Abstract

The sensory nervous system of the vertebrate head comprises the three paired sense organs, the eye, ear and olfactory epithelium, and the cranial sensory ganglia. It receives contribution from two cell populations: neural crest cells and sensory placodes. The latter are specialised neurogenic epithelia outside of the central nervous system, which arise from unique multipotential cells in the pre-placodal region. This review summarises our current understanding of how sensory placode progenitors are specified from non-committed embryonic ectoderm and how sensory placodes with characteristic identity are induced from those progenitors. In particular, it focuses on how different signalling pathways converge and are used repeatedly to impart distinct fates.

Highlights

  • The vertebrate head ectoderm contains unique neurogenic regions outside the central nervous system (CNS): the cranial sensory placodes

  • While at gastrula stages placode precursors are widely dispersed (Garcia-Martinez et al, 1993, Hatada and Stern, 1994, Streit, unpublished), a continuous placode territory can first be defined at neurula stages (Kozlowski et al, 1997, Streit, 2002, Bhattacharyya et al, 2004, Xu et al, 2008): all placode precursors locate to a band of ectoderm surrounding the neural plate from forebrain to hindbrain levels

  • Of the many transcription factors that are co-expressed in the pre-placodal region (PPR), only members of the Six and eyes absent (Eya) families coincide precisely with the position of placode precursors (Mishima and Tomarev, 1998, Esteve and Bovolenta, 1999, Kobayashi et al, 2000, Pandur and Moody, 2000, McLarren et al, 2003, Bessarab et al, 2004, Schlosser and Ahrens, 2004, Litsiou et al, 2005, Ishihara et al, 2008)

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Summary

Introduction

The vertebrate head ectoderm contains unique neurogenic regions outside the central nervous system (CNS): the cranial sensory placodes. Like the neural plate, which generates the CNS, they are transient columnar epithelia. They form at distinct rostro-caudal positions next to the neural tube and contribute to the special sense organs associated with hearing, balance, olfaction and vision and to the distal parts of the cranial sensory ganglia (see Figure 1). Their derivatives generate a large variety of cell types ranging from simple lens fibre cells to sensory receptors and neurons. This review concentrates on the molecular mechanisms that specify placode precursors in the embryonic ectoderm and lead to their segregation from other ectodermal derivatives, and the events that generate cell diversity among placode progenitors

Sensory placodes and their derivatives
A unique territory of multipotent sensory progenitors
Six and Eya genes and their role in placode development
Induction of the placode territory through combinatorial signalling
FGF pathway
Bmp pathway
Wnt pathway
Sequential subdivision of the placode territory
Lens suppression by Fgfs and neural crest cells
Signalling pathways in placode induction
Conclusions
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