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Principles of Development   Chapter 8
Early Concepts: Preformation vsEpigenesis  The  question of how a zygote   becomes an animal has been   asked for centuri...
Early Concepts: Preformation vsEpigenesis  Kaspar  Friederich Wolff (1759)   demonstrated there was no preformed   chick ...
Early Concepts: Preformation vsEpigenesis  Epigenesis    is the concept that the   fertilized egg contains building mater...
Key Events in Development  Development  describes the  changes in an  organism from its  earliest beginnings  through mat...
Key Events in Development  Specializationof cell types occurs as a   hierarchy of developmental decisions.    Cell types...
Key Events in Development  The two basic processes responsible for  this progressive subdivision:    Cytoplasmic   local...
Fertilization  Fertilization               is the initial event in   development in sexual reproduction.     Union of ma...
Fertilization  Oocyte     Maturation     Egg   grows in size by accumulating yolk.       Containsmuch mRNA, ribosomes, ...
Fertilization  Mostof these preparations in the egg  occur during the prolonged prophase I.    In   mammals  Oocyte    ...
Fertilization and Activation A  century of   research has   been   conducted on   marine   invertebrates.    Especially ...
Contact Between Sperm & Egg    Broadcast spawners     often release a     chemotactic factor that     attracts sperm to e...
Fertilization in Sea Urchins    Prevention of     polyspermy – only one     sperm can enter.        Fast block          ...
Fertilization in Sea Urchins  The  cortical reaction follows the fusion of   thousands of enzyme-rich cortical granules  ...
Fertilization in Sea Urchins
Fertilization in Sea Urchins    One cortical granule     enzyme causes the     vitelline envelope to     harden.        ...
Fertilization in Sea Urchins  The  increased Ca2+   concentration in the   egg after the cortical   reaction results in a...
Fusion of Pronuclei  Aftersperm and egg membranes fuse,   the sperm loses its flagellum.  Enlarged sperm nucleus is the ...
Cleavage  Cleavage   – rapid  cell divisions  following fertilization.    Very  little growth     occurs.    Each cell ...
Polarity  The eggs and zygotes of many animals (not   mammals) have a definite polarity.  The polarity is defined by the...
Body Axes  The  development of body axes  in frogs is influenced by the  polarity of the egg.                The polarity...
Amount of Yolk   Different types of animals    have different amounts of    yolk in their eggs.       Isolecithal – very...
Cleavage in Frogs    Cleavage planes usually     follow a specific pattern that     is relative to the animal and     veg...
Cleavage in Birds  Meroblastic   cleavage,   incomplete division   of the egg.    Occurs   in species     with yolk-rich...
Direct vs. Indirect Development    When lots of nourishing yolk is present, embryos     develop into a miniature adult.  ...
Blastula A  fluid filled cavity, the blastocoel, forms   within the embryo – a hollow ball of cells now   called a blastu...
Gastrulation  The  morphogenetic   process called   gastrulation   rearranges the cells of   a blastula into a three-   l...
Gastrulation  The three tissue layers produced by   gastrulation are called embryonic germ layers.      The ectoderm for...
Gastrulation – Sea Urchin  Gastrulation in a sea urchin produces an   embryo with a primitive gut (archenteron) and   thr...
Gastrulation - Frog  Result– embryo with gut & 3 germ layers.  More complicated:    Yolk laden cells in vegetal hemisph...
Gastrulation - Chick  Gastrulation     in the chick is affected by the large   amounts of yolk in the egg.  Primitive st...
Gastrulation - Chick    Blastoderm consists of     two layers:      Epiblast  and       hypoblast      Layers separated...
Gastrulation - Mouse  Inmammals the blastula is called a   blastocyst.    Inner   cell mass will become the embryo while...
Suites of Developmental Characters  Two   major groups of triploblastic animals:    Protostomes    Deuterostomes  Diff...
Deuterostome Development  Deuterostomes   include echinoderms  (sea urchins, sea stars etc) and  chordates.    Radial   ...
Deuterostome Development  Regulative  development – the fate of a cell  depends on its interactions with neighbors, not  ...
Deuterostome Development  Deuterostome means second mouth.  The blastopore becomes the anus and the   mouth develops as ...
Deuterostome Development  The  coelom is a body cavity completely   surrounded by mesoderm.    Mesoderm   & coelom form ...
Deuterostome Development  Typicaldeuterostomes have coeloms  that develop by enterocoely.    Vertebrates   use a modifie...
Protostome Development  Protostomes  include flatworms,  annelids and molluscs.    Spiral   cleavage
Protostome Development  Mosaic   development  – cell fate is  determined by the  components of the  cytoplasm found in  e...
Protostome Development  Protostome means first mouth.  Blastopore becomes the mouth.  The second opening will become th...
Protostome Development    In protostomes, a mesodermal band of tissue forms     before the coelom is formed.    The meso...
Two Clades of Protostomes  Lophotrochozoan    protostomes include   annelid worms, molluscs, & some small phyla.    Loph...
Two Clades of Protostomes  The ecdysozoan protostomes include  arthropods, roundworms, and other taxa  that molt their ex...
Building a Body Plan  An organism’s development is   determined by the genome of the zygote   and also by differences tha...
Building a Body Plan  Uneven   distribution of   substances in the egg   called cytoplasmic   determinants results   in s...
Restriction of Cellular Potency  In many species that have cytoplasmic   determinants only the zygote is   totipotent, ca...
Restriction of Cellular Potency  Unevenly distributed cytoplasmic   determinants in the egg cell:    Are important in es...
Restriction of Cellular Potency  As embryonic development proceeds,   the potency of cells becomes   progressively more l...
Cell Fate Determination and PatternFormation by Inductive Signals  Once   embryonic cell division creates   cells that di...
Induction  Induction   is the   capacity of some   cells to cause other   cells to develop in a   certain way.  Dorsal l...
Spemann-Mangold Experiment Transplanting   a piece  of dorsal blastopore  lip from a salamander  gastrula to a ventral  o...
Building a Body Plan  Cell differentiation – the specialization   of cells in their structure and function.  Morphogenes...
Building a Body Plan  The   sequence includes    Cell movement    Changes in adhesion    Cell proliferation  There  i...
Hox Genes    Hox genes control the     subdivision of embryos     into regions of different     developmental fates     a...
Formation of the VertebrateLimb  Inductivesignals play a major role in   pattern formation – the development of   an anim...
Formation of the VertebrateLimb  The molecular cues that control pattern   formation, called positional   information:   ...
Formation of the VertebrateLimb  The  wings and legs of chicks, like all   vertebrate limbs begin as bumps of tissue   ca...
Formation of the VertebrateLimb  One limb-bud organizer region is the apical   ectodermal ridge (AER).      A thickened ...
Morphogenesis  Morphogenesis  is a major aspect of  development in both plants and animals  but only in animals does it i...
The Cytoskeleton, Cell Motility, andConvergent Extension  Changes   in the shape of a cell usually   involve reorganizati...
Changes in Cell Shape  The formation of the  neural tube is  affected by  microtubules and  microfilaments.
Cell Migration  The cytoskeleton also drives cell   migration, or cell crawling.    The    active movement of cells from...
Evo-Devo  Evolutionary  developmental biology -  evolution is a process in which  organisms become different as a result ...
Evo-Devo  Insteadof evolution proceeding by the  gradual accumulation of numerous small  mutations, could it proceed by r...
The Common Vertebrate Heritage    Vertebrates share a     common ancestry and     a common pattern of     early developme...
Amniotes  The embryos of birds, reptiles, and  mammals develop within a fluid-filled sac  that is contained within a shel...
Amniotes  In these three types of organisms, the   three germ layers also give rise to the   four extraembryonic membrane...
Amniotes  Amnion    – fluid filled   membranous sac   that encloses the   embryo. Protects   embryo from shock.  Yolk sa...
Amniotes  Allantois - storage of metabolic wastes during   development.  Chorion - lies beneath the eggshell and   enclo...
The Mammalian Placenta and EarlyMammalian Development  Most mammalian embryos do not   develop within an egg shell.    D...
The Mammalian Placenta and EarlyMammalian Development  Marsupials    Embryosborn at an early stage of    development and...
Mammalian Development  The   eggs of placental mammals:    Are small and store few nutrients.    Exhibit holoblastic cl...
Mammalian Development  Gastrulation and organogenesis  resemble the processes in birds and  other reptiles.
Mammalian Development  Early embryonic development   in a human proceeds through   four stages:    Blastocyst   reaches ...
Mammalian Development The  extraembryonic membranes in mammals  are homologous to those of birds and other  reptiles and ...
Mammalian Development   Amnion        Surrounds embryo        Secretes fluid in which         embryo floats   Yolk sac...
Organogenesis  Variousregions of the three embryonic  germ layers develop into the rudiments  of organs during the proces...
Organogenesis Many  different structures are derived from the three embryonic germ layers during organogenesis.
Derivatives of Ectoderm: NervousSystem and Nerve Growth    Just above the notochord     (mesoderm), the ectoderm     thic...
Derivatives of Ectoderm: NervousSystem and Nerve Growth  Neural       crest cells pinch off from the neural   tube.     ...
Derivatives of Endoderm: DigestiveTube and Survival of Gill Arches    During gastrulation, the     archenteron forms as  ...
Derivatives of Endoderm: DigestiveTube and Survival of Gill Arches  Pharyngeal   pouches are derivatives of the   digesti...
Derivatives of Mesoderm: Support,Movement and the Beating Heart  Most  muscles arise   from mesoderm   along each side of...
Derivatives of Mesoderm: Support,Movement and the Beating Heart    The splitting, fusion and     migration of somites    ...
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Processes on animal development

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Transcript of "Processes on animal development"

  1. 1. Principles of Development Chapter 8
  2. 2. Early Concepts: Preformation vsEpigenesis  The question of how a zygote becomes an animal has been asked for centuries.  As recently as the 18th century, the prevailing theory was a notion called preformation – the idea that the egg or sperm contains an embryo. A preformed miniature infant, or “homunculus,” that simply becomes larger during development.
  3. 3. Early Concepts: Preformation vsEpigenesis  Kaspar Friederich Wolff (1759) demonstrated there was no preformed chick in the early egg.  Undifferentiated granular material became arranged into layers.  The layers thickened, thinned, and folded to produce the embryo.
  4. 4. Early Concepts: Preformation vsEpigenesis  Epigenesis is the concept that the fertilized egg contains building materials only, somehow assembled by an unknown directing force.  Although current ideas of development are essentially epigenetic in concept, far more is known about what directs growth and differentiation.
  5. 5. Key Events in Development  Development describes the changes in an organism from its earliest beginnings through maturity.  Search for commonalities.
  6. 6. Key Events in Development  Specializationof cell types occurs as a hierarchy of developmental decisions.  Cell types arise from conditions created in preceding stages.  Interactions become increasingly restrictive.  With each new stage:  Each stage limits developmental fate.  Cells lose option to become something different  Said to be determined.
  7. 7. Key Events in Development  The two basic processes responsible for this progressive subdivision:  Cytoplasmic localization  Induction
  8. 8. Fertilization  Fertilization is the initial event in development in sexual reproduction.  Union of male and female gametes  Provides for recombination of paternal and maternal genes.  Restores the diploid number.  Activates the egg to begin development.
  9. 9. Fertilization  Oocyte Maturation  Egg grows in size by accumulating yolk.  Containsmuch mRNA, ribosomes, tRNA and elements for protein synthesis.  Morphogenetic determinants direct the activation and repression of specific genes later in post-fertilization development.  Egg nucleus grows in size, bloated with RNA.  Now called the germinal vesicle.
  10. 10. Fertilization  Mostof these preparations in the egg occur during the prolonged prophase I.  In mammals  Oocyte now has a highly structured system.  Afterfertilization it will support nutritional requirements of the embryo and direct its development through cleavage.  Aftermeiosis resumes, the egg is ready to fuse its nucleus with the sperm nucleus.
  11. 11. Fertilization and Activation A century of research has been conducted on marine invertebrates.  Especially sea urchins
  12. 12. Contact Between Sperm & Egg  Broadcast spawners often release a chemotactic factor that attracts sperm to eggs.  Species specific  Sperm enter the jelly layer.  Egg-recognition proteins on the acrosomal process bind to species- specific sperm receptors on the vitelline envelope.
  13. 13. Fertilization in Sea Urchins  Prevention of polyspermy – only one sperm can enter.  Fast block  Depolarization of membrane  Slow block  Cortical reaction resulting in fertilization membrane
  14. 14. Fertilization in Sea Urchins  The cortical reaction follows the fusion of thousands of enzyme-rich cortical granules with the egg membrane.  Cortical granules release contents between the membrane and vitelline envelope.  Creates an osmotic gradient  Water rushes into space  Elevates the envelope  Lifts away all bound sperm except the one sperm that has successfully fused with the egg plasma membrane.
  15. 15. Fertilization in Sea Urchins
  16. 16. Fertilization in Sea Urchins  One cortical granule enzyme causes the vitelline envelope to harden.  Now called the fertilization membrane.  Block to polyspermy is now complete.  Similar process occurs in mammals.
  17. 17. Fertilization in Sea Urchins  The increased Ca2+ concentration in the egg after the cortical reaction results in an increase in the rates of cellular respiration and protein synthesis.  The egg is activated.
  18. 18. Fusion of Pronuclei  Aftersperm and egg membranes fuse, the sperm loses its flagellum.  Enlarged sperm nucleus is the male pronucleus and migrates inward to contact the female pronucleus.  Fusion of male and female pronuclei forms a diploid zygote nucleus.
  19. 19. Cleavage  Cleavage – rapid cell divisions following fertilization.  Very little growth occurs.  Each cell called a blastomere.  Morula – solid ball of cells. First 5-7 divisions.
  20. 20. Polarity  The eggs and zygotes of many animals (not mammals) have a definite polarity.  The polarity is defined by the distribution of yolk.  The vegetal pole has the most yolk and the animal pole has the least.
  21. 21. Body Axes  The development of body axes in frogs is influenced by the polarity of the egg. The polarity of the egg determines the anterior-posterior axis before fertilization. At fertilization, the pigmented cortex slides over the underlying cytoplasm toward the point of sperm entry. This rotation (red arrow) exposes a region of lighter-colored cytoplasm, the gray crescent, which is a marker of the dorsal side. The first cleavage division bisects the gray crescent. Once the anterior- posterior and dorsal-ventral axes are defined, so is the left-right axis.
  22. 22. Amount of Yolk Different types of animals have different amounts of yolk in their eggs.  Isolecithal – very little yolk, even distribution.  Mesolecithal – moderate amount of yolk concentrated at vegetal pole.  Telolecithal – Lots of yolk at vegetal pole.  Centrolecithal – lots of yolk, centrally located.
  23. 23. Cleavage in Frogs  Cleavage planes usually follow a specific pattern that is relative to the animal and vegetal poles of the zygote.  Animal pole blastomeres are smaller.  Blastocoel in animal hemisphere.  Little yolk, cleavage furrows complete.  Holoblastic cleavage
  24. 24. Cleavage in Birds  Meroblastic cleavage, incomplete division of the egg.  Occurs in species with yolk-rich eggs, such as reptiles and birds.  Blastoderm – cap of cells on top of yolk.
  25. 25. Direct vs. Indirect Development  When lots of nourishing yolk is present, embryos develop into a miniature adult.  Direct development  When little yolk is present, young develop into larval stages that can feed.  Indirect development  Mammals have little yolk, but nourish the embryo via the placenta.
  26. 26. Blastula A fluid filled cavity, the blastocoel, forms within the embryo – a hollow ball of cells now called a blastula.
  27. 27. Gastrulation  The morphogenetic process called gastrulation rearranges the cells of a blastula into a three- layered (triploblastic) embryo, called a gastrula, that has a primitive gut.  Diploblastic organisms have two germ layers.
  28. 28. Gastrulation  The three tissue layers produced by gastrulation are called embryonic germ layers.  The ectoderm forms the outer layer of the gastrula.  Outer surfaces, neural tissue  The endoderm lines the embryonic digestive tract.  The mesoderm partly fills the space between the endoderm and ectoderm.  Muscles, reproductive system
  29. 29. Gastrulation – Sea Urchin  Gastrulation in a sea urchin produces an embryo with a primitive gut (archenteron) and three germ layers.  Blastopore – open end of gut, becomes anus in deuterostomes.
  30. 30. Gastrulation - Frog  Result– embryo with gut & 3 germ layers.  More complicated:  Yolk laden cells in vegetal hemisphere.  Blastula wall more than one cell thick.
  31. 31. Gastrulation - Chick  Gastrulation in the chick is affected by the large amounts of yolk in the egg.  Primitive streak – a groove on the surface along the future anterior-posterior axis.  Functionally equivalent to blastopore lip in frog.
  32. 32. Gastrulation - Chick  Blastoderm consists of two layers:  Epiblast and hypoblast  Layers separated by a blastocoel  Epiblast forms endoderm and mesoderm.  Cells on surface of embryo form ectoderm.
  33. 33. Gastrulation - Mouse  Inmammals the blastula is called a blastocyst.  Inner cell mass will become the embryo while trophoblast becomes part of the placenta.  Notice that the gastrula is similar to that of the chick.
  34. 34. Suites of Developmental Characters  Two major groups of triploblastic animals:  Protostomes  Deuterostomes  Differentiated by:  Spiral vs. radial cleavage  Regulative vs. mosaic cleavage  Blastopore becomes mouth vs. anus  Schizocoelous vs. enterocoelous coelom formation.
  35. 35. Deuterostome Development  Deuterostomes include echinoderms (sea urchins, sea stars etc) and chordates.  Radial cleavage
  36. 36. Deuterostome Development  Regulative development – the fate of a cell depends on its interactions with neighbors, not what piece of cytoplasm it has. A blastomere isolated early in cleavage is able to from a whole individual.
  37. 37. Deuterostome Development  Deuterostome means second mouth.  The blastopore becomes the anus and the mouth develops as the second opening.
  38. 38. Deuterostome Development  The coelom is a body cavity completely surrounded by mesoderm.  Mesoderm & coelom form simultaneously.  Inenterocoely, the coelom forms as outpocketing of the gut.
  39. 39. Deuterostome Development  Typicaldeuterostomes have coeloms that develop by enterocoely.  Vertebrates use a modified version of schizocoely.
  40. 40. Protostome Development  Protostomes include flatworms, annelids and molluscs.  Spiral cleavage
  41. 41. Protostome Development  Mosaic development – cell fate is determined by the components of the cytoplasm found in each blastomere.  Morphogenetic determinants.  An isolated blastomere can’t develop.
  42. 42. Protostome Development  Protostome means first mouth.  Blastopore becomes the mouth.  The second opening will become the anus.
  43. 43. Protostome Development  In protostomes, a mesodermal band of tissue forms before the coelom is formed.  The mesoderm splits to form a coelom.  Schizocoely  Not all protostomes have a true coelom.  Pseudocoelomates have a body cavity between mesoderm and endoderm.  Acoelomates have no body cavity at all other than the gut.
  44. 44. Two Clades of Protostomes  Lophotrochozoan protostomes include annelid worms, molluscs, & some small phyla.  Lophophore – horseshoe shaped feeding structure.  Trochophore larva  Feature all four protostome characteristics.
  45. 45. Two Clades of Protostomes  The ecdysozoan protostomes include arthropods, roundworms, and other taxa that molt their exoskeletons.  Ecdysis – shedding of the cuticle.  Many do not show spiral cleavage.
  46. 46. Building a Body Plan  An organism’s development is determined by the genome of the zygote and also by differences that arise between early embryonic cells.  Different genes will be expressed in different cells.
  47. 47. Building a Body Plan  Uneven distribution of substances in the egg called cytoplasmic determinants results in some of these differences.  Position of cells in the early embryo result in differences as well.  Induction
  48. 48. Restriction of Cellular Potency  In many species that have cytoplasmic determinants only the zygote is totipotent, capable of developing into all the cell types found in the adult.
  49. 49. Restriction of Cellular Potency  Unevenly distributed cytoplasmic determinants in the egg cell:  Are important in establishing the body axes.  Set up differences in blastomeres resulting from cleavage.
  50. 50. Restriction of Cellular Potency  As embryonic development proceeds, the potency of cells becomes progressively more limited in all species.
  51. 51. Cell Fate Determination and PatternFormation by Inductive Signals  Once embryonic cell division creates cells that differ from each other,  The cells begin to influence each other’s fates by induction.
  52. 52. Induction  Induction is the capacity of some cells to cause other cells to develop in a certain way.  Dorsal lip of the blastopore induces neural development.  Primary organizer
  53. 53. Spemann-Mangold Experiment Transplanting a piece of dorsal blastopore lip from a salamander gastrula to a ventral or lateral position in another gastrula developed into a notochord & somites and it induced the host ectoderm to form a neural tube.
  54. 54. Building a Body Plan  Cell differentiation – the specialization of cells in their structure and function.  Morphogenesis – the process by which an animal takes shape and differentiated cells end up in their appropriate locations.
  55. 55. Building a Body Plan  The sequence includes  Cell movement  Changes in adhesion  Cell proliferation  There is no “hard-wired” master control panel directing development.  Sequence of local patterns in which one step in development is a subunit of another.  Each step in the developmental hierarchy is a necessary preliminary for the next.
  56. 56. Hox Genes  Hox genes control the subdivision of embryos into regions of different developmental fates along the anteroposterior axis.  Homologous in diverse organisms.  These are master genes that control expression of subordinate genes.
  57. 57. Formation of the VertebrateLimb  Inductivesignals play a major role in pattern formation – the development of an animal’s spatial organization.
  58. 58. Formation of the VertebrateLimb  The molecular cues that control pattern formation, called positional information:  Tella cell where it is with respect to the animal’s body axes.  Determine how the cell and its descendents respond to future molecular signals.
  59. 59. Formation of the VertebrateLimb  The wings and legs of chicks, like all vertebrate limbs begin as bumps of tissue called limb buds.  The embryonic cells within a limb bud respond to positional information indicating location along three axes.
  60. 60. Formation of the VertebrateLimb  One limb-bud organizer region is the apical ectodermal ridge (AER).  A thickened area of ectoderm at the tip of the bud.  The second major limb-bud organizer region is the zone of polarizing activity (ZPA).  A block of mesodermal tissue located underneath the ectoderm where the posterior side of the bud is attached to the body.
  61. 61. Morphogenesis  Morphogenesis is a major aspect of development in both plants and animals but only in animals does it involve the movement of cells.
  62. 62. The Cytoskeleton, Cell Motility, andConvergent Extension  Changes in the shape of a cell usually involve reorganization of the cytoskeleton.
  63. 63. Changes in Cell Shape  The formation of the neural tube is affected by microtubules and microfilaments.
  64. 64. Cell Migration  The cytoskeleton also drives cell migration, or cell crawling.  The active movement of cells from one place to another.  Ingastrulation, tissue invagination is caused by changes in both cell shape and cell migration.
  65. 65. Evo-Devo  Evolutionary developmental biology - evolution is a process in which organisms become different as a result of changes in the genetic control of development.  Genes that control development are similar in diverse groups of animals.  Hox genes
  66. 66. Evo-Devo  Insteadof evolution proceeding by the gradual accumulation of numerous small mutations, could it proceed by relatively few mutations in a few developmental genes?  The induction of legs or eyes by a mutation in one gene suggests that these and other organs can develop as modules.
  67. 67. The Common Vertebrate Heritage  Vertebrates share a common ancestry and a common pattern of early development.  Vertebrate hallmarks all present briefly.  Dorsal neural tube  Notochord  Pharyngeal gill pouches  Postanal tail
  68. 68. Amniotes  The embryos of birds, reptiles, and mammals develop within a fluid-filled sac that is contained within a shell or the uterus.  Organisms with these adaptations form a monophyletic group called amniotes.  Allows for embryo to develop away from water.
  69. 69. Amniotes  In these three types of organisms, the three germ layers also give rise to the four extraembryonic membranes that surround the developing embryo.
  70. 70. Amniotes  Amnion – fluid filled membranous sac that encloses the embryo. Protects embryo from shock.  Yolk sac – stores yolk and pre-dates the amniotes by millions of years.
  71. 71. Amniotes  Allantois - storage of metabolic wastes during development.  Chorion - lies beneath the eggshell and encloses the embryo and other extraembryonic membrane.  As embryo grows, the need for oxygen increases.  Allantois and chorion fuse to form a respiratory surface, the chorioallantoic membrane.  Evolution of the shelled amniotic egg made internal fertilization a requirement.
  72. 72. The Mammalian Placenta and EarlyMammalian Development  Most mammalian embryos do not develop within an egg shell.  Develop within the mother’s body.  Most retained in the mother’s body.  Monotremes  Primitive mammals that lay eggs.  Large yolky eggs resembling bird eggs.  Duck-billed platypus and spiny anteater.
  73. 73. The Mammalian Placenta and EarlyMammalian Development  Marsupials  Embryosborn at an early stage of development and continue development in abdominal pouch of mother.  Placental Mammals  Represent 94% of the class Mammalia.  Evolution of the placenta required:  Reconstruction of extraembryonic membranes.  Modification of oviduct - expanded region formed a uterus.
  74. 74. Mammalian Development  The eggs of placental mammals:  Are small and store few nutrients.  Exhibit holoblastic cleavage.  Show no obvious polarity.
  75. 75. Mammalian Development  Gastrulation and organogenesis resemble the processes in birds and other reptiles.
  76. 76. Mammalian Development  Early embryonic development in a human proceeds through four stages:  Blastocyst reaches uterus.  Blastocyst implants.  Extraembryonic membranes start to form and gastrulation begins.  Gastrulation has produced a 3-layered embryo.
  77. 77. Mammalian Development The extraembryonic membranes in mammals are homologous to those of birds and other reptiles and have similar functions.
  78. 78. Mammalian Development Amnion  Surrounds embryo  Secretes fluid in which embryo floats Yolk sac  Contains no yolk  Source of stem cells that give rise to blood and lymphoid cells  Stem cells migrate to into the developing embryo Allantois  Not needed to store wastes  Contributes to the formation of the umbilical cord Chorion  Forms most of the placenta
  79. 79. Organogenesis  Variousregions of the three embryonic germ layers develop into the rudiments of organs during the process of organogenesis.
  80. 80. Organogenesis Many different structures are derived from the three embryonic germ layers during organogenesis.
  81. 81. Derivatives of Ectoderm: NervousSystem and Nerve Growth  Just above the notochord (mesoderm), the ectoderm thickens to form a neural plate.  Edges of the neural plate fold up to create an elongated, hollow neural tube.  Anterior end of neural tube enlarges to form the brain and cranial nerves.  Posterior end forms the spinal cord and spinal motor nerves.
  82. 82. Derivatives of Ectoderm: NervousSystem and Nerve Growth  Neural crest cells pinch off from the neural tube.  Give rise to  Portions of cranial nerves  Pigment cells  Cartilage  Bone  Ganglia of the autonomic system  Medulla of the adrenal gland  Parts of other endocrine glands  Neural crest cells are unique to vertebrates.  Important in evolution of the vertebrate head and jaws.
  83. 83. Derivatives of Endoderm: DigestiveTube and Survival of Gill Arches  During gastrulation, the archenteron forms as the primitive gut.  This endodermal cavity eventually produces:  Digestive tract  Lining of pharynx and lungs  Most of the liver and pancreas  Thyroid, parathyroid glands and thymus
  84. 84. Derivatives of Endoderm: DigestiveTube and Survival of Gill Arches  Pharyngeal pouches are derivatives of the digestive tract.  Arise in early embryonic development of all vertebrates.  During development, endodermally-lined pharyngeal pouches interact with overlying ectoderm to form gill arches.  In fish, gill arches develop into gills.  In terrestrial vertebrates:  No respiratory function  1st arch and endoderm-lined pouch form upper and lower jaws, and inner ear.  2nd, 3rd, and 4th gill pouches form tonsils, parathyroid gland and thymus.
  85. 85. Derivatives of Mesoderm: Support,Movement and the Beating Heart  Most muscles arise from mesoderm along each side of the neural tube.  The mesoderm divides into a linear series of somites (38 in humans).
  86. 86. Derivatives of Mesoderm: Support,Movement and the Beating Heart  The splitting, fusion and migration of somites produce the:  Axial skeleton  Dermis of dorsal skin  Muscles of the back, body wall, and limbs  Heart  Lateral to the somites the mesoderm splits to form the coelom.
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