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Title: Principles of Development


1
Principles of Development
  • Chapter 8

2
Early Concepts Preformation vs Epigenesis
  • 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
Early Concepts Preformation vs Epigenesis
  • 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
Early Concepts Preformation vs Epigenesis
  • 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
Key Events in Development
  • Development describes the changes in an organism
    from its earliest beginnings through maturity.
  • Search for commonalities.

6
Key Events in Development
  • Specialization of 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
Key Events in Development
  • The two basic processes responsible for this
    progressive subdivision
  • Cytoplasmic localization
  • Induction

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
Fertilization
  • Oocyte Maturation
  • Egg grows in size by accumulating yolk.
  • Contains much 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
Fertilization
  • Most of these preparations in the egg occur
    during the prolonged prophase I.
  • In mammals
  • Oocyte now has a highly structured system.
  • After fertilization it will support nutritional
    requirements of the embryo and direct its
    development through cleavage.
  • After meiosis resumes, the egg is ready to fuse
    its nucleus with the sperm nucleus.

11
Fertilization and Activation
  • A century of research has been conducted on
    marine invertebrates.
  • Especially sea urchins

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
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
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
Fertilization in Sea Urchins
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
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
Fusion of Pronuclei
  • After sperm 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
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
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
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
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
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
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
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
Blastula
  • A fluid filled cavity, the blastocoel, forms
    within the embryo a hollow ball of cells now
    called a blastula.

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
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
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
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
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
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
Gastrulation - Mouse
  • In mammals 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
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
Deuterostome Development
  • Deuterostomes include echinoderms (sea urchins,
    sea stars etc) and chordates.
  • Radial cleavage

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
Deuterostome Development
  • Deuterostome means second mouth.
  • The blastopore becomes the anus and the mouth
    develops as the second opening.

38
Deuterostome Development
  • The coelom is a body cavity completely surrounded
    by mesoderm.
  • Mesoderm coelom form simultaneously.
  • In enterocoely, the coelom forms as outpocketing
    of the gut.

39
Deuterostome Development
  • Typical deuterostomes have coeloms that develop
    by enterocoely.
  • Vertebrates use a modified version of
    schizocoely.

40
Protostome Development
  • Protostomes include flatworms, annelids and
    molluscs.
  • Spiral cleavage

41
Protostome Development
  • Mosaic development cell fate is determined by
    the components of the cytoplasm found in each
    blastomere.
  • Morphogenetic determinants.
  • An isolated blastomere cant develop.

42
Protostome Development
  • Protostome means first mouth.
  • Blastopore becomes the mouth.
  • The second opening will become the anus.

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
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
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
Building a Body Plan
  • An organisms 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
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
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
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
Restriction of Cellular Potency
  • As embryonic development proceeds, the potency of
    cells becomes progressively more limited in all
    species.

51
Cell Fate Determination and Pattern Formation by
Inductive Signals
  • Once embryonic cell division creates cells that
    differ from each other,
  • The cells begin to influence each others fates
    by induction.

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
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
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
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
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
Formation of the Vertebrate Limb
  • Inductive signals play a major role in pattern
    formation the development of an animals
    spatial organization.

58
Formation of the Vertebrate Limb
  • The molecular cues that control pattern
    formation, called positional information
  • Tell a cell where it is with respect to the
    animals body axes.
  • Determine how the cell and its descendents
    respond to future molecular signals.

59
Formation of the Vertebrate Limb
  • 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
Formation of the Vertebrate Limb
  • 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
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
The Cytoskeleton, Cell Motility, and Convergent
Extension
  • Changes in the shape of a cell usually involve
    reorganization of the cytoskeleton.

63
Changes in Cell Shape
  • The formation of the neural tube is affected by
    microtubules and microfilaments.

64
Cell Migration
  • The cytoskeleton also drives cell migration, or
    cell crawling.
  • The active movement of cells from one place to
    another.
  • In gastrulation, tissue invagination is caused by
    changes in both cell shape and cell migration.

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
Evo-Devo
  • Instead of 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
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
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
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
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
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
The Mammalian Placenta and Early Mammalian
Development
  • Most mammalian embryos do not develop within an
    egg shell.
  • Develop within the mothers body.
  • Most retained in the mothers body.
  • Monotremes
  • Primitive mammals that lay eggs.
  • Large yolky eggs resembling bird eggs.
  • Duck-billed platypus and spiny anteater.

73
The Mammalian Placenta and Early Mammalian
Development
  • Marsupials
  • Embryos born 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
Mammalian Development
  • The eggs of placental mammals
  • Are small and store few nutrients.
  • Exhibit holoblastic cleavage.
  • Show no obvious polarity.

75
Mammalian Development
  • Gastrulation and organogenesis resemble the
    processes in birds and other reptiles.

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
Mammalian Development
  • The extraembryonic membranes in mammals are
    homologous to those of birds and other reptiles
    and have similar functions.

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
Organogenesis
  • Various regions of the three embryonic germ
    layers develop into the rudiments of organs
    during the process of organogenesis.

80
Organogenesis
  • Many different structures are derived from the
    three embryonic germ layers during organogenesis.

81
Derivatives of Ectoderm Nervous System 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
Derivatives of Ectoderm Nervous System 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
Derivatives of Endoderm Digestive Tube 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
Derivatives of Endoderm Digestive Tube 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
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
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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