Plasticity of axial identity among somites: Cranial somites can generate vertebrae without expressing Hox genes appropriate to the trunk

Rajiv Kant, Ronald S. Goldstein

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Classic studies have shown that the presomitic mesoderm is already committed to a specific morphological fate, for example, the ability to generate a rib. Hox gene expression in the paraxial mesoderm has also been shown to be fixed early and not susceptible to modulation by an ectopic environment. This is in contrast to the plasticity of Hox expression in neuroectodermal derivatives. We reexamine here the potential of somites for morphological plasticity by transplanting the cranial (occipital) somites 1- 4, that normally produce small contributions to the skull, to the trunk of avian embryos. Surprisingly, the transposed cranial somites are able to form reasonably normal vertebral anlage. In addition, the cranial somitic mesoderm produces intervertebral disks, structures not normally found in the skull. These somites are however unable to generate some elements of the vertebrae, such as the costal process. In contrast to the morphogenetic plasticity of the occipital somites, their characteristic inability to support survival of dorsal root ganglia was not significantly modified by posterior transplantation. Dorsal root ganglia initially developed and then degenerated with the same morphological stages as normally observed. In striking contrast to the plasticity of morphology, we found that all four members of the of the fourth paralogous group of Hox genes that are expressed endogenously at the level of the graft are not upregulated in the caudad-transposed cranial mesoderm. It therefore appears that genes other than those of the Hox family normally expressed at this axial level control the position-specific morphogenesis of ectopic vertebrae formed from cranial somites. In evolutionary terms, the present results imply that occipital somites that were incorporated into the 'New Head' retain the ability to develop according to their original morphogenetic fate, into vertebrae.

Original languageEnglish
Pages (from-to)507-520
Number of pages14
JournalDevelopmental Biology
Issue number2
StatePublished - 15 Dec 1999

Bibliographical note

Funding Information:
First, we express our gratitude to B. Motro, for insightful comments on (and criticisms of) the manuscript and gracious help with the in situs. We are indebted to C. Kalcheim for invaluable discussions and suggestions. Thanks also to Karen Anderson, Camila Avivi and Yael Khafi for assistance with some of the experiments. Plasmids containing probes for Hox b4,c4, and d4 were generously provided by C. Tabin, and Hox a4 by K. Yutzey. This work was supported by grants from the Dysautonomia Foundation Inc., the Israel Institute for Psychobiology-Charles Smith Foundation, and the Health Science Center and Research Authority of Bar-Ilan University, The Aviv Fund for Neuroscience Research. Monoclonal antibody QCPN was obtained from the Developmental Studies Hybridoma Bank developed under the auspices of the NICHD and maintained by the Department of Biological Services, University of Iowa, Iowa City, Iowa.


  • Avian chimeras
  • Hox genes
  • Morphogenesis
  • Plasticity
  • Somites
  • Vertebra


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