CHOXC-8 AND CHOXD-13 EXPRESSION IN EMBRYONIC CHICK SKIN AND CUTANEOUSAPPENDAGE SPECIFICATION

We studied the expression of two distantly clustered Hox genes which c ould, respectively, be involved in specification of dorsal feather-and foot scale-forming skin in the chick embryo: cHoxc-8, a median paralo g, and cHoxd-13, located at the 5' extremity of the HoxD cluster. The cHoxc-8 transcripts are present at embryonic day 3.5 (E3.5) in the sem itic cells, which give rise to the dorsal dermis by E5, and at E6.5-8. 5 in the dorsal dermal and epidermal cells during the first stages of feather morphogenesis. The cHoxd-13 transcripts are present at E4.5-9. 5 in the autopodial mesenchyme and at E10.5-12.5 in the plantar dermis during the initiation of reticulate scale morphogenesis. Both the cHo xc-8 and cHoxd-13 transcripts are no longer detectable after the anlag en stage of cutaneous appendage morphogenesis, Furthermore, heterotopi c dermal-epidermal recombinations of dorsal, plantar, and apteric tiss ues revealed that the epidermal ability or inability to form feathers is already established by the time of skin formation. Retinoic acid (R A) treatment at E11 induces after 12 hr an inhibition of cHoxd-13 expr ession in the plantar dermis, followed by the formation of feather fil aments on the reticulate scales. When E7.5 dorsal explants are treated with RA for 8 days, they form scale-like structures where the Hox tra nscripts are no more detectable. Protein analysis revealed that the pl antar filaments, made up of feather beta-keratins, corresponded to a h omeotic transformation, whereas the scale-like structures, composed al so of feather beta-keratins, were teratoid. These results strengthen t he hypothesis that different homeobox genes play a significant role in specifying the regional identity of the different epidermal territori es. (C) 1997 Wiley-Liss, Inc.