Study of Skeletal Myogenesis in Cultures of Unsegmented Paraxial Mesoderm

Recent developments in the field of skeletal myogenesis have identified signaling tissues that control the determination and the differentiation of muscle cells in unsegmented paraxial mesoderm of vertebrate embryos (1 –3 ). Unsegmented mesoderm, called presomitic mesoderm in mouse embryos and segmental plate in avian embryos, requires signals from the notochord, the dorsal neural tube, and the surface ectoderm to positively initiate muscle formation (4 –10 ), whereas signals from the lateral plate and the ventral neural tube negatively control muscle formation (10 –12 ). Together, these positive and negative signals define and specify a localized group of cells within the presomitic mesoderm that differentiate into muscle. Although many of these studies were performed using in vivo microsurgeries (10 –15 ), tissue-culture experiments have provided a useful minimal system to investigate the functions of signals involved in myogenesis during development (4 –9 ,(16 ). Identification of the specific signaling molecules controlling muscle formation is in progress, and for many of these studies, involve culture of unsegmented mesoderm in the presence of embryonic inducing tissues; or in the presence of conditioned medium of cells stably transfected with candidate signal gene (16 ); or in the presence of purified molecules (17 ); or following infection with retroviral vector expressing specific signal molecules (18 ).