MYOGENESIS IN PARAXIAL MESODERM - PREFERENTIAL INDUCTION BY DORSAL NEURAL-TUBE AND BY CELLS EXPRESSING WNT-1

Previous studies have demonstrated that the neural tube/notochord comp lex is required for skeletal muscle development within somites. In ord er to explore the localization of myogenic inducing signals within the neural tube, dorsal or ventral neural tube halves were cultured in co ntact with single somites or pieces of segmental plate mesoderm. Somit es and segmental plates cultured with the dorsal half of the neural tu be exhibited 70% and 85% myogenic response rates, as determined by imm unostaining for myosin heavy chain, This response was slightly lower t han the 100% response to whole neural tube/notochord, but was much gre ater than the 30% and 10% myogenic response to ventral neural tube wit h and without notochord. These results demonstrate that the dorsal neu ral tube emits a potent myogenic inducing signal which accounts for mo st of the inductive activity of whole neural tube/notochord. However, a role for ventral neural tube/notochord in somite myogenic induction was clearly evident from the larger number of myogenic cells induced w hen both dorsal neural tube and ventral neural tube/notochord were pre sent. To address the role of a specific dorsal neural tube factor in s omite myogenic induction, we tested the ability of Wnt-1-expressing fi broblasts to promote paraxial mesoderm myogenesis in vitro. We found t hat cells expressing Wnt-1 induced a small number of somite and segmen tal plate cells to undergo myogenesis. This finding is consistent with the localized dorsal neural tube inductive activity described above, but since the ventral neural tube/notochord also possesses myogenic in ductive capacity yet does not express Wnt-1, additional inductive fact ors are likely involved.