EARLY EVENTS DURING AVIAN SKIN APPENDAGE REGENERATION - DEPENDENCE ONEPITHELIAL-MESENCHYMAL INTERACTION AND ORDER OF MOLECULAR REAPPEARANCE

Early molecular events during the development and regeneration of skin appendages were studied using cultured chicken skin explants with epi thelial-mesenchymal recombination. The explant epithelium was separate d from the mesenchyme, rotated 90 degrees or 180 degrees, recombined w ith the mesenchyme, and cultured, After this procedure, existing feath er buds disappeared and new buds were regenerated, The location of the new buds is determined by the original dermal condensations, whereas the orientation is dictated by the original epithelium, The temporal e xpression of key morphogenetic molecules was examined 3, 6, and 20 h a fter recombination by wholemount in situ hybridization and immunostain ing. The results showed the following, (i) Placode formation and the e xpression of wingless-int (Wnt) 7a and Msx-1 in the placode epithelium are mesenchyme dependent, (ii) Hox C6 and neural cell adhesion molecu le (NCAM) expression in the anterior mesenchyme is placode epithelium dependent, (iii) Bone morphogenetic protein (BMP)-2, BMP-4, and fibrob last growth factor (FGF)-4 expression in the original dermal condensat ions was unaffected by recombination, (iv) Old dermal condensations ca n induce new placodes with new Wnt 7a, sonic hedgehog (Shh), and Msx-1 and -2 expression, (v) The new placode epithelium can then induce new Hox C6 and NCAM microgradients in the feather bud mesenchyme, (vi) Th e order of appearance can be classified into four groups in the follow ing order: BMP-2, BMP-4, and FGF-4 (peptide growth factors); Wnt 7a an d Shh (Drosophila segment polarity gene homologs); Msx-1 and Msx-2 (Ms x class homeobox genes); and then Hox C6 (Hox class homeobox genes) an d NCAM (adhesion molecules). These results suggest an order for the mo lecular cascade during the inductive phase of skin appendage developme nt.