HOXA-2 RESTRICTS THE CHONDROGENIC DOMAIN AND INHIBITS BONE-FORMATION DURING DEVELOPMENT OF THE BRANCHIAL AREA

In Hoxa-2(-/-) embryos, the normal skeletal elements of the second bra nchial arch are replaced by a duplicated set of first arch elements, W e show here that Hoxa-2 directs proper skeletal formation in the secon d arch by preventing chondrogenesis and intramembranous ossification, In normal embryos, Hoxa-2 is expressed throughout the second arch mese nchyme, but is excluded from the chondrogenic condensations. In the ab sence of Hoxa-2, chondrogenesis is activated ectopically within the ro stral Hoxa-2 expression domain to form the mutant set of cartilages. I n Hoxa-2(-/-) embryos the Sox9 expression domain is shifted into the n ormal Hoxa-2 domain. Misexpression of Sox9 in this area produces a phe notype resembling that of the Hoxa-2 mutants. These results indicate t hat Hoxa-2 acts at early stages of the chondrogenic pathway, upstream of Sox9 induction. We also show that Hoxa-2 inhibits dermal bone forma tion when misexpressed in its precursors. Furthermore, molecular analy ses indicate that Cbfa1 is upregulated in the second branchial arches of Hoxa-2 mutant embryos suggesting that prevention of Cbfa1 induction might mediate Hoxa-2 inhibition of dermal bone formation during norma l second arch development. The implications of these results on the pa tterning of the branchial area are discussed.