavity called a blastocoel.
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    (a) Fertilized egg(b) Four-cell stage (c) Early blastula (d) Later blastula
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    Copyright © 2008Pearson Education, Inc., publishing as Pearson Benjamin Cummings • The eggs and zygotes of many animals, except mammals, have a definite polarity. • The polarity is defined by distribution of yolk (stored nutrients). • The vegetal pole has more yolk; the animal pole has less yolk.
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    Copyright © 2008Pearson Education, Inc., publishing as Pearson Benjamin Cummings • The three body axes are established by the egg’s polarity and by a cortical rotation following binding of the sperm. • Cortical rotation exposes a gray crescent opposite to the point of sperm entry.
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    (a) The threeaxes of the fully developed embryo (b) Establishing the axes Pigmented cortex Right First cleavage Dorsal Left Posterior Ventral Anterior Gray crescent Future dorsal side Vegetal hemisphere Vegetal pole - yolk Animal pole Animal hemisphere Point of sperm nucleus entry
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    Copyright © 2008Pearson Education, Inc., publishing as Pearson Benjamin Cummings • Cleavage planes usually follow a pattern that is relative to the zygote’s animal and vegetal poles. • Cell division is slowed by yolk. Yolk can cause uneven cell division at the poles. • 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.
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    Copyright © 2008Pearson Education, Inc., publishing as Pearson Benjamin Cummings Gastrulation • Gastrulation rearranges the cells of a blastula into a three-layered embryo, called a gastrula, which has a primitive gut. • 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.
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    Copyright © 2008Pearson Education, Inc., publishing as Pearson Benjamin Cummings The blastula consists of a single layer of cells surrounding the blastocoel. Mesenchyme cells migrate from the vegetal pole into the blastocoel. The vegetal plate forms from the remaining cells of the vegetal pole and buckles inward through invagination. The newly formed cavity is called the archenteron. This opens through the blastopore, which will become the anus. Gastrulation in the sea urchin embryo:
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    Gastrulation in asea urchin embryo Future ectoderm Key Future endoderm Digestive tube (endoderm) Mouth Ectoderm Mesenchyme (mesoderm forms future skeleton) Anus (from blastopore) Future mesoderm Blastocoel Archenteron - cavity Blastopore Blastopore Mesenchyme cells Blastocoel Blastocoel Mesenchyme cells Vegetal Pole Invagination Vegetal plate Vegetal pole Animal pole Filopodia pulling archenteron tip 50 µm
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    Copyright © 2008Pearson Education, Inc., publishing as Pearson Benjamin Cummings • The frog blastula is many cell layers thick. Cells of the dorsal lip originate in the gray crescent and invaginate to create the archenteron. • Cells continue to move from the embryo surface into the embryo by involution. These cells become the endoderm and mesoderm. – The blastopore encircles a yolk plug when gastrulation is completed. – The surface of the embryo is now ectoderm, the innermost layer is endoderm, and the middle layer is mesoderm. Gastrulation in the frog
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    Gastrulation in a frog embryo Futureectoderm Key Future endoderm Future mesoderm SURFACE VIEW Animal pole Vegetal pole Early gastrula Blastopore Blastocoel Dorsal lip of blasto- pore CROSS SECTION Dorsal lip of blastopore Late gastrula Blastocoel shrinking Archenteron Blastocoel remnant Archenteron Blastopore Blastopore Yolk plug Ectoderm Mesoderm Endoderm
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    Copyright © 2008Pearson Education, Inc., publishing as Pearson Benjamin Cummings • The embryo forms from a blastoderm and sits on top of a large yolk mass. • During gastrulation, the upper layer of the blastoderm (epiblast) moves toward the midline of the blastoderm and then into the embryo toward the yolk. • The midline thickens and is called the primitive streak. • The movement of different epiblast cells gives rise to the endoderm, mesoderm, and ectoderm. Gastrulation in the chick
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    Gastrulation in a chick embryo Endoderm Future ectoderm Migrating cells (mesoderm) Hypoblast DorsalFertilized egg Blastocoel YOLK Anterior Right Ventral Posterior Left Epiblast Primitive streak Embryo Yolk Primitive streak
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    Copyright © 2008Pearson Education, Inc., publishing as Pearson Benjamin Cummings Organogenesis • During organogenesis, various regions of the germ layers develop into rudimentary organs. • The frog is used as a model for organogenesis. • Early in vertebrate organogenesis, the notochord forms from mesoderm, and the neural plate forms from ectoderm.
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    Early organogenesis ina frog embryo Neural folds Tail bud Neural tube (b) Neural tube formation Neural fold Neural plate Neural fold Neural plate Neural crest cells Neural crest cells Outer layer of ectoderm Mesoderm Notochord Archenteron Ectoderm Endoderm (a) Neural plate formation (c) Somites Neural tube Coelom Notochord 1 mm 1 mm SEM Somite Neural crest cells Archenteron (digestive cavity) Somites Eye
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    Copyright © 2008Pearson Education, Inc., publishing as Pearson Benjamin Cummings • The neural plate soon curves inward, forming the neural tube. The neural tube will become the central nervous system = brain and spinal cord. • Neural crest cells develop along the neural tube of vertebrates and form various parts of the embryo: nerves, parts of teeth, skull bones ... • Mesoderm lateral to the notochord forms blocks called somites. • Lateral to the somites, the mesoderm splits to form the coelom.
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    Organogenesis in achick embryo is similar to that in a frog Endoderm (a) Early organogenesis Neural tube Coelom Notochord These layers form extraembryonic membranes YOLK Heart Eye Neural tube Somite Archenteron Mesoderm Ectoderm Lateral fold Yolk stalk Yolk sac (b) Late organogenesis Somites Forebrain Blood vessels
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    ECTODERM MESODERM ENDODERM Epidermisof skin and its derivatives (including sweat glands, hair follicles) Epithelial lining of mouth and anus Cornea and lens of eye Nervous system Sensory receptors in epidermis Adrenal medulla Tooth enamel Epithelium of pineal and pituitary glands Notochord Skeletal system Muscular system Muscular layer of stomach and intestine Excretory system Circulatory and lymphatic systems Reproductive system (except germ cells) Dermis of skin Lining of body cavity Adrenal cortex Epithelial lining of digestive tract Epithelial lining of respiratory system Lining of urethra, urinary bladder, and reproductive system Liver Pancreas Thymus Thyroid and parathyroid glands
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    Copyright © 2008Pearson Education, Inc., publishing as Pearson Benjamin Cummings 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. • Amniotes develop extra-embryonic membranes to support the embryo.
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    Copyright © 2008Pearson Education, Inc., publishing as Pearson Benjamin Cummings • During amniote development, four extraembryonic membranes form around the embryo: – The chorion outermost membrane / functions in gas exchange. – The amnion encloses the amniotic fluid. – The yolk sac encloses the yolk. – The allantois disposes of nitrogenous waste products and contributes to gas exchange. Amniote ExtraEmbryonic Membranes
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    Copyright © 2008Pearson Education, Inc., publishing as Pearson Benjamin Cummings Mammalian Development • The eggs of placental mammals – Are small yolk 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.
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    Copyright © 2008Pearson Education, Inc., publishing as Pearson Benjamin Cummings • At completion of cleavage, the blastocyst forms. • A group of cells called the inner cell mass develops into the embryo and forms the extraembryonic membranes. • The trophoblast, the outer epithelium of the blastocyst, initiates implantation in the uterus, and the inner cell mass of the blastocyst forms a flat disk of cells. • As implantation is completed, gastrulation begins.
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    Early embryonic developmentof a human Blastocoel Trophoblast Uterus Endometrial epithelium (uterine lining) Inner cell mass
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    Early embryonic developmentof a human Trophoblast Hypoblast Maternal blood vessel Expanding region of trophoblast Epiblast
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    Copyright © 2008Pearson Education, Inc., publishing as Pearson Benjamin Cummings • The epiblast cells invaginate through a primitive streak to form mesoderm and endoderm. • The placenta is formed from the trophoblast, mesodermal cells from the epiblast, and adjacent endometrial tissue. • The placenta allows for the exchange of materials between the mother and embryo. • By the end of gastrulation, the embryonic germ layers have formed. The extraembryonic membranes in mammals are homologous to those of birds and other reptiles and develop in a similar way.
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    Early embryonic developmentof a human Yolk sac (from hypoblast) Hypoblast Expanding region of trophoblast Amniotic cavity Epiblast Extraembryonic mesoderm cells (from epiblast) Chorion (from trophoblast)
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    Early embryonic developmentof a human Yolk sac Mesoderm Amnion Chorion Ectoderm Extraembryonic mesoderm Atlantois Endoderm
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    Four stages inearly embryonic development of a human Yolk sac Mesoderm Amnion Chorion Ectoderm Extraembryonic mesoderm Trophoblast Endoderm Hypoblast Expanding region of trophoblast Epiblast Maternal blood vessel Allantois Trophoblast Hypoblast Endometrial epithelium (uterine lining) Inner cell mass Blastocoel Uterus Epiblast Amniotic cavity Expanding region of trophoblast Yolk sac (from hypoblast) Chorion (from trophoblast) Extraembryonic mesoderm cells (from epiblast)
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    Copyright © 2008Pearson Education, Inc., publishing as Pearson Benjamin Cummings Morphogenesis in animals involves specific changes in cell shape, position, and adhesion • Morphogenesis is a major aspect of development in plants and animals. • Only in animals does it involve the movement of cells.
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    Copyright © 2008Pearson Education, Inc., publishing as Pearson 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.
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    Change in cellshape during morphogenesis Neural tube Actin filaments Microtubules Ectoderm Neural plate
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    Copyright © 2008Pearson Education, Inc., publishing as Pearson 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.
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    Copyright © 2008Pearson Education, Inc., publishing as Pearson Benjamin Cummings Role of Cell Adhesion Molecules and the Extracellular Matrix • Cell adhesion molecules located on cell surfaces 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.
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    Cadherin is requiredfor development of the blastula Control embryo Embryo without EP cadherin 0.25 mm 0.25 mm RESULTS
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    Copyright © 2008Pearson Education, Inc., publishing as Pearson Benjamin Cummings The developmental fate of cells depends on their history and on • Cells in a multicellular organism share the same genome. • Differences in cell types is the result of differentiation, the expression of different genes = differential gene expression.
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    Copyright © 2008Pearson Education, Inc., publishing as Pearson Benjamin Cummings 1. During early cleavage divisions, embryonic cells must become different from one another. – If the egg’s cytoplasm is heterogenous, dividing cells vary in the cytoplasmic determinants they contain. 2. After cell asymmetries are set up, interactions among embryonic cells influence their fate, usually causing changes in gene expression – This mechanism is called induction, and is mediated by diffusible chemicals or cell-cell interactions. Two general principles underlie differentiation
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    Copyright © 2008Pearson Education, Inc., publishing as Pearson Benjamin Cummings • 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. • To understand how embryonic cells acquire their fates, think about how basic axes of the embryo are established.
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    Fate Mapping fortwo chordates Epidermis (b) Cell lineage analysis in a tunicate (a) Fate map of a frog embryo Epidermis Blastula Neural tube stage (transverse section) Central nervous system Notochord Mesoderm Endoderm 64-cell embryos Larvae Blastomeres injected with dye
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    Copyright © 2008Pearson Education, Inc., publishing as Pearson 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 the body axes. • 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.
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    Copyright © 2008Pearson Education, Inc., publishing as Pearson 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. • As embryonic development proceeds, potency of cells becomes more limited. • After embryonic cell division creates cells that differ from each other, the cells begin to influence each other’s fates by induction signals.
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    How does distributionof the gray crescent affect the development potential of the two daughter cells? Thread Gray crescent Experimental egg (side view) Gray crescent Control egg (dorsal view) EXPERIMENT Normal Belly piece Normal RESULTS
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    Copyright © 2008Pearson Education, Inc., publishing as Pearson Benjamin Cummings The Dorsal Lip = “Organizer” of Spemann and Mangold • Based on their famous experiment, Hans Spemann and Hilde Mangold concluded that the blastopore’s dorsal lip is an organizer of the embryo. • The Spemann organizer initiates inductions that result in formation of the notochord, neural tube, and other organs.
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    Can the dorsallip of the blastopore induce cells in another part of the amphibian embryo to change their developmental fate? Primary structures: Neural tube Dorsal lip of blastopore Secondary (induced) embryo Notochord Pigmented gastrula (donor embryo) EXPERIMENT Primary embryo RESULTS Nonpigmented gastrula (recipient embryo) Secondary structures: Notochord (pigmented cells) Neural tube (mostly nonpigmented cells)
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    Copyright © 2008Pearson Education, Inc., publishing as Pearson Benjamin Cummings Formation of the Vertebrate Limb • Inductive signals play a major role in pattern formation, development of spatial organization. • 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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    Copyright © 2008Pearson Education, Inc., publishing as Pearson Benjamin Cummings • The wings and legs of chicks, like all vertebrate limbs, begin as bumps of tissue called limb buds. • The embryonic cells in a limb bud respond to positional information indicating location along three axes – Proximal-distal axis – Anterior-posterior axis – Dorsal-ventral axis
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    Vertebrate limb development (a) Organizer regions Apical ectodermal ridge(AER) Digits Limb buds (b) Wing of chick embryo Posterior Anterior Limb bud AER ZPA 50 µm Anterior 2 3 4 Posterior Ventral Distal Dorsal Proximal
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    Copyright © 2008Pearson Education, Inc., publishing as Pearson 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. • Hox genes also play roles during limb pattern formation.
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    Review Sperm-egg fusion and depolarization ofegg membrane (fast block to polyspermy) Cortical granule release (cortical reaction) Formation of fertilization envelope (slow block to polyspermy)
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    Review: Gastrulation /Early Embryonic Development Sea urchin Frog Chick/bird
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    Review: Early Organogenesis Neuraltube Coelom Notochord Coelom Notochord Neural tube
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    Review: Fate Mapof Frog Embryo Species: Stage:
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    Copyright © 2008Pearson Education, Inc., publishing as Pearson Benjamin Cummings You should now be able to: 1. Describe the acrosomal reaction. 2. Describe the cortical reaction. 3. Distinguish among meroblastic cleavage and holoblastic cleavage. 4. Compare the formation of a blastula and gastrulation in a sea urchin, a frog, and a chick. 5. List and explain the functions of the extraembryonic membranes.
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    Copyright © 2008Pearson Education, Inc., publishing as Pearson Benjamin Cummings 6. Describe the role of the extracellular matrix in embryonic development. 7. Describe two general principles that integrate our knowledge of the genetic and cellular mechanisms underlying differentiation. 8. Explain the significance of Spemann’s organizer in amphibian development. 9. Explain pattern formation in a developing chick limb.