THE EXPRESSION AND REGULATION OF FOLLISTATIN AND A FOLLISTATIN-LIKE GENE DURING AVIAN SOMITE COMPARTMENTALIZATION AND MYOGENESIS

We report on the normal and experimentally altered expression of two s tructurally related genes, Follistatin and Follistatin-like (Flik), in the somites of avian embryos. In normal chick embryos, Follistatin ex pression can first be seen in the cells of the dorsolateral somite qua rter. During somite maturation, the cells of the dorsomedial quarter a lso express this gene. Within the dermomyotome it seems that only the muscle precursors are Follistatin-positive. The migrating precursors o f limb and tongue muscle as well as the myotome cells show Follistatin expression. The manipulation experiments reveal that the expression o f Follistatin in the somites can be inhibited by notochord signals. Th is effect can be mimicked by sonic hedgehog protein. Flik is expressed in the dorsomedial compartment of the somite and later on in the myot ome. Unlike follistatin, Flik expression requires signals emanating fr om the neural tube. Notochordal influences do not alter Flik expressio n. The expression of both genes does not depend on signals of intermed iate or lateral mesoderm. Since the products of both genes are propose d to antagonize TGF-beta superfamily proteins during gastrulation and neuralization, we postulate that during myogenesis follistatin and fli k counteract inhibiting effects of related molecules on muscle differe ntiation. (C) 1996 Academic Press, Inc.