Developmental expression of chick Twist and its regulation during limb patterning

We have isolated a chick Twist gene (cTwist) and examined its expression pa ttern during development by whole mount in situ hybridization. In early emb ryos, cTwist transcripts are found in the developing somites, lateral plate mesoderm, limb mesenchyme, branchial arches and head mesenchyme. At later stages, cTwist expression is found in the sclerotome and dermatome, limb bu d mesenchyme, interdigital regions, and distal mesenchyme of the maxillary and mandibular processes. In the developing feathers, cTwist is expressed i n the mesenchyme of the buds and becomes restricted to the proximal region of the feather filaments. Additionally, we report that the expression of cT wist in the limb mesenchyme is regulated by the AER, FGFs, RA and SHH. The FGFs secreted by the AER seem to have a critical role in maintaining cTwist expression. SHH is also able to maintain cTwist expression but only in the presence of the AER. Overall, our results provide new evidence that reinfo rce the existence of an interplay between the cTwist and FGF signalling pat hways.