Differentiation of somites into sclerotome, dermatome, and myotome is controlled by a complex set of inductive interactions. The ability of axial midline tissues, the notochord and floor plate, to induce sclerotome has been well documented and has led to models in which ventral somite identity is specified by signals derived from the notochord and floor plate. Herein, we provide evidence that Sonic hedgehog, a vertebrate homolog of the Drosophila segment polarity gene hedgehog, is a signal produced by the notochord and floor plate that directs ventral somite differentiation. Sonic hedgehog is expressed in ventral midline tissues at critical times during somite specification and has the ability, when ectopically expressed, to enhance the formation of sclerotome and antagonize the development of dermatome. introduction

A characteristic feature of vertebrate embryos is the transient presence of segmented paraxial mesoderm, termed somites, which give rise to mesodermal derivatives of the trunk and tail (reviewed by Keynes and Stern, 1988; Tam and Trainor, 1994). Cells destined to contribute to somitic mesoderm invaginate through the primitive streak and come to reside in sheets of cells that flank axial structures, the notochord and neural tube. These precursors then irndergo a mesenchymal to epithelial transition to form spherical balls of cells termed epithelial somites. Subsequently, ventral-medial cells of the epithelial somite delaminate to form the sclerotome, whose derivatives include the vertebral column and ribs of the mature organism. Dorsal-lateral cells of the epithelial somite comprise the dermomyotome, which later differentiates into dermatome and subjacent myotome. Cells derived from the lateral edge of the dermomyotome disperse and contribute to limb musculature, dermatome cells form the dermis of the back, and the myotome is responsible for the formation of axial muscle.