Development of the abdominal musculature in the chicken embryo.

The abdominal musculature is of prime importance for stabilization and flexion movements of the trunk, and of high clinical relevance in hernia surgery. Despite a number of severe congenital malformations involving defective abdominal muscle formation, little is known about abdominal muscle development during embryogenesis. Here, we used the chicken model system to investigate anatomy, morphogenesis, segmental origin, and fiber formation of the abdominal musculature. The specific anatomy of the chicken abdominal musculature was first determined by macroscopic dissections of adult specimens. We then describe the sculpting of the external oblique, internal oblique, transversus, and rectus muscles from a uniform hypaxial muscle blastema on the basis of whole-mount embryos and sections. We show that abdominal muscles arise from somites 24-28, that all abdominal muscles receive muscle cells from multiple somites, and that somites 25 and 26 provide the major source of abdominal muscle cells. We find that the contribution of individual somites to distinct muscle portions is heterogeneous, with a roughly segmental arrangement of muscle fibers in the transversus muscle, and a random mix of fibers of different segmental origin in the rectus muscle. We furthermore show that despite extensive fiber mixing, there is no fusion between fibers of different segments, so that the segmental identity of individual fibers in the abdominal muscle sheets is maintained. We present abdominal muscle formation as a paradigm for the development of large, segment-crossing trunk muscles from segmental myotomal anlagen.