2 REGULATORY GENES, CNKX5-1 AND CPAX2, SHOW DIFFERENT RESPONSES TO LOCAL SIGNALS DURING OTIC PLACODE AND VESICLE FORMATION IN THE CHICK-EMBRYO

The early stages of otic placode development depend on signals from ne ighbouring tissues including the hindbrain, The identity of these sign als and of the responding placodal genes, however, is not known, We ha ve identified a chick homeobox gene cNkx5-1, which is expressed in the otic placode beginning at stage 10 and exhibits a dynamic expression pattern during formation and further differentiation of the otic vesic le, In a series of heterotopic transplantation experiments, we demonst rate that cNkx5-1 can be activated in ectopic positions. However, sign ificant differences in otic development and cNkx5-1 gene activity were observed when placodes were transplanted into the more rostral positi ons within the head mesenchyme or into the wing buds of older hosts, T hese results indicate that only the rostral tissues were able to induc e and/or maintain ear development, Ectopically induced cNkx5-1 express ion always reproduced the endogenous pattern within the lateral wall o f the otocyst that is destined to form vestibular structures, In contr ast, cPax2 which is expressed in the medial wall of the early otic ves icle later forming the cochlea never resumed its correct expression pa ttern after transplantation. Our experiments illustrate that only some aspects of gene expression and presumably pattern formation during in ner ear development can be established and maintained ectopically, In particular, the dorsal vestibular structures seem to be programmed ear lier and differently from the ventral cochlear part.