The actions of Frzb and the Frizzled receptor protein show how embryonic development represents a ba

(a) Structures of the Frizzled receptor protein and Frzb protein. The Frizzled receptor contains both cysteine-rich domains required for binding the morphogen Wnt and transmembrane domains required for localizing in the cell's plasma membrane. Frzb contains the cysteine-rich domains but lacks the transmembrane domains. (b) The binding of Wnt to the Frizzled receptor in the plasma membrane induces mesoderm development. When Frzb, which is expressed in the Spemann organizer of amphibians or node of mammals, binds to molecules of free Wnt, the effects of Wnt on mesoderm induction are prevented. It is the ratios in which Frzb and Frizzled receptor proteins are expressed during development that fine-tune the inductive activity of the Wnt morphogen.