Crescent, a novel chick gene encoding a Frizzled-like cysteine-rich domain, is expressed in anterior regions during early embryogenesis

We describe the isolation of a novel chicken gene that we have termed crescent, based on the most distinctive stage of its highly dynamic expression pattern during early embryogenesis. Crescent encodes a protein that in its N-terminal half shows the characteristic invariant 9 cysteine residues of the cysteine-rich domain (CRD) found in the Frizzled family of proteins, in Smoothened and in Collagen XVIII. The CRD of several Frizzled proteins have recently been shown to bind to Wg. Unlike Frizzled proteins, crescent does not contain a transmembrane domain and thus can not function as a receptor. Crescent expression is first found at stage XII (E-G and K) in the center of the area pellucida. On primitive streak formation, expression is detected in the entire anterior half of the area pellucida in the hypoblast layer. At maximal streak extension, crescent transcripts are localized primarily to the germinal crescent, where the primordial germ cells reside. During head process and head fold stages, crescent labels the anteriormost endodermal cells which will give rise to prospective foregut. With the commencement of somitogenesis, crescent expression rapidly wanes.