13.4 Sunday, Jan. 4 Body Wall Formation in Lamprey TULENKO, FJ*; KUSAKABE, R; KURATANI, S; BURKE, AC; Wesleyan University; Kobe University; RIKEN Center for Developmental Biology; Wesleyan University email@example.com
Patterning of the musculoskeletal system from embryonic mesoderm has diverged considerably over more than 350 million years of vertebrate evolution, resulting in extraordinary morphological variation across taxonomic groups. In vertebrates, skeletal muscle derives from somitic mesoderm. Whereas most axial musculature is primaxial and forms from somitic cells that differentiate in a somitic environment, muscles of the limbs, diaphragm, and abdominal body wall are abaxial and derive from somitic migratory cells that enter the lateral plate. The lateral somitic frontier (LSF) marks the interface between the primaxial and abaxial domains. Although the LSF has been mapped in mouse and chick, its position in anamniotes is unknown. Lamprey lack jaws and paired fins, and diverged from other vertebrates prior to the radiation of gnathostomes. Thus, lamprey are a key model system for gaining insight into the ontogeny of basal vertebrates and the evolutionary innovations of gnathostomes. We use DiI to label presumptive lateral plate cells in the Japanese River Lamprey (Lethenteron japonicum) to determine if a boundary consistent with our definition of the LSF exists in an agnathan vertebrate. Embryos were injected shortly after somitogenesis and fixed at various developmental stages, up to and including ammocoetes. Myotomes were labeled with the skeletal muscle marker MF20. Preliminary results indicate that DiI labeled cells contribute to the lining of the body coelom and invest the ventral margin of the growing myotome, but do not mix with somitic myofibers. These data suggest that the primaxial myotome displaces the lateral mesoderm ventrally, and lateral plate cells do not contribute to the post-branchial lateral body wall. This supports the hypothesis that the primitive vertebrate trunk was primaxial, and paired appendages evolved as an innovative expansion of the abaxial domain.