Developmental expression of the chick <i>four‐jointed</i> homologue

<jats:title>Abstract</jats:title><jats:p>Four‐jointed is a type II transmembrane protein that is thought to be cleaved to give rise to a secreted protein. In <jats:italic>Drosophila</jats:italic>, 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 <jats:italic>Drosophila</jats:italic> and mice, Fj is regulated by notch signaling. Here, we have determined the expression of the chick four‐jointed (<jats:italic>fjx</jats:italic>) homologue during embryonic development. We show that <jats:italic>fjx</jats:italic> 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 <jats:italic>fjx</jats:italic> expression in the developing limb bud showed that initially <jats:italic>fjx</jats:italic> is expressed throughout the limb bud, but as the limb develops, highest levels of <jats:italic>fjx</jats:italic> transcripts are found distally. However, by stage 27, <jats:italic>fjx</jats:italic> expression is predominantly found in the central core of the limb bud. Finally, <jats:italic>fjx</jats:italic> expression becomes confined to the developing tendons, ligaments, articular cartilage, and arteries but not the veins. Comparison with <jats:italic>scleraxis</jats:italic> (<jats:italic>scx</jats:italic>), a marker of tendons and ligaments, revealed that they are coexpressed in the majority of tendons but that <jats:italic>fjx</jats:italic> is expressed after <jats:italic>scx</jats:italic>, 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. Developmental Dynamics 235:3085–3091, 2006. © 2006 Wiley‐Liss, Inc.</jats:p>