Competence, specification and commitment in otic placode induction.

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Competence, specification and commitment in otic
placode induction.

• Groves, A.K. and M. Bronner-Fraser
• Development 127, 3489-3499 (2000)
• Presented by Jessica Butler

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Background Information
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Induction

• Definition an interaction between an inducing
  and a responding tissue that alters the path of
  differentiation of the responding tissue (Groves
  and Bronner-Fraser, 2000).

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Discovery of Induction

• Discovered by Spemann and Mangold in 1924

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Methods of induction
Fig. 1.20, Wolpert et al.
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Definitions

• Competence a cell or tissue which can adopt a
  particular fate in an appropriate environment,
  even if it doesnt normally.
• Specification a cell or tissue which, after
  receiving inducing signals can adopt a fate in a
  neutral environment without any further
  signaling.

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Definitions (cont.)

• Commitment a cell or tissue which repeats a
  particular fate, regardless of the environment it
  is in.

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Otic placode

• Patch of placodal ectoderm that forms as an
  ectodermal thickening
• Located next to the hindbrain
• Gives rise to the entire inner ear
• Formation requires inductive effects from the
  mesoderm and developing nervous system

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Otic Placode
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Somites
Fig. 4.2, Wolpert et al.
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The Study
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Methods

• Tissue preparation
• Quail-chick grafts
• Isolation of epiblast tissue
• Collagen gel cultures
• Analysis
• Immunocytochemistry
• In situ hybridization

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Genes studied

• Pax-2
• Sox-3
• BMP-7
• Notch
• Quail tissue examined using QCPN

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Molecular markers of the developing otic placode

• All four genes showed early expression
• Pax-2 at 4-5ss
• Sox-3 at 6ss
• BMP-7 at 7ss
• Notch at 9-10ss
• The genes were only expressed in the ectoderm at
  the otic placode area

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Competence of embryonic ectoderm to form otic
placode

• Quail otic ectoderm grafted to chick ectoderm
• Ectoderm from anterior epiblast of quail grafted
  to chick
• ectoderm

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Grafting results

• Otic ectoderm from quail incorporated into chick
  ectoderm
• 88 of grafts from anterior region expressed
  Pax-2
• 88 expressed BMP-7, 87 Sox-3, and 80 Notch

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Specification of otic placode fate in embryonic
ectoderm

• Pieces of ectoderm from chick cultured on
  collagen gels
• Expression (as of grafts)
• Pax-2 ? 61 at 5-6ss
• BMP-7 ? 53 at 7-8ss

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Pax-2 Expression
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Commitment of embryonic ectoderm to otic placode
fate

• Ectoderm from quail grafted to trunk of chick
  hosts
• Young ectoderm did not express Pax-2
• 5-6ss ectoderm did express
• the gene

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Older ectoderm grafts

• At 9-10ss, almost 100 of grafts expressed Pax-2
• Key
• Pax-2 forming vesicle
• Pax-2 not forming vesicle

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Location of Pax-2-inducing activity in the embryo

• Quail tissue grafted to 7 different locations on
  chick host
• Only locations B,C,D
• expressed Pax-2

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Key Conclusions

• Competence to form the otic placode is found in
  ectoderm that does not normally give rise to it
• Specification of the otic placode w.r.t Pax-2
  BMP-7 happens close to when the genes are
  expressed in vivo
• Otic ectoderm is committed to form the otic
  placode around the time when the tissue becomes
  specified

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Key Conclusions II

• Otic placode induction occurs as a series of
  steps
• All 4 genes are expressed before the otic placode
  is identifiable
• Chick otic placode induction occurs earlier than
  previously thought

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Future Studies

• When does otic placode induction occur?
• The study suggests it could happen at the end of
  gastrulation
• Homozygous mutation of Pax-2 causes inner ear
  defects in mice.
• What about defects in the other genes?
• What tissues give the inducing signals?