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Limb development and evolution: a frog embryo with no apical ectodermal ridge (AER)

Published online by Cambridge University Press:  01 April 1998

MICHAEL K. RICHARDSON
Affiliation:
Department of Anatomy, St George's Hospital Medical School, London, UK
TIMOTHY F. CARL
Affiliation:
Department of Environmental, Population, and Organismic Biology, University of Colorado, Boulder, CO, USA
JAMES HANKEN
Affiliation:
Department of Environmental, Population, and Organismic Biology, University of Colorado, Boulder, CO, USA
RICHARD P. ELINSON
Affiliation:
Department of Zoology, University of Toronto, Toronto, Ontario, Canada
CELIA COPE
Affiliation:
Department of Anatomy, St George's Hospital Medical School, London, UK
PETER BAGLEY
Affiliation:
Department of Anatomy, St George's Hospital Medical School, London, UK
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Abstract

The treefrog Eleutherodactylus coqui is a direct developer — it has no tadpole stage. The limb buds develop earlier than in metamorphosing species (indirect developers, such as Xenopus laevis). Previous molecular studies suggest that at least some mechanisms of limb development in E. coqui are similar to those of other vertebrates and we wished to see how limb morphogenesis in this species compares with that in other vertebrates. We found that the hind limb buds are larger and more advanced than the forelimbs at all stages examined, thus differing from the typical amniote pattern. The limb buds were also small compared to those in the chick. Scanning and transmission electron microscopy showed that although the apical ectoderm is thickened, there was no apical ectodermal ridge (AER). In addition, the limb buds lacked the dorsoventral flattening seen in many amniotes. These findings could suggest a mechanical function for the AER in maintaining dorsoventral flattening, although not all data are consistent with this view. Removal of distal ectoderm from E. coqui hindlimb buds does not stop outgrowth, although it does produce anterior defects in the skeletal pattern. The defects are less severe when the excisions are performed earlier. These results contrast with the chick, in which AER excision leads to loss of distal structures. We suggest that an AER was present in the common ancestor of anurans and amniotes and has been lost in at least some direct developers including E. coqui.

Type
Research Article
Copyright
© Anatomical Society of Great Britain and Ireland 1998

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