LATERAL AND AXIAL SIGNALS INVOLVED IN AVIAN SOMITE PATTERNING - A ROLE FOR BMP4

In vertebrates, muscles of the limbs and body wall derive from the lat eral compartment of the embryonic somites, and axial muscles derive fr om the medial compartment. Whereas the mechanisms that direct patterni ng of somites along the dorsoventral axis are beginning to be understo od, little is known about the tissue interactions and signaling molecu les that direct somite patterning along the mediolateral axis. We repo rt the identification of a specific marker for the lateral semitic com partment and its early derivatives, cSim1, an avian homolog of the Dro sophila single minded gene. Using this marker, we provide evidence tha t specification of the lateral semitic lineage results from the antago nistic actions of a diffusible medializing signal from the neural tube and a diffusible lateralizing signal from the lateral plate mesoderm, and we implicate bone morphogenetic protein 4 (BMP4) in directing thi s lateralization.