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 structurally related genes, Follistatin and Follistatin-like (Flik), in the somites of avian embryos. In normal chick embryos, Follistatin expression can first be seen in the cells of the dorsolateral somite quarter. During somite maturation, the cells of the dorsomedial quarter also express this gene. Within the dermomyotome it seems that only the muscle precursors are Follistatin-positive. The migrating precursors of limb and tongue muscle as well as the myotome cells show Follistatin expression. The manipulation experiments reveal that the expression of Follistatin in the somites can be inhibited by notochord signals. This effect can be mimicked by sonic hedgehog protein. Flik is expressed in the dorsomedial compartment of the somite and later on in the myotome. Unlike Follistatin, Flik expression requires signals emanating from the neural tube. Notochordal influences do not alter Flik expression. The expression of both genes does not depend on signals of intermediate or lateral mesoderm. Since the products of both genes are proposed to antagonize TGF-[Beta] superfamily proteins during gastrulation and neuralization, we postulate that during myogenesis follistatin and flik counteract inhibiting effects of related molecules on muscle differentiation.