Origin and segregation of cranial placodes in Xenopus laevis

Dev Biol. 2011 Dec 15;360(2):257-75. doi: 10.1016/j.ydbio.2011.09.024. Epub 2011 Oct 2.

Abstract

Cranial placodes are local thickenings of the vertebrate head ectoderm that contribute to the paired sense organs (olfactory epithelium, lens, inner ear, lateral line), cranial ganglia and the adenohypophysis. Here we use tissue grafting and dye injections to generated fate maps of the dorsal cranial part of the non-neural ectoderm for Xenopus embryos between neural plate and early tailbud stages. We show that all placodes arise from a crescent-shaped area located around the anterior neural plate, the pre-placodal ectoderm. In agreement with proposed roles of Six1 and Pax genes in the specification of a panplacodal primordium and different placodal areas, respectively, we show that Six1 is expressed uniformly throughout most of the pre-placodal ectoderm, while Pax6, Pax3, Pax8 and Pax2 each are confined to specific subregions encompassing the precursors of different subsets of placodes. However, the precursors of the vagal epibranchial and posterior lateral line placodes, which arise from the posteriormost pre-placodal ectoderm, upregulate Six1 and Pax8/Pax2 only at tailbud stages. Whereas our fate map suggests that regions of origin for different placodes overlap extensively with each other and with other ectodermal fates at neural plate stages, analysis of co-labeled placodes reveals that the actual degree of overlap is much smaller. Time lapse imaging of the pre-placodal ectoderm at single cell resolution demonstrates that no directed, large-scale cell rearrangements occur, when the pre-placodal region segregates into distinct placodes at subsequent stages. Our results indicate that individuation of placodes from the pre-placodal ectoderm does not involve large-scale cell sorting in Xenopus.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Animals
  • Ectoderm / cytology
  • Ectoderm / metabolism
  • Embryo, Nonmammalian / metabolism
  • Homeodomain Proteins / genetics
  • Homeodomain Proteins / metabolism
  • Neural Plate / embryology
  • PAX2 Transcription Factor / genetics
  • PAX2 Transcription Factor / metabolism
  • PAX8 Transcription Factor
  • Paired Box Transcription Factors / genetics
  • Paired Box Transcription Factors / metabolism
  • Skull / cytology
  • Skull / embryology*
  • Skull / metabolism
  • Up-Regulation
  • Xenopus Proteins / genetics
  • Xenopus Proteins / metabolism*
  • Xenopus laevis / embryology*

Substances

  • Homeodomain Proteins
  • PAX2 Transcription Factor
  • PAX8 Transcription Factor
  • Paired Box Transcription Factors
  • Pax8 protein, Xenopus
  • Six1 protein, Xenopus
  • Xenopus Proteins