Involvement of Frzb-1 in mesenchymal condensation and cartilage differentiation in the chick limb bud

In developing limb bud, mesenchymal cells form cellular aggregates called " mesenchymal condensations". These condensations show the prepattern of skel etal elements of the limb prior to cartilage differentiation. Roles of vari ous signaling molecules in chondrogenesis in the limb bud have been reporte d. One group of signalling factors includes the Wnt proteins, which have be en shown to have an inhibitory effect on chondrogenesis in the limb bud. Th erefore, regulation of Wnt activity may be important in regulating cartilag e differentiation. Here we show that Frzb-1, which encodes a secreted frizz led-related protein that can bind to Wnt proteins and can antagonize the ac tivity of some Wnts, is expressed in the developing limb bud. At early stag es of limb development, Frzb-1 is expressed in the ventral core mesenchyme of the limb bud, and later Frzb-1 expression becomes restricted to the cent ral core region where mesenchymal condensations occur. At these stages, a c hondrogenic marker gene, aggrecan, is not yet expressed. As limb developmen t proceeds, expression of Frzb-1 is detected in cartilage primordial cells, although ultimately Frzb-1 expression is down-regulated. Similar results w ere obtained in the recombinant limb bud, which was constructed from dissoc iated and re-aggregated mesenchymal cells and an ectodermal jacket with the apical ectodermal ridge. In addition, Frzb-1 expression preceded aggrecan expression in micromass cultures. These results suggest that Frzb-1 has a r ole in condensation formation and cartilage differentiation by regulating W nt activity in the limb bud.