Competence, specification, and commitment in otic placode induction

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Competence, specification, and commitment in otic
placode induction

• Groves AK and Bonner-Fraser M.
• Development 127, 3489-3499 (2000)

Carrie Niziolek carrien HST.721 22 nov 2005
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Introduction

• Competence
• Specification
• Commitment
• Inner ear is derived from the otic placode, but
  much of ectoderm is competent to generate an
  inner ear
• Earlier studies used morphology need an earlier
  marker for successful induction

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Objective

• Determine the timescale of development with
  classic grafting experiments using molecular
  markers
• Pax-2
• Sox-3
• BMP-7
• Notch

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Methods

• Quail-chick grafts
• Isolate regions of ectoderm in quail implant in
  chick embryo
• Visualize molecular markers to determine
  persistence of competence, time course of
  specification and commitment

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Molecular markers

• Expression of molecular markers (assayed by in
  situ hybridization and immunostaining)
• Pax-2 expression at 4-5ss
• Sox-3 at 6ss
• BMP-7 at 7ss
• Notch at 9-10ss

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Results competence

• Which regions of embryo are competent to give
  rise to the otic placode?
• Grafts of quail at HH stage 3-4 do contribute to
  otic placode tissue (they express Pax-2)

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Time-course of competence

• Graft ectoderm from different stages of quail
  embryo
• Competence of non-otic regions declines rapidly
  with age
• 6ss ? no Pax-2 vesicle
• 10ss ? no contributionto invaginating placode

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Immunostaining for competence
3 ss
3 ss
9 ss
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Results specification

• Isolate chick placodal ectoderm at different
  embryonic ages
• Significant specification of Pax-2 at 5-6ss of
  BMP-7 at 7-8ss (restricted to a small part of the
  ectoderm)

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Specification, assayed in situ
Pax-2 expression
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Results commitment

• Graft presumptive otic ectoderm from quail to
  lateral trunk of chick (where it will receive
  different inducing signals)
• Commitment started from 5-6ss, increased with age

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Immunostaining in lateral trunk
(4ss)
(10ss)
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Map of Pax-2-inducing activity

• Tightly-restricted region of Pax-2 expression
  axial level of hindbrain

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Duration of inducing activity

• Graft quail tissue to chick otic placode region
  at later devt stages (11-22ss)
• Quail tissue forms epithelial vesicle, but
  frequently does not express Pax-2 experimental
  decoupling of otic placode differentiation

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Summary

• Map developmental characteristics of the otic
  placode with molecular probes
• Competence of non-otic regions declines with age
  (6-10ss cutoff)
• Isolated tissue can be specified as early as
  5-6ss (as assayed by Pax-2 expression) increases
  with age
• Commitment increases with age (otic tissue
  implanted to other regions is more likely to
  express genetic markers the older it is)

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Further questions

• Gene expression and vesicle formation can be
  experimentally uncoupled ? independently
  regulated
• Successive waves of inducing signals?
• Progressive differentiation?