Developmental expression of the chick four-jointed homologue

Four-jointed is a type II transmembrane protein that is thought to be cleaved to give rise to a secreted protein. In Drosophila, four-jointed controls outgrowth, vein patterning, and bristle polarity in the developing limb together with the polarity of the ommatidia in the developing eye. In Drosophila and mice, Fj is regulated by notch signaling. Here, we have determined the expression of the chick four-jointed (fjx) homologue during embryonic development. We show that fjx is expressed in the limb bud; facial primordia; the proliferating zone of the lens, feather buds, the neural tube; and neural crest derivatives such as the dorsal root ganglia. Analysis of the fjx expression in the developing limb bud showed that initially fjx is expressed throughout the limb bud, but as the limb develops, highest levels of fjx transcripts are found distally. However, by stage 27, fjx expression is predominantly found in the central core of the limb bud. Finally, fjx expression becomes confined to the developing tendons, ligaments, articular cartilage, and arteries but not the veins. Comparison with scleraxis (scx), a marker of tendons and ligaments, revealed that they are coexpressed in the majority of tendons but that fjx is expressed after scx, when the tendons have begun to differentiate. These data suggest that fjx has two roles during limb development: the first controlling outgrowth and the second tissue differentiation.