Vertebrate Development

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Vertebrate Development

• Biology II
• Form and Function

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The six stages of vertebrate development

• Fertilization
• Cleavage
• Gastrulation
• Neurulation
• Neural crest formation
• Organogenesis

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Stage of vertebrate development (I)
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Stage of vertebrate development (II)
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Fertilization (I)

• Entry of sperm cell induces activation
• prevents other sperm from entering
• Intitiates second meiotic division of egg nucleus
• Induces polarity

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Fertilization (II)
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Fertilization in sea urchins
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Sperm penetration
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Polarity in early embryos
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Cleavage

• Division of first cell to many within ball of
  same volume (morula) is followed by hollowing of
  that ball to a blastula. Form of cleavage and
  blastulation depends on orientation of yolk and
  nucleus
• In primitive chordates, division is even, towards
  a symmetrical blastula composed of cells of equal
  size
• In amphibians, holoblastic cleavage leads to
  assymetrical blastula
• In reptiles and birds, meroblastic cleavage
  occurs, resulting in a cap of cells on top of the
  yolk
• In mammals, holoblastic cleavage occurs, creating
  a trophoblast containing a blastocoel, with inner
  disc of cells equivalent to a blastodisc

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Yolk distribution in amniotic eggs affects
blastula development
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Holoblastic cleavage

• Cells with little yolk, and central nucleus,
  develop evenly

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Uneven cleavage

• In frog cells, there is more yolk, and nucleus of
  fertilized egg is to one side
• Yolk slows division, so areas of low yolk content
  divide quicker, and create smaller cells (see
  here, front)
• Areas of high yolk content divide more slowly,
  and give rise to larger cells

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Meroblastic cleavage

• Occurring in reptiles, birds and mammals, an
  uneven division of cells causes a cap of cells on
  top of the yolk

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Blastula of mammals and birds

• Cap of cells develops into a blastodisc
• Blastocoel develops in mammals, surrounded by
  trophoblast

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Gastrulation

• Invagination of outer layer of cells to inside of
  the blastula is known as gastrulation, resulting
  in the formation of the gastrula
• Type of gastrulation is a function of type of
  blastula
• End result is three types of germ layer tissue -
  endoderm, mesoderm and ectoderm

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Gastrulation in the lancelet
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Gastrulation in the frog
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Gastrulation in birds
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Gastrulation in mammals
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Neurulation and neural crest formation

• Formation of neural fold (primitive streak) above
  notocord, begins a channel that eventually seals
  on the dorsal surface, forming neural groove
• Mesoderm derived tissue close to notocord develop
  into somites, giving rise to muscles, connective
  tissue and vertebrae
• Layer of cells on dorsal surface of groove form
  neural crest, responsible for formation of
  several important organs
• Associated patches of ectoderm tissue derive into
  placodes, which evetually result in important
  neurally related organs

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Neural tube formation (I)
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Neural tube formation (II)
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Induction

• Proximity of a cell to certain other cells
  controls its development - Spemann and Mangold
• Cells whose fate can be predicted are termed
  determined
• Cells whose fate cannot be reversed are termed
  committed
• Since all cells initially can become any
  tissue... ...Development is a process of
  progressive restriction of gene expression

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Spemann and Mangolds dorsal lip transplant
experiment
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Induction of the vertebrate eye
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Organogenesis

• Ontogeny recapitulates phylogeny
• (and a quick word about extraembryonic membranes)

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Derivation of major tissue types
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Embryonic development of vertebrates (I)
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Embryonic development of vertebrates (II)
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Extraembryonic membranes - Chick embryo
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Extraembryonic membranes - mammalian embryo (I)
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Extraembryonic membranes - mammalian embryo (II)
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The placenta
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Human development
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Developing human at 4 weeks
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Developing human at 7 weeks
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Developing human at 3 months
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Developing human at 4 months
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Ultrasound at 5 months
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Delivery position of foetus