Creighton University, Department Biomedical Sciences, Omaha, NE 68178, USA. FRITZSCH@CREIGHTON.EDU
An overview is provided of the structural and molecular events causing the transformation of undifferentiated epidermal cells together with the underlying mesenchyme to become the complex, three-dimensional ear. While tremendous progress has been achieved in a few model systems, enough is not yet known about the comparative embryology of ear development to provide causal explanations of the adult structural differences among species. It is hoped that the changes in selector and/or structural genes, as well as changes in the spatiotemporal induction of structural gene activation, and possible changes in the interaction between the various embryonic sources which contribute to the ear, will soon be understood. The most promising new avenue for research appears to be studies which combine classical transplantation tissue experiments with modern gene expression analyses and modern in vitro assays of the role of putative morphogens or trophic factors. It is emphasized that it is not understood what is missing in the developmental program of those salamanders which have lost a basilar papilla. Direct comparison of gene expression patterns and xenoplastic transplantations in salamanders of comparable stages which either do or do not develop this organ should help to clarify the molecular events that have led to this major evolutionary novel feature of the vertebrate ear.