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Limb development and evolution: a frog embryo with no apical ectodermal ridge (AER)
M K Richardson1, T F Carl, J Hanken
1Department of Anatomy, St George's Hospital Medical School, London, UK. m.richardson@sghms.ac.uk
Journal of Anatomy
|August 4, 1998
Summary
Direct developing frogs like Eleutherodactylus coqui show unique limb development. Their limb buds lack an apical ectodermal ridge (AER), suggesting this structure may have been lost in some direct developers.
Area of Science:
- Developmental biology
- Comparative embryology
- Herpetology
Background:
- Eleutherodactylus coqui is a direct-developing frog, lacking a tadpole stage.
- Limb bud development in E. coqui occurs earlier than in indirect-developing species.
- Previous studies suggest conserved molecular mechanisms in vertebrate limb development.
Purpose of the Study:
- To compare limb morphogenesis in E. coqui with other vertebrates.
- To investigate the role of the apical ectodermal ridge (AER) in direct-developing frogs.
- To understand evolutionary changes in limb development.
Main Methods:
- Comparative morphological analysis of limb bud development.
- Scanning and transmission electron microscopy of limb buds.
- Experimental manipulation: distal ectoderm removal from hindlimb buds.
Main Results:
- Hindlimb buds were larger and more advanced than forelimb buds, differing from the amniote pattern.
- Limb buds lacked a distinct apical ectodermal ridge (AER) and dorsoventral flattening.
- Distal ectoderm removal did not halt outgrowth but caused anterior skeletal defects, less severe when done earlier.
Conclusions:
- The apical ectodermal ridge (AER) may not be essential for limb outgrowth in E. coqui, potentially indicating its loss in direct developers.
- The AER's role in maintaining limb shape might be mechanical and less critical in species lacking it.
- An AER was likely present in the common ancestor of anurans and amniotes, with subsequent loss in some direct-developing lineages.